ALT-JAVA(1)

🌸 alt-java(1) — JDK Commands 🌸

Hiii bestie 💖 okay so welcome to alt-java(1), from the JDK Commands fam. This is the hardened java binary, and she's the one you call when you're launching untrusted code from the Web — like, say, javaws 🙈. Sit down, grab a strawberry drink 🍓, we're doing this.

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✨ NAME ✨

alt-java — the hardened java binary, and honestly she's the one you want on the door when the code is sketchy 👑. Recommended for launching untrusted code from the Web, e.g. javaws. No cap, this is the secure one 💅.

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🎀 SYNOPSIS 🎀

Okay so there are like, four whole ways to summon her. Pick your fighter ✨

To launch a class file:

alt-java [options] mainclass [args ...]

To launch the main class in a JAR file:

alt-java [options] -jar jarfile [args ...]

To launch the main class in a module:

alt-java [options] -m module[/mainclass] [args ...]

or

alt-java [options] --module module[/mainclass] [args ...]

To launch a source-file program:

alt-java [options] source-file [args ...]

Now the pieces that go inside those brackets, one by one 👇

• options — Optional bestie! Command-line options, separated by spaces. There's a whole "Overview of Java Options" situation that describes what's available — go read that for the full menu 🍓.
• mainclass — the name of the class to be launched, periodt. Any command-line entries that come after the classname? Those become the arguments for the main method 👑.
• -jar jarfile — runs a program that's all bundled up inside a JAR file 💅. The jarfile argument is the name of a JAR file whose manifest contains a line in the form Main-Class:classname — that's the line that defines the class holding the public static void main(String[] args) method, aka your application's starting point ✨. Heads up though: when you use -jar, that specified JAR file is the source of all user classes, and every other class path setting gets ignored 🫶. Using JAR files? Then go see jar.
• -m or --module module[/mainclass] — executes the main class in a module, specified by mainclass if you gave one, or — if you didn't — the value in the module 🌸. In other words: mainclass is there for when the module doesn't specify it, or to override the value when it does. Slay. See Standard Options for Java 💜.
• source-file — only used to launch a source-file program! Specifies the source file containing the main class when you're in source-file mode 🦋. See "Using Source-File Mode to Launch Source-Code Programs."
• args ... — Optional: arguments that follow mainclass, source-file, -jar jarfile, and -m or --module module/mainclass get passed as arguments to the main class ✨.

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💖 DESCRIPTION 💖

The alt-java command starts a Java application — that's her whole thing 🔥. How? She starts the Java Virtual Machine (JVM), loads the specified class, and calls that class's main() method. Iconic behavior, honestly.

But there are RULES, bestie. That method must be declared public and static, it must not return any value, and it must accept a String array as a parameter 😤. The declaration looks exactly like this:

public static void main(String[] args)

In source-file mode, the alt-java command can launch a class declared in a source file 🎀. See "Using Source-File Mode to Launch Source-Code Programs" for the whole description of source-file mode.

Note 💌: You can use the JDK_JAVA_OPTIONS launcher environment variable to prepend its content to the actual command line of the alt-java launcher. See "Using the JDK_JAVA_OPTIONS Launcher Environment Variable."

By default, the first argument that isn't an option of the alt-java command is the fully qualified name of the class to be called ✨. And if -jar is specified, then its argument is the name of the JAR file containing the class and resource files for the application — and the startup class must be indicated by the Main-Class manifest header in its manifest file 👑

Arguments after the class file name or the JAR file name? Straight to the main() method. As a treat 🍓.

🌈 alt-javaw 🌈

Windows: the alt-javaw command is identical to alt-java, except that with alt-javaw there's no associated console window 😌. Use alt-javaw when you don't want a command prompt window to appear — she's lowkey, she's mysterious. The alt-javaw launcher will, however, display a dialog box with error information if a launch fails 💀. So like, she still tells you when things go wrong. Communication queen 🫶.

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🧚 USING SOURCE-FILE MODE TO LAUNCH SOURCE-CODE PROGRAMS 🧚

To launch a class declared in a source file, run the alt-java launcher in source-file mode 💅. Whether you enter source-file mode is decided by two items on the alt-java command line:

• The first item on the command line that is not an option or part of an option 🦋. In other words: the item on the command line that would otherwise be the main class name.
• The --source version option, if present ✨.

If the class identifies an existing file that has a .java extension, or if the --source option is specified, then source-file mode is selected 👑. The source file then gets compiled and run. The --source option can be used to specify the source version, or N, of the source code — and that determines the API that can be used 🌸. When you set --source N, you can only use the public API that was defined in JDK N.

Note 💌: The valid values of N change for each release — new values get added and old values get removed, that's just how she grows 😭. You'll get an error message if you use a value of N that is no longer supported. The supported values of N are the current Java SE release (27) and a limited number of previous releases, detailed in the command-line help for alt-javac, under the --source and --release options.

If the file does not have the .java extension, the --source option must be used to tell the alt-java command to use source-file mode 🔥. This is for those cases when the source file is a "script" to be executed and the name of the source file doesn't follow the normal naming conventions for Java source files 💀 — she's flexible like that.

In source-file mode, the effect is as though the source file is compiled into memory, and the first class found in the source file is executed ✨. Any arguments placed after the name of the source file in the original command line get passed to the compiled class when it's executed 🎀.

For example, if a file were named HelloWorld.java and contained a class named HelloWorld, then the source-file mode command to launch the class would be:

alt-java HelloWorld.java

This use of source-file mode is informally equivalent to using the following two commands:

javac -d <memory> --source-path <source-root> HelloWorld.java
java --class-path <memory> HelloWorld

where <source-root> is computed 💫

In source-file mode, any additional command-line options are processed as follows:

• The launcher scans the options specified before the source file for any that are relevant in order to compile the source file 👑. This includes: --class-path, --module-path, --add-exports, --add-modules, --limit-modules, --patch-module, --upgrade-module-path, and any variant forms of those options. It also includes the new --enable-preview option, described in JEP 12 🌸.
• No provision is made to pass any additional options to the compiler, such as -processor or -Werror 🙈. (Yeah, that's just how it is, bestie.)
• Command-line argument files (@-files) may be used in the standard way 🦋. Long lists of arguments for either the VM or the program being invoked may be placed in files specified on the command line by prefixing the filename with an @ character.

In source-file mode, compilation proceeds as follows:

• Any command-line options that are relevant to the compilation environment are taken into account 💜. These include: --class-path/-classpath/-cp, --module-path/-p, --add-exports, --add-modules, --limit-modules, --patch-module, --upgrade-module-path, --enable-preview.
• The root of the source tree, <source-root>, is computed from the package of the class being launched ✨. For example, if HelloWorld.java declared its classes to be in the hello package, then the file HelloWorld.java is expected to reside in the directory somedir/hello/. In this case, somedir is computed to be the root of the source tree 🌸.
• The root of the source tree serves as the source path for compilation, so that other source files found in that tree and needed by HelloWorld could be compiled 🎀.
• Annotation processing is disabled, as if -proc:none is in effect 😌.
• If a version is specified, via the --source option, the value is used as the argument for an implicit --release option for the compilation 👑. This sets both the source version accepted by the compiler and the system API that may be used by the code in the source file.
• If --enable-preview is specified, the --source N arguments can be omitted 💅. If the Java runtime version is N, then --release N is implied when compiling source files.
• If a module-info.java file exists in the <source-root> directory, its module declaration is used to define a named module that will contain all the classes compiled from .java files in the source tree 🌈. If module-info.java does not exist, all the classes compiled from source files will be compiled in the context of the unnamed module.
• The source file that is launched should contain one or more top-level classes, the first of which is taken as the class to be executed ⭐.
• For the source file that is launched, the compiler does not enforce the optional restriction defined at the end of JLS 7.6 — the one saying a type in a named package should exist in a file whose name is composed from the type name followed by the .java extension 🫶. (She's chill about it.)
• If a source file contains errors, appropriate error messages are written to the standard error stream, and the launcher exits with a non-zero exit code 💀. Rude but fair.

In source-file mode, execution proceeds as follows:

• The class to be executed is the first top-level class found in the source file 👑. It must contain a declaration of an entry main method.
• The compiled classes are loaded by a custom class loader, that delegates to the application class loader 🦋. This implies that classes appearing on the application class path cannot refer to any classes declared in source files. Yeah. That's the rule.
• If a module-info.java file exists in the <source-root> directory, then all the classes compiled from .java files in the source tree will be in that module, which will serve as the root module for the execution of the program ✨. If module-info.java does not exist, the compiled classes are executed in the context of an unnamed module, as though --add-modules=ALL-DEFAULT is in effect 🌸. This is in addition to any other --add-modules options that may have been specified on the command line.
• Any arguments appearing after the name of the file on the command line are passed to the main method in the obvious way 💅.
• It is an error if there is a class on the application class path whose name is the same as that of the class to be executed 😭. Don't do it. She will notice.

See JEP 458: Launch Multi-File Source-Code Programs [https://openjdk.org/jeps/458] for complete details 💖.

🦋 USING THE JDK_JAVA_OPTIONS LAUNCHER ENVIRONMENT VARIABLE 🦋

okay so this env var is a whole mood 💅 — JDK_JAVA_OPTIONS prepends its content to the options parsed from the command line. Like, it gets first dibs.

The content of JDK_JAVA_OPTIONS is a list of arguments separated by white-space characters (as determined by isspace()). These get prepended to the command line arguments passed to the alt-java launcher. The encoding requirement for the environment variable is the same as the alt-java command line on the system. And the content of JDK_JAVA_OPTIONS is treated in the same manner as what you'd specify on the command line. fr fr.

Single (') or double (") quotes can be used to enclose arguments that contain whitespace characters. All content between the open quote and the first matching close quote is preserved by simply removing the pair of quotes. In case a matching quote is not found, the launcher will abort with an error message. no cap, it will just dip 💀

@-files are supported as they are specified in the command line. Any wildcard literal * in the JDK_JAVA_OPTIONS environment variable content isn't expanded and is passed as-is to the starting VM.

So listen — to mitigate potential misuse of JDK_JAVA_OPTIONS behavior, options that specify the main class (such as -jar) or cause the alt-java launcher to exit without executing the main class (such as -h) are disallowed in the environment variable. If any of these options appear in the environment variable, the launcher will abort with an error message. 🙈

When JDK_JAVA_OPTIONS is set, the launcher prints a message to stderr as a reminder. (yes, every time, it's not being clingy, it's being helpful 🫶)

Example:

$ export JDK_JAVA_OPTIONS='-g @file1 -Dprop=value @file2 -Dws.prop="white spaces"'
$ alt-java -Xint @file3

is equivalent to the command line:

java -g @file1 -Dprop=value @file2 -Dws.prop="white spaces" -Xint @file3

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✨ OVERVIEW OF JAVA OPTIONS ✨

The alt-java command supports a wide range of options in the following categories:

• Standard Options for Java: Options guaranteed to be supported by all implementations of the Java Virtual Machine (JVM). They're used for common actions, such as checking the version of the JRE, setting the class path, enabling verbose output, and so on. the basics, the essentials 💖
• Extra Options for Java: General purpose options that are specific to the Java HotSpot Virtual Machine. They aren't guaranteed to be supported by all JVM implementations, and are subject to change. These options start with -X.

The advanced options aren't recommended for casual use. These are developer options used for tuning specific areas of the Java HotSpot Virtual Machine operation that often have specific system requirements and may require privileged access to system configuration parameters. Several examples of performance tuning are provided in Performance Tuning Examples. These options aren't guaranteed to be supported by all JVM implementations and are subject to change. Advanced options start with -XX.

• Advanced Runtime Options for Java: Control the runtime behavior of the Java HotSpot VM.
• [Advanced JIT Compiler Options for alt-java]: Control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM.
• Advanced Serviceability Options for Java: Enable gathering system information and performing extensive debugging.
• Advanced Garbage Collection Options for Java: Control how garbage collection (GC) is performed by the Java HotSpot

Boolean options are used to either enable a feature that's disabled by default or disable a feature that's enabled by default. Such options don't require a parameter. Boolean -XX options are enabled using the plus sign (-XX:+OptionName) and disabled using the minus sign (-XX:-OptionName). cute, right? ➕➖

For options that require an argument, the argument may be separated from the option name by a space, a colon (:), or an equal sign (=), or the argument may directly follow the option (the exact syntax differs for each option). If you're expected to specify the size in bytes, then you can use no suffix, or use the suffix k or K for kilobytes (KB), m or M for megabytes (MB), or g or G for gigabytes (GB). For example, to set the size to 8 GB, you can specify either 8g, 8192m, 8388608k, or 8589934592 as the argument. If you are expected to specify the percentage, then use a number from 0 to 1. For example, specify 0.25 for 25%.

The following sections describe the options that are deprecated, obsolete, and removed:

• Deprecated Java Options: Accepted and acted upon --- a warning is issued when they're used. 👀 still works, but it's on notice
• Obsolete Java Options: Accepted but ignored --- a warning is issued when they're used. bestie it's literally not listening to you 😭
• Removed Java Options: Removed --- using them results in an error. gone gone, don't even try 💀

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💅 STANDARD OPTIONS FOR JAVA 💅

These are the most commonly used options supported by all implementations of the JVM. the starter pack, if you will 🎀

Note: To specify an argument for a long option, you can use either --name=value or --name value.

• -agentlib:libname[=options] — Loads the specified native agent library. After the library name, a comma-separated list of options specific to the library can be used. If the option -agentlib:foo is specified, then the JVM attempts to load the library named foo using the platform specific naming conventions and locations:
  • Linux and other POSIX-like platforms: The JVM attempts to load the library named libfoo.so in the location specified by the LD_LIBRARY_PATH system variable.
  • macOS: The JVM attempts to load the library named libfoo.dylib in the location specified by the DYLD_LIBRARY_PATH system variable.
  • Windows: The JVM attempts to load the library named foo.dll in the location specified by the PATH system variable.

The following example shows how to load the Java Debug Wire Protocol (JDWP) library and listen for the socket connection on port 8000, suspending the JVM before the main class loads:

-agentlib:jdwp=transport=dt_socket,server=y,address=8000

• -agentpath:pathname[=options] — Loads the native agent library specified by the absolute path name. This option is equivalent to -agentlib but uses the full path and file name of the library. same vibe, but with the whole address 🗺️
• --class-path classpath, -classpath classpath, or -cp classpath — Specifies a list of directories, JAR files, and ZIP archives to search for class files.

On Windows, semicolons (;) separate entities in this list; on other platforms it is a colon (:).

Specifying classpath overrides any setting of the CLASSPATH environment variable. If the class path option isn't used and classpath isn't set, then the user class path consists of the current directory (.).

As a special convenience, a class path element that contains a base name of an asterisk (*) is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR . A Java program can't tell the difference between the two invocations. For example, if the directory mydir contains a.jar and b.JAR, then the class path element mydir/* is expanded to A.jar:b.JAR, except that the order of JAR files is unspecified. All .jar files in the specified directory, even hidden ones, are included in the list. A class path entry consisting of an asterisk (*) expands to a list of all the jar files in the current directory. The CLASSPATH environment variable, where defined, is similarly expanded. Any class path wildcard expansion that occurs before the Java VM is started. Java programs never see wildcards that aren't expanded except by querying the environment, such as by calling System.getenv("CLASSPATH"). ✨ the wildcard ate, honestly

• --disable-@files — Can be used anywhere on the command line, including in an argument file, to prevent further @filename expansion. This option stops expanding @-argfiles after the option. like a "no more, we're done here" button 🛑
• --enable-preview — Allows classes to depend on preview features of the release. peek at the future, lowkey 👀 https://docs.oracle.com/en/java/javase/12/language/index.html#JS-LAN-GUID-5A82FE0E-0CA4-4F1F-B075-564874FE2823
• --enable-native-access module[,module...] — Native access involves access to code or data outside the Java runtime. This is generally unsafe and, if done incorrectly, might crash the JVM or result in memory corruption. 💀 Native access can occur as a result of calling a method that is either restricted (https://openjdk.org/jeps/454#Safety), or native. This option allows code in the specified modules to perform native access. Native access occurring in a module that has not been explicitly enabled is deemed illegal.

module can be a module name, or ALL-UNNAMED to indicate code on the class path.

• ---illegal-native-access=parameter — This option specifies a mode for how illegal native access is handled:

Note: This option will be removed in a future release.

• allow: This mode allows illegal native access in all modules, without any warnings.
• warn: This mode is identical to allow except that a warning message is issued for the first illegal native access found in a module. This mode is the default for the current JDK but will change in a future release.
• deny: This mode disables illegal native access. That is, any illegal native access causes an IllegalCallerException. This mode will become the default in a future release.

To verify that your application is ready for a future version of the JDK, run it with --illegal-native-access=deny along with any necessary --enable-native-access options. future-proofing, we love to see it 🌈

• --enable-final-field-mutation module[,module...] — Mutation of final fields is possible with the reflection API of the Java Platform. However, it compromises safety and performance in all programs. This option allows code in the specified modules to mutate final fields by reflection. Attempts by code in any other module to mutate final fields by reflection are deemed illegal.

module can be the name of a module on the module path, or ALL-UNNAMED to indicate code on the class path.

• --illegal-final-field-mutation=parameter — tells the JVM how to handle illegal final field mutation, and you get to pick the parameter ✨

💀 heads up bestie: this option is getting yeeted in a future release. Like, gone gone.

The modes, in order:

• allow — illegal final field mutation is allowed in all modules, zero warnings, zero drama. Live your truth.
• warn — same as allow except it throws a warning the first time an illegal final field mutation happens in a module. This is the default for the current JDK 🌸 but it's changing in a future release, so don't get too comfy.
• debug — same as allow except it prints a warning and a stack trace for every single illegal final field mutation. Maximum receipts 🔥
• deny — final field mutation is straight up disabled. Any illegal final field mutation access throws an IllegalAccessException. This becomes the default in a future release 👑

Wanna vibe-check that your app is ready for a future JDK? Run it with --illegal-final-field-mutation=deny plus whatever --enable-final-field-mutation options you need. Future-proofing slay 💅

• --finalization=value — controls whether the JVM does finalization of objects. Valid values are "enabled" and "disabled". Finalization is enabled by default, so "enabled" literally does nothing (iconic useless king). "disabled" turns it off, so no finalizers get invoked, periodt.
• --module-path modulepath... or -p modulepath — tells the launcher where to find your application modules, as a list of path elements. Each element can be a file path to a module or a directory containing modules, and each module is either a modular JAR or an exploded-module directory 🎀

Platform check: on Windows, semicolons (;) separate the path elements in this list; everywhere else it's a colon (:).

• --upgrade-module-path modulepath... — same idea but for finding module replacements of upgradeable modules in the runtime image, as a list of path elements. Elements can be a file path to a module or a directory containing modules. Each module is either a modular JAR or an exploded-module directory ✨

Same deal with separators: on Windows it's semicolons (;), on other platforms it's a colon (:).

• --add-modules module[,module...] — names the root modules to resolve in addition to the initial module. And plot twist, module can also be ALL-DEFAULT, ALL-SYSTEM, or ALL-MODULE-PATH 👑
• --list-modules — lists all the observable modules and then dips. That's it, that's the whole job 💅
• -d module_name or --describe-module module_name — describes the module you name and then exits. Tell me about yourself bestie 🫶
• --dry-run — creates the VM but doesn't run the main method. Super useful for validating command-line options like the module system config without actually committing to the bit 🙈
• --validate-modules — validates all the modules and exits. Genuinely helpful for sniffing out conflicts and other errors with modules on the module path 🔍
• -D property=value — sets a system property value. The property part is a string with no spaces representing the property's name; the value part is a string representing the property's value. If your value has spaces, wrap it in quotation marks, like -Dfoo="foo bar" ✨
• -disableassertions[:[packagename]...|:classname] or -da[:[packagename]...|:classname] — disables assertions. By default assertions are disabled in all packages and classes anyway, so this is the "turn it off (again)" button 🎀

No arguments? -disableassertions (-da) disables assertions in all packages and classes.

Got a packagename argument ending in ...? The switch disables assertions in that package and any subpackages. If the argument is literally just ..., then you're disabling assertions in the unnamed package in the current working directory.

Got a classname argument? It disables assertions in that specified class.

Now, the -disableassertions (-da) option applies to all class loaders and to system classes (which don't have a class loader at all, main character energy). There's one exception to this rule: if you pass the option with no arguments, it doesn't apply to system classes. That's the whole point — it makes it easy to disable assertions in all classes except system classes 💅

The -disablesystemassertions option is your separate switch for disabling assertions in all system classes.

Wanna explicitly enable assertions in specific packages or classes? Use -enableassertions (-ea). Both options can be used at the same time, no beef 🫶 For example, to run the MyClass application with assertions enabled in the package com.wombat.fruitbat (and any subpackages) but disabled in the class com.wombat.fruitbat.Brickbat, you'd do:

alt-java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass

• -disablesystemassertions or -dsa — disables assertions in all system classes. Short and iconic.
• -enableassertions[:[packagename]...|:classname] or -ea[:[packagename]...|:classname] — enables assertions ✨ (yes, even though by default assertions are disabled in all packages and classes — this is the glow-up button)

No arguments? -enableassertions (-ea) enables assertions in all packages and classes.

packagename argument ending in ...? It enables assertions in that package and any subpackages. Just ... by itself? Unnamed package in the current working directory, we see you.

classname argument? Assertions enabled in that specified class, slay.

The -enableassertions (-ea) option applies to all class loaders and to system classes (which, again, don't have a class loader). One exception to the rule: if you provide the option with no arguments, then it doesn't apply to system classes. Which conveniently makes it easy to enable assertions in every class except the system classes 💅

-enablesystemassertions is the separate switch for enabling assertions in all system classes.

To explicitly disable assertions in specific packages or classes, use the -disableassertions (-da) option. And important: if a single command contains multiple instances of these switches, they're processed in order, before any classes get loaded. So like, order matters, no cap. For example, to run the MyClass application with assertions enabled only in the package com.wombat.fruitbat (and any subpackages) but disabled in the class com.wombat.fruitbat.Brickbat, use:

alt-java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass

• -enablesystemassertions or -esa — enables assertions in all system classes. The counterpart to -dsa, we stan a balanced duo 🧚
• -help, -h, or -? — prints the help message to the error stream. (Yes, error stream. I know. That's just the vibe.)
• --help — prints the help message to the output stream. See the difference? Two dashes = output stream 💅
• -alt-javaagent: jarpath[=options] — loads the specified Java programming language agent. Check out java.lang.instrument for more ✨
• --show-version — prints the product version to the output stream and then keeps going. Continues, doesn't bail.
• -showversion — prints the product version to the error stream and then keeps going. Same energy, different stream 😭
• --show-module-resolution — shows module resolution output during startup. Watch it cook 🔥
• -splash: imagepath — shows the splash screen with the image specified by imagepath. HiDPI scaled images are automatically supported and used if they're available, which is highkey a flex. Ngl, always pass the unscaled image file name as the argument to -splash, like image.ext. The most appropriate scaled image that's provided gets picked up automatically 🎀

For example, to show the splash.gif file from the images directory when starting your app, use:

-splash:images/splash.gif

See the SplashScreen API documentation for more info 🌸

• -verbose:class — displays info about each loaded class. Every single one. Spammy but in an informative way 💖
• -verbose:gc — displays info about each garbage collection (GC) event. Watching the GC work is weirdly hypnotic, ngl.
• -verbose:jni — displays info about the use of native methods and other Java Native Interface (JNI) activity 🦋
• -verbose:module — displays info about the modules in use.
• --version — prints the product version to the output stream and exits. Two dashes = output stream, exits. Clean ✨
• -version — prints the product version to the error stream and exits. One dash = error stream, exits. Vintage 😭
• -X — prints the help on extra options to the error stream. There's a whole extra menu, bestie 👀
• --help-extra — prints the help on extra options to the output stream. Fancy version of -X 🎀
• @argfile — specifies one or more argument files prefixed by @ that the alt-java command uses. Real talk: alt-java command lines get very long because of all the .jar files you need in the classpath, and @argfile overcomes command-line length limitations by enabling the launcher to expand the contents of argument files after shell expansion but before argument processing. Iconic pipeline moment 💅

The contents get expanded because otherwise they'd just sit there being specified on the command line until the --disable-@files option was encountered. So, yeah.

Also, argument files can contain the main class name and all the options. If an argument file contains every option the alt-java command needs, your command line can simply be:

alt-java @argfile

See alt-java Command-Line Argument Files for a description and examples of using @-argfiles 📚

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🔥 EXTRA OPTIONS FOR JAVA 🔥

The following alt-java options are general purpose options that are specific to the Java HotSpot Virtual Machine. Buckle up, bestie, this is where it gets juicy 🌈

• -Xbatch — turns off background compilation, bestie. Normally the JVM compiles a method as a background task and keeps running that method in interpreter mode until the background compilation finishes. With -Xbatch, no more of that — compilation of all methods happens as a foreground task, all the way till it's done. This one is equivalent to -XX:-BackgroundCompilation. ✨
• -Xbootclasspath/a: directories | zip | JAR-files — gives you a list of directories, JAR files, and ZIP archives to append to the end of the default bootstrap class path. 📚

On Windows, semicolons (;) separate entities in this list; on other platforms it's a colon (:).

• -Xcheck:jni — does extra checks for Java Native Interface (JNI) functions. 🕵️‍♀️

These following checks are considered indicative of significant problems with the native code, and the JVM terminates with an irrecoverable error in such cases:

• The thread doing the call is not attached to the JVM.
• The thread doing the call is using the JNIEnv belonging to another thread.
• A parameter validation check fails:
  • A jfieldID, or jmethodID, is detected as being invalid. For example:
    • Of the wrong type
    • Associated with the wrong class
  • A parameter of the wrong type is detected.
  • An invalid parameter value is detected. For example:
    • NULL where not permitted
    • An out-of-bounds array index, or frame capacity
    • A non-UTF-8 string
    • An invalid JNI reference
    • An attempt to use a ReleaseXXX function on a parameter not produced by the corresponding GetXXX function

The following checks only result in warnings being printed:

• A JNI call was made without checking for a pending exception from a previous JNI call, and the current call is not safe when an exception may be pending.
• A class descriptor is in decorated format (Lname;) when it should not be.
• A NULL parameter is allowed, but its use is questionable.
• Calling other JNI functions in the scope of Get/ReleasePrimitiveArrayCritical or Get/ReleaseStringCritical.
• A JNI call was made to mutate a final field.

Expect a performance degradation when this option is used. 😭 like, it's checking everything, it's gonna be slower, that's the tradeoff bestie.

• -Xcomp — testing mode to exercise JIT compilers. 🧪

This option should not be used in production environments. literally do not, I'm begging.

• -Xdebug — does nothing; deprecated for removal in a future release. 💀 it's giving placeholder. it does nothing. iconic of it honestly.
• -Xdiag — shows additional diagnostic messages. 🔍
• -Xint — runs the application in interpreted-only mode. Compilation to native code is disabled, and all bytecode is executed by the interpreter. The performance benefits offered by the just-in-time (JIT) compiler aren't present in this mode. 🐌 ngl she's slow but she's thorough.
• -Xinternalversion — displays more detailed JVM version information than the -version option, and then exits. 👑 more detail, then ✌️ out.
• -Xlog: option — configure or enable logging with the Java Virtual Machine (JVM) unified logging framework. See Enable Logging with the JVM Unified Logging Framework. 📝
• -Xmixed — executes all bytecode by the interpreter except for hot methods, which are compiled to native code. On by default. Use -Xint to switch off. 🔥 this is the main character default fr.
• -Xmn size — sets the initial and maximum size (in bytes) of the heap for the young generation (nursery) in the generational collectors. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. 🌱

The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too small, then a lot of minor garbage collections are performed. If the size is too large, then only full garbage collections are performed, which can take a long time to complete. It's recommended that you do not set the size for the young generation for the G1 collector, and keep the size for the young generation greater than 25% and less than 50% of the overall heap size for other collectors. 🎯

The following examples show how to set the initial and maximum size of young generation to 256 MB using various units:

-Xmn256m
-Xmn262144k
-Xmn268435456

Instead of the -Xmn option to set both the initial and maximum size of the heap for the young generation, you can use -XX:NewSize to set the initial size and -XX:MaxNewSize to set the maximum size. 🫶

• -Xms size — sets the minimum and the initial size (in bytes) of the heap. This value must be a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. 📏

The following examples show how to set the size of allocated memory to 6 MB using various units:

-Xms6291456
-Xms6144k
-Xms6m

If you do not set this option, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation. The initial size of the heap for the young generation can be set using the -Xmn option or the -XX:NewSize option. 🧮

Note that the -XX:InitialHeapSize option can also be used to set the initial heap size. If it appears after -Xms on the command line, then the initial heap size gets set to the value specified with -XX:InitialHeapSize. order matters bestie, it's giving last-one-wins energy 💅

• -Xmx size — specifies the maximum size (in bytes) of the heap. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. 🎛️

For server deployments, -Xms and -Xmx are often set to the same value. The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:

-Xmx83886080
-Xmx81920k
-Xmx80m

The -Xmx option is equivalent to -XX:MaxHeapSize. ✨

• -Xnoclassgc — disables garbage collection (GC) of classes. This can save some GC time, which shortens interruptions during the application run. ⏱️

When you specify -Xnoclassgc at startup, the class objects in the application are left untouched during GC and are always be considered live. This can result in more memory being permanently occupied which, if not used carefully, throws an out-of-memory exception. 😬 so like… memory just stays. forever. be careful.

• -Xrs — reduces the use of operating system signals by the JVM. Shutdown hooks enable the orderly shutdown of a Java application by running user cleanup code (such as closing database connections) at shutdown, even if the JVM terminates abruptly. 🛑
  • Non-Windows:
    • The JVM catches signals to implement shutdown hooks for unexpected termination. The JVM uses SIGHUP, SIGINT, and SIGTERM to initiate the running of shutdown hooks.
    • Applications embedding the JVM frequently need to trap signals such as SIGINT or SIGTERM, which can lead to interference with the JVM signal handlers. The -Xrs option is available to address this issue. When -Xrs is used, the signal masks for SIGINT, SIGTERM, SIGHUP, and SIGQUIT aren't changed by the JVM, and signal handlers for these signals aren't installed.
  • Windows:
    • The JVM watches for console control events to implement shutdown hooks for unexpected termination. Specifically, the JVM registers a console control handler that begins shutdown-hook processing and returns TRUE for CTRL_C_EVENT, CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, and CTRL_SHUTDOWN_EVENT.
    • The JVM uses a similar mechanism to implement the feature of dumping thread stacks for debugging purposes. The JVM uses CTRL_BREAK_EVENT to perform thread dumps.
    • If the JVM is run as a service (for example, as a servlet engine for a web server), then it can receive CTRL_LOGOFF_EVENT but shouldn't initiate shutdown because the operating system doesn't actually terminate the process. To avoid possible interference such as this, the -Xrs option can be used. When the -Xrs option is used, the JVM doesn't install a console control handler, implying that it doesn't watch for or process CTRL_C_EVENT, CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, or CTRL_SHUTDOWN_EVENT.

There are two consequences of specifying -Xrs:

• Non-Windows: SIGQUIT thread dumps aren't available. 😭
• Windows: Ctrl + Break thread dumps aren't available.

User code is responsible for causing shutdown hooks to run, for example, by calling System.exit() when the JVM is to be terminated. 🫡

• -Xshare: mode — sets the class data sharing (CDS) mode. 🎀

Possible mode arguments for this option include the following:

• auto — use shared class data if possible (default).
• on — require using shared class data, otherwise fail.

Note: The -Xshare:on option is used for testing purposes only. It may cause the VM to unexpectedly exit during start-up when the CDS archive cannot be used (for example, when certain VM parameters are changed, or when a different JDK is used). This option should not be used in production environments. 💀 seriously, testing only.

• off — do not attempt to use shared class data.

• -XshowSettings — shows all settings and then continues. 👀 all of them. every last one.
• -XshowSettings: category — shows settings and continues. Possible category arguments for this option include the following: 🌈
• all — shows every category of settings, in glorious verbose detail ✨
• locale — shows the settings that are all about locale 🌍
• properties — shows the system-properties side of things 🧾
• security — shows every security-related setting (yes, all of them) 🔐

the sub-category arguments for security are these cuties:

• security:all — shows all security settings 💅
• security:properties — shows security properties
• security:providers — shows static security provider settings
• security:tls — shows TLS related security settings 🦋

• vm — shows the settings of the JVM 🧚
• system — Linux only! 👑 Shows host system or container configuration and continues.

---

• -Xss size — sets the thread stack size, in bytes 🧵. Append the letter k or K to mean KB, m or M to mean MB, or g or G to mean GB. ngl the actual size might get rounded up to a multiple of the system page size, because the operating system said so. The default value depends on the platform — for example:
  • Linux/x64: 1024 KB
  • Linux/Aarch64: 2048 KB
  • macOS/x64: 1024 KB
  • macOS/Aarch64: 2048 KB
  • Windows: the default value depends on virtual memory 😭

These examples all set the thread stack size to 1024 KB, just in different units (slay, unit conversion):

-Xss1m
-Xss1024k
-Xss1048576

This option is similar to -XX:ThreadStackSize 🫶

• --add-reads module=target-module(,target-module)* — updates module so it reads target-module, regardless of the module declaration 💫. target-module can be ALL-UNNAMED to read all unnamed modules.
• --add-exports module/package=target-module(,target-module)* — updates module to export package to target-module, regardless of module declaration 🔥. target-module can be ALL-UNNAMED to export to all unnamed modules.
• --add-opens module/package=target-module(,target-module)* — updates module to open package to target-module, regardless of module declaration 🚪✨
• --limit-modules module[,module...] — specifies the limit of the universe of observable modules 🌌 (yes, "universe", that's the actual word, iconic).
• --patch-module module=file(;file)* — overrides or augments a module with classes and resources inside JAR files or directories 🩹
• --source version — sets the version of the source in source-file mode 📼
• --sun-misc-unsafe-memory-access= value — allow or deny usage of the unsupported API sun.misc.Unsafe 💀. value is one of these:
  • allow — allow use of the memory-access methods with no warnings at run time 🤫
  • warn — allow use of the memory-access methods, but issue a warning on the first occasion that any memory-access method is used. At most one warning is issued ⚠️
  • debug — allow use of the memory-access methods, but issue a one-line warning and a stack trace when any memory-access method is used 🔍
  • deny — disallow use of the memory-access methods by throwing an UnsupportedOperationException on every usage 🙅‍♀️

When the option isn't specified at all, the default value is warn.

---

🍎 EXTRA OPTIONS FOR MACOS

These extra options are macOS specific, bestie. Intel-and-Apple-Silicon girlies only 💻🌸

• -XstartOnFirstThread — runs the main() method on the first (AppKit) thread 🧵🍏
• -Xdock:name= application_name — overrides the default application name displayed in dock 🏷️
• -Xdock:icon= path_to_icon_file — overrides the default icon displayed in dock 🖼️

---

👑 ADVANCED OPTIONS FOR JAVA

These alt-java options are how you unlock the other advanced options. It's giving secret menu 🔓✨

• -XX:+UnlockDiagnosticVMOptions — unlocks the options intended for diagnosing the JVM 🩺. By default, this option is disabled and diagnostic options aren't available.

⚠️ Command line options that get enabled by using this option are not supported. If you hit issues while using any of these options, it's very likely you'll be required to reproduce the problem without any of these unsupported options before Oracle Support can assist with an investigation 😭. It's also possible that any of these options may be removed, or their behavior changed, without any warning. fr fr, tread carefully.

• -XX:+UnlockExperimentalVMOptions — unlocks the options that provide experimental features in the JVM 🧪. By default, this option is disabled and experimental features aren't available.

---

🔥 ADVANCED RUNTIME OPTIONS FOR JAVA

These alt-java options control the runtime behavior of the Java HotSpot VM. main character energy only 👑

• -XX:ActiveProcessorCount= x — overrides the number of CPUs that the VM will use to calculate the size of the thread pools it uses for various operations, like Garbage Collection and ForkJoinPool 🧮.

Normally the VM figures out the number of available processors from the operating system. This flag is lowkey useful for partitioning CPU resources when you're running multiple Java processes in docker containers 🐳. This flag is honored even if UseContainerSupport is not enabled. See -XX:-UseContainerSupport for a description of enabling and disabling container support 👀.

• -XX:AllocateHeapAt= path — takes a path to the file system and uses memory mapping to allocate the object heap on that memory device 💾. Using this option enables the HotSpot VM to allocate the Java object heap on an alternative memory device — like an NV-DIMM — specified by the user.

Alternative memory devices that have the same semantics as DRAM, including the semantics of atomic operations, can be used instead of DRAM for the object heap without changing the existing application code ✨. All the other memory structures (like the code heap, metaspace, and thread stacks) still reside in DRAM.

Some operating systems expose non-DRAM memory through the file system. Memory-mapped files in these file systems bypass the page cache and give you a direct mapping of virtual memory to the physical memory on the device 🗺️. The existing heap related flags (like -Xmx and -Xms) and garbage-collection related flags keep working exactly as before.

• -XX:-CompactStrings — disables the Compact Strings feature 🧵💅. By default, this option is enabled. When it's enabled, Java Strings containing only single-byte characters are internally represented and stored as single-byte-per-character Strings using ISO-8859-1 / Latin-1 encoding. That reduces, by 50%, the amount of space required for Strings containing only single-byte characters 🔥. For Java Strings containing at least one multibyte character: those get represented and stored as 2 bytes per character using UTF-16 encoding. Disabling Compact Strings forces UTF-16 encoding as the internal representation for all Java Strings.

Cases where it may be beneficial to disable Compact Strings include:

• When it's known that an application overwhelmingly will be allocating multibyte character Strings 😬
• In the unexpected event where a performance regression is observed when migrating from Java SE 8 to Java SE 9, and an analysis shows that Compact Strings is what introduced the regression 🕵️‍♀️

In both of these scenarios, disabling Compact Strings makes sense. periodt.

• -XX:ErrorFile= filename — specifies the path and file name that error data gets written to when an irrecoverable error occurs 💀. By default, this file is created in the current working directory and named hs_err_pidpid.log, where pid is the identifier of the process that encountered the error.

This example shows how to set the default log file (note that the identifier of the process is specified as %p):

-XX:ErrorFile=./hs_err_pid%p.log

• Non-Windows: this example shows how to set the error log to /var/log/java/java_error.log:

-XX:ErrorFile=/var/log/java/java_error.log

• Windows: this example shows how to set the error log file to C:/log/java/java_error.log:

-XX:ErrorFile=C:/log/java/java_error.log

If the file exists and is writeable, then it gets overwritten 👋. Otherwise, if the file can't be created in the specified directory (insufficient space, permission problem, or some other issue), then the file gets created in the temporary directory for the operating system:

• Non-Windows: the temporary directory is /tmp 🌸
• Windows: the temporary directory is specified by the value of the TMP environment variable; if that environment variable isn't defined, then the value of the TEMP environment variable is used.

• -XX:+ExtensiveErrorReports — enables the reporting of more extensive error information in the ErrorFile 📋. You can turn this option on in environments where maximal information is desired — even if the resulting logs may be quite large and/or contain information that might be considered sensitive 🙈. The information can vary from release to release, and across different platforms. By default this option is disabled.
• -XX:FlightRecorderOptions= parameter=value (or) -XX:FlightRecorderOptions:parameter=value — sets the parameters that control the behavior of JFR 🎥. -XX:FlightRecorderOptions:help prints the available options, the default redaction filters, and example command lines 🧾. Multiple parameters can be specified by separating them with a comma.

The following list contains the available JFR parameter=value entries:

• globalbuffersize=size — Specifies the total amount of primary memory used for data retention. The default value is based on the value specified for memorysize. Change the memorysize parameter to alter the size of global buffers. slay
• maxchunksize=size — Specifies the maximum size (in bytes) of the data chunks in a recording. Append m or M to specify the size in megabytes (MB), or g or G to specify the size in gigabytes (GB). By default, the maximum size of data chunks is set to 12 MB. The minimum allowed is 1 MB.
• memorysize=size — Determines how much buffer memory should be used, and sets the globalbuffersize and numglobalbuffers parameters based on the size specified. Append m or M to specify the size in megabytes (MB), or g or G to specify the size in gigabytes (GB). By default, the memory size is set to 10 MB.
• numglobalbuffers — Specifies the number of global buffers used. The default value is based on the memory size specified. Change the memorysize parameter to alter the number of global buffers.
• old-object-queue-size=number-of-objects — Maximum number of old objects to track. By default, the number of objects is set to 256.
• preserve-repository={true|false} — Specifies whether files stored in the disk repository should be kept after the JVM has exited. If false, files are deleted. By default, this parameter is disabled.
• repository=path — Specifies the repository (a directory) for temporary disk storage. By default, the system's temporary directory is used.
• retransform={true|false} — Specifies whether event classes should be retransformed using JVMTI. If false, instrumentation is added when event classes are loaded. By default, this parameter is enabled.
• redact-argument=argument-filter — Replace command-line arguments that match a semicolon-separated list of glob patterns, for example, *secret*;password*. Matching is case-insensitive, and the supported wildcards are * and ?.
  • To redact multiple arguments, use a literal space (' ') as a separator. For example, to match the two arguments --auth username:token, use the filter --auth *:*.
  • Filters containing spaces must be quoted as a single command-line argument, for example, -XX:FlightRecorderOptions='redact-argument=--auth *:*'.
  • Arguments containing spaces might not be matched as expected.
  • To load patterns from a file (one per line) use @<filename>.
  • To add to the default patterns instead of replacing them, prefix the whole list with +, for example, +*foo*;@redact.txt.
  • Use none (lowercase) to disable all redaction filters for command-line arguments.
  • Redacted arguments will be replaced with [REDACTED].
  • The option redact-argument is best-effort and applies only to command-line arguments in the jdk.JVMInformation event and to the java.command system property in the jdk.InitialSystemProperty event, and to matching command-line argument text in the values of jdk.InitialEnvironmentVariable events.
  • Other events, such as jdk.ProcessStart (child processes), are not redacted.
  • Use -XX:FlightRecorderOptions:help to see the default filters used by the redact-argument option.
• redact-key=key-filter — Replace the value of environment variables and system properties whose key matches a semicolon-separated list of glob patterns, for example, *password*;*token*. Matching is case-insensitive, and the supported wildcards are * and ?.
  • To load patterns from a file (one per line), use @<filename>.
  • To add to the default patterns instead of replacing them, prefix the whole list with +, for example, +*cred*;@keys.txt.
  • Use none (lowercase) to disable all redaction filters for key matching.
  • Redacted values will be replaced with [REDACTED].
  • The option redact-key is best-effort and applies only to the jdk.InitialSystemProperty, jdk.InitialEnvironmentVariable and jdk.JVMInformation (-Dkey=...) events.
  • Other events, such as jdk.InitialSecurityProperty, are not redacted.
  • Use -XX:FlightRecorderOptions:help to see the default filters used by the redact-key option.
• stackdepth=depth — Stack depth for stack traces. By default, the depth is set to 64 method calls. The maximum is 2048. Values greater than 64 could create significant overhead and reduce performance.
• threadbuffersize=size — Specifies the per-thread local buffer size (in bytes). By default, the local buffer size is set to 8 kilobytes, with a minimum value of 4 kilobytes. Overriding this parameter could reduce performance and is not recommended.
• -XX:LargePageSizeInBytes=size — Sets the maximum large page size (in bytes) used by the JVM. The size argument must be a valid page size supported by the environment to have any effect. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM will use the default large page size for the environment as the maximum size for large pages. See Large Pages.

Setting it to 1 gigabyte (GB) is a whole vibe:

-XX:LargePageSizeInBytes=1g

• -XX:MaxDirectMemorySize=size — Sets the maximum total size (in bytes) of the java.nio package, direct-buffer allocations. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. If not set, the flag is ignored and the JVM chooses the size for NIO direct-buffer allocations automatically.

The following examples illustrate how to set the NIO size to 1024 KB in different units:

-XX:MaxDirectMemorySize=1m
-XX:MaxDirectMemorySize=1024k
-XX:MaxDirectMemorySize=1048576

• -XX:-MaxFDLimit — Disables the attempt to set the soft limit for the number of open file descriptors to the hard limit. By default, this option is enabled on all platforms, but is ignored on Windows. The only time that you may need to disable this is on macOS, where its use imposes a maximum of 10240, which is lower than the actual system maximum.
• -XX:NativeMemoryTracking=mode — Specifies the mode for tracking JVM native memory usage. Possible mode arguments for this option include the following:
  • off — Instructs not to track JVM native memory usage. This is the default behavior if you don't specify the -XX:NativeMemoryTracking option.
  • summary — Tracks memory usage only by JVM subsystems, such as Java heap, class, code, and thread.
  • detail — In addition to tracking memory usage by JVM subsystems, track memory usage by individual CallSite, individual virtual memory region and its committed regions.
• -XX:TrimNativeHeapInterval=millis — Interval, in ms, at which the JVM will trim the native heap. Lower values will reclaim memory more eagerly at the cost of higher overhead. A value of 0 (default) disables native heap trimming. Native heap trimming is performed in a dedicated thread.

This option is only supported on Linux with GNU C Library (glibc).

• -XX:ObjectAlignmentInBytes=alignment — Sets the memory alignment of Java objects (in bytes). By default, the value is set to 8 bytes. The specified value should be a power of 2, and must be within the range of 8 and 256 (inclusive). This option makes it possible to use compressed pointers with large Java heap sizes.

The heap size limit in bytes is calculated as:

4GB * ObjectAlignmentInBytes

⚠️ Note: As the alignment value increases, the unused space between objects also increases. As a result, you may not realize any benefits from using compressed pointers with large Java heap sizes.

• -XX:OnError=string — Sets a custom command or a series of semicolon-separated commands to run when an irrecoverable error occurs. If the string contains spaces, then it must be enclosed in quotation marks.
  • Non-Windows: The following example shows how the -XX:OnError option can be used to run the gcore command to create a core image, and start the gdb debugger to attach to the process in case of an irrecoverable error (the %p designates the current process identifier):

-XX:OnError="gcore %p;gdb -p %p"

• Windows: The following example shows how the -XX:OnError option can be used to run the userdump.exe utility to obtain a crash dump in case of an irrecoverable error (the %p designates the current process identifier). This example assumes that the path to the userdump.exe utility is specified in the PATH environment variable:

-XX:OnError="userdump.exe %p"

• -XX:OnOutOfMemoryError= string — sets a custom command, or a whole semicolon-separated list of commands, to run the first time an OutOfMemoryError exception gets thrown by the JVM. If the string has spaces, bestie, wrap it in quotation marks. For an example command string, peek at the description of the -XX:OnError option.

Heads up: this only applies to OutOfMemoryError exceptions caused by Java Heap exhaustion. It does not apply to OutOfMemoryError exceptions thrown directly from Java code, and it also doesn't apply to ones thrown by the JVM for other types of resource exhaustion (such as native thread creation errors).

• -XX:+PrintCommandLineFlags — turns on printing of ergonomically selected JVM flags that showed up on the command line. It's lowkey useful for knowing the ergonomic values the JVM picked, like heap space size and the selected garbage collector. By default, this option is disabled and flags aren't printed.
• -XX:+PreserveFramePointer — picks between using the RBP register as a general purpose register (-XX:-PreserveFramePointer) and using the RBP register to hold the frame pointer of the currently executing method (-XX:+PreserveFramePointer). If the frame pointer is available, external profiling tools (for example, Linux perf) can construct more accurate stack traces.
• -XX:+PrintNMTStatistics — enables printing of collected native memory tracking data at JVM exit when native memory tracking is enabled (see -XX:NativeMemoryTracking). By default, this option is disabled and native memory tracking data isn't printed.
• -XX:SharedArchiveFile= path — specifies the path and name of the class data sharing (CDS) archive file.

See Application Class Data Sharing.

• -XX:+VerifySharedSpaces — if this option is specified, the JVM will load a CDS archive file only if it passes an integrity check based on CRC32 checksums. The purpose of this flag is to check for unintentional damage to CDS archive files in transmission or storage.

To guarantee the security and proper operation of CDS, the user must ensure that the CDS archive files used by Java applications cannot be modified without proper authorization.

• -XX:SharedArchiveConfigFile= shared_config_file — specifies additional shared data added to the archive file.
• -XX:SharedClassListFile= file_name — specifies the text file that contains the names of the classes to store in the class data sharing (CDS) archive. This file contains the full name of one class per line, except slashes (/) replace dots (.). For example, to specify the classes java.lang.Object and hello.Main, create a text file that contains the following two lines:

java/lang/Object
hello/Main

The classes that you specify in this text file should include the classes that are commonly used by the application. They may include any classes from the application, extension, or bootstrap class paths.

See Application Class Data Sharing.

• -XX:+ShowCodeDetailsInExceptionMessages — enables printing of improved NullPointerException messages. When an application throws a NullPointerException, the option enables the JVM to analyze the program's bytecode instructions to determine precisely which reference is null, and describes the source with a null-detail message. The null-detail message is calculated and returned by NullPointerException.getMessage(), and will be printed as the exception message along with the method, filename, and line number. By default, this option is enabled.
• -XX:+ShowMessageBoxOnError — enables the display of a dialog box when the JVM experiences an irrecoverable error. This prevents the JVM from exiting and keeps the process active so that you can attach a debugger to it to investigate the cause of the error. By default, this option is disabled.
• -XX:StartFlightRecording: parameter=value — starts a JFR recording for the Java application. This option is equivalent to the JFR.start diagnostic command that starts a recording during runtime. -XX:StartFlightRecording:help prints available options and example command lines. You can set the following parameter=value entries when starting a JFR recording:
  • delay= time — specifies the delay between the Java application launch time and the start of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 10m means 10 minutes). By default, there's no delay, and this parameter is set to 0.
  • disk={true|false} — specifies whether to write data to disk while recording. By default, this parameter is enabled.
  • dumponexit={true|false} — specifies if the running recording is dumped when the JVM shuts down. If enabled and a filename is not entered, the recording is written to a file in the directory where the process was started. The file name is a system-generated name that contains the process ID, recording ID, and current timestamp, similar to hotspot-pid-47496-id-1-2018_01_25_19_10_41.jfr. By default, this parameter is disabled.
  • duration= time — specifies the duration of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 5h means 5 hours). By default, the duration isn't limited, and this parameter is set to 0.
  • filename= path — specifies the path and name of the file to which the recording is written when the recording is stopped, for example:
    • recording.jfr
    • /home/user/recordings/recording.jfr
    • c:\recordings\recording.jfr

If %p and/or %t is specified in the filename, it expands to the JVM's PID and the current timestamp, respectively. The filename may also be a directory in which case, the filename is generated from the PID and the current date in the specified directory.

• name= identifier — takes both the name and the identifier of a recording.
• maxage= time — specifies the maximum age of disk data to keep for the recording. This parameter is valid only when the disk parameter is set to true. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 30s means 30 seconds). By default, the maximum age isn't limited, and this parameter is set to 0s.
• maxsize= size — specifies the maximum size (in bytes) of disk data to keep for the recording. This parameter is valid only when the disk parameter is set to true. The value must not be less than the value for the maxchunksize parameter set with -XX:FlightRecorderOptions. Append m or M to specify the size in megabytes, or g or G to specify the size in gigabytes. By default, the maximum size of disk data isn't limited, and this parameter is set to 0.
• path-to-gc-roots={true|false} — specifies whether to collect the path to garbage collection (GC) roots at the end of a recording. By default, this parameter is disabled.

The path to GC roots is useful for finding memory leaks, but collecting it is time-consuming. Enable this option only when you start a recording for an application that you suspect has a memory leak. If the settings parameter is set to profile, the stack trace from where the potential leaking object was allocated is included in the information collected.

• report-on-exit= identifier — specifies the name of the view to display when the Java Virtual Machine (JVM) shuts down. To specify more than one view, use the report-on-exit parameter repeatedly. This option is not available if the disk option is set to false. For a list of available views, see jfr help view. By default, no report is generated.
• settings= path — specifies the path and name of the event settings file (of type JFC). By default, the default.jfc file is used, which is located in JAVA_HOME/lib/jfr. This default settings file collects a predefined set of information with low overhead, so it has minimal impact on performance and can be used with recordings that run continuously.

A second settings file is also provided, profile.jfc, which provides more data than the default configuration, but can have more overhead and impact performance. Use this configuration for short periods of time when more information is needed.

You can specify values for multiple parameters by separating them with a comma. Event settings and .jfc options can be specified using the following syntax:

• option=value — specifies the option value to modify. To list available options, use the JAVA_HOME/bin/jfr tool.
• event-setting=value — specifies the event setting value to modify. Use the form: <event-name>#<setting-name>=<value>. To add a new event setting, prefix the event name with '+'.

okay so you can stack multiple event settings and .jfc options by separating them with a comma, bestie. If a parameter and a .jfc option are beefing, the parameter takes precedence — periodt. The whitespace delimiter can be left out for timespan values, i.e. 20ms. Wanna know the settings syntax? Check the Javadoc of the jdk.jfr package. And if you only want warnings and errors from JFR during startup, set -Xlog:jfr+startup=warning.

• -XX:ThreadStackSize=size — sets the Java thread stack size in kilobytes. Using a scaling suffix like k scales the kilobytes value, so -XX:ThreadStackSize=1k sets the Java thread stack size to 1024*1024 bytes or 1 megabyte. The default depends on the platform, bestie:
  • Linux/x64: 1024 KB
  • Linux/Aarch64: 2048 KB
  • macOS/x64: 1024 KB
  • macOS/Aarch64: 2048 KB
  • Windows: the default value depends on virtual memory

Here's how to set the thread stack size to 1 megabyte in different units:

-XX:ThreadStackSize=1k
-XX:ThreadStackSize=1024

This option is similar to -Xss.

• -XX:-UseCompactObjectHeaders — disables compact object headers. By default this option is enabled, and compact object headers are used. Using compact object headers reduces memory footprint in the Java heap by 4 bytes per object (on average) and often improves performance. You can use this option if performance regressions are suspected. In a future release compact object headers is expected to become the only mode of operation.
• -XX:-UseCompressedOops — disables the use of compressed pointers. By default this option is enabled, and compressed pointers are used. This automatically limits the maximum ergonomically determined Java heap size to the maximum amount of memory that can be covered by compressed pointers. By default this range is 32 GB. With compressed oops enabled, object references are represented as 32-bit offsets instead of 64-bit pointers, which typically increases performance when running the application with Java heap sizes smaller than the compressed oops pointer range. This option works only for 64-bit JVMs. It's possible to use compressed pointers with Java heap sizes greater than 32 GB — see the -XX:ObjectAlignmentInBytes option.
• -XX:-UseContainerSupport — Linux only: the VM now provides automatic container detection support, which lets the VM determine the amount of memory and number of processors available to a Java process running in docker containers. It uses this info to allocate system resources. The default for this flag is true, and container support is enabled by default. It can be disabled with -XX:-UseContainerSupport. Unified Logging is available to help diagnose issues related to this support. Use -Xlog:os+container=trace for maximum logging of container information. See Enable Logging with the JVM Unified Logging Framework for a description of using Unified Logging.
• -XX:+UseLargePages — enables the use of large page memory. By default this option is disabled and large page memory isn't used. See Large Pages.
• -XX:+UseTransparentHugePages — Linux only: enables the use of large pages that can dynamically grow or shrink. This option is disabled by default. You may encounter performance problems with transparent huge pages as the OS moves other pages around to create huge pages; this option is made available for experimentation.
• -XX:+AllowUserSignalHandlers — Non-Windows: enables installation of signal handlers by the application. By default this option is disabled and the application isn't allowed to install signal handlers.
• -XX:VMOptionsFile=filename — lets the user specify VM options in a file, for example, alt-java -XX:VMOptionsFile=/var/my_vm_options HelloWorld.
• -XX:UseBranchProtection=mode — Linux AArch64 only: specifies the branch protection mode. All options other than none require the VM to have been built with branch protection enabled. In addition, for full protection, any native libraries provided by applications should be compiled with the same level of protection. Possible mode arguments for this option include:
  • none — do not use branch protection. This is the default value.
  • standard — enables all branch protection modes available on the current platform.
  • pac-ret — enables protection against ROP based attacks. (AArch64 8.3+ only)

---

✨ ADVANCED JIT COMPILER OPTIONS FOR JAVA ✨

These alt-java options control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM. Very main character energy, fr fr.

• -XX:AllocateInstancePrefetchLines=lines — sets the number of lines to prefetch ahead of the instance allocation pointer. By default, the number of lines to prefetch is set to 1:

-XX:AllocateInstancePrefetchLines=1

• -XX:AllocatePrefetchDistance=size — sets the size (in bytes) of the prefetch distance for object allocation. Memory about to be written with the value of new objects is prefetched up to this distance starting from the address of the last allocated object. Each Java thread has its own allocation point. Negative values denote that prefetch distance is chosen based on the platform. Positive values are bytes to prefetch. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value is set to -1. Example time — setting the prefetch distance to 1024 bytes:

-XX:AllocatePrefetchDistance=1024

• -XX:AllocatePrefetchInstr=instruction — sets the prefetch instruction to prefetch ahead of the allocation pointer. Possible values are from 0 to 3. The actual instructions behind the values depend on the platform. By default, the prefetch instruction is set to 0:

-XX:AllocatePrefetchInstr=0

• -XX:AllocatePrefetchLines=lines — sets the number of cache lines to load after the last object allocation by using the prefetch instructions generated in compiled code. The default value is 1 if the last allocated object was an instance, and 3 if it was an array. Here's how to set the number of loaded cache lines to 5:

-XX:AllocatePrefetchLines=5

• -XX:AllocatePrefetchStepSize=size — sets the step size (in bytes) for sequential prefetch instructions. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the step size is set to 16 bytes:

-XX:AllocatePrefetchStepSize=16

• -XX:AllocatePrefetchStyle=style — sets the generated code style for prefetch instructions. The style argument is an integer from 0 to 3:
  • 0 — don't generate prefetch instructions.
  • 1 — execute prefetch instructions after each allocation. This is the default setting.
  • 2 — use the thread-local allocation block (TLAB) watermark pointer to determine when prefetch instructions are executed.
  • 3 — generate one prefetch instruction per cache line.
• -XX:+BackgroundCompilation — enables background compilation. This option is enabled by default. To disable background compilation, specify -XX:-BackgroundCompilation (this is equivalent to specifying -Xbatch).
• -XX:CICompilerCount=threads — sets the number of compiler threads to use for compilation. By default, the number of compiler threads is selected automatically depending on the number of CPUs and memory available for compiled code. Example — setting the number of threads to 2:

-XX:CICompilerCount=2

• -XX:+UseDynamicNumberOfCompilerThreads — dynamically create compiler thread up to the limit specified by -XX:CICompilerCount. This option is enabled by default.
• -XX:CompileCommand=command,method[,option] — specifies a command to perform on a method. For example, to exclude the indexOf() method of the String class from being compiled, use this:

-XX:CompileCommand=exclude,java/lang/String.indexOf

Note that the full class name is specified, including all packages and subpackages separated by a slash (/). For easier cut-and-paste operations, it's also possible to use the method name format produced by the -XX:+PrintCompilation and -XX:+LogCompilation options:

-XX:CompileCommand=exclude,java.lang.String::indexOf

If the method is specified without the signature, then the command is applied to all methods with the specified name. However, you can also specify the signature of the method in the class file format. In this case, you should enclose the arguments in quotation marks, because otherwise the shell treats the semicolon as a command end. For example, if you want to exclude only the indexOf(String) method of the String class from being compiled, use this:

-XX:CompileCommand="exclude,java/lang/String.indexOf,(Ljava/lang/String;)I"

So yeah — back to the compile commands themselves 💅

You can also use the asterisk (*) as a wildcard for class and method names. Like, say you want to exclude every single indexOf() method in every class from being compiled — boom:

-XX:CompileCommand=exclude,*.indexOf

The commas and periods are lowkey just aliases for spaces, which makes it way easier to pass compiler commands through a shell. So you can pass arguments to -XX:CompileCommand using actual spaces as separators, as long as you wrap the whole thing in quotation marks:

-XX:CompileCommand="exclude java/lang/String indexOf"

Ok but here's the tea 🫖 — after it parses the commands you passed on the command line with the -XX:CompileCommand options, the JIT compiler then reads commands from the .hotspot_compiler file. You can add commands to that file, or point at a whole different file with the -XX:CompileCommandFile option.

Want several commands? Either specify the -XX:CompileCommand option multiple times, or separate each argument with the newline separator (\n). The following commands are available:

• break — sets a breakpoint when you're debugging the JVM, so it stops at the beginning of compilation of the method you specified.
• compileonly — excludes all methods from compilation except the one you specified. As an alternative, you can use the -XX:CompileOnly option, which lets you specify several methods at once.
• dontinline — prevents the specified method from being inlined.
• exclude — excludes the specified method from compilation.
• help — prints a help message for the -XX:CompileCommand option.
• inline — attempts to inline the specified method.
• log — excludes compilation logging (the kind you get with the -XX:+LogCompilation option) for all methods except the one you specified. By default, logging is performed for all compiled methods.
• option — passes a JIT compilation option to the specified method in place of the last argument (option). The compilation option is set at the end, after the method name. For example, to enable the BlockLayoutByFrequency option for the append() method of the StringBuffer class, use the following:

-XX:CompileCommand=option,java/lang/StringBuffer.append,BlockLayoutByFrequency

You can specify multiple compilation options, separated by commas or spaces.

• print — prints generated assembler code after compilation of the specified method.
• quiet — instructs it not to print the compile commands. By default, the commands you specify with the -XX:CompileCommand option are printed; so for example, if you exclude the indexOf() method of the String class from compilation, then this gets printed to standard output:

CompilerOracle: exclude java/lang/String.indexOf

You can suppress that by specifying the -XX:CompileCommand=quiet option before your other -XX:CompileCommand options.

---

Compilation levels can be specified in the compileonly, exclude, print, and break commands using a bitmask as an optional value:

-XX:CompileCommand=exclude,java/lang/String.indexOf,1011
-XX:CompileCommand=compileonly,java/lang/String.indexOf,100
-XX:CompileCommand=print,java/lang/String.indexOf,100
-XX:CompileCommand=break,java/lang/StringBuffer.append,1000

The bitmask is calculated by summing the desired compilation level values:

• 1 — C1 JIT compiler without profiling.
• 10 — C1 JIT compiler with limited profiling.
• 100 — C1 JIT compiler with full profiling.
• 1000 — C2 JIT compiler: no profiling, full optimization.

If the bitmask isn't specified, all levels are assumed.

Note: excluding specific compilation levels may disrupt normal state transitions between the levels, because the VM won't automatically work around the excluded ones. So like, be careful babes 💀

---

[-XX:CompileCommandFile=]filename 🗂️

Sets the file from which JIT compiler commands are read. By default, the .hotspot_compiler file is used to store commands performed by the JIT compiler.

Each line in the command file represents a command, a class name, and a method name for which the command is used. For example, this line prints assembly code for the toString() method of the String class:

print java/lang/String toString

If you're using commands for the JIT compiler to perform on methods, then see the -XX:CompileCommand option.

---

[-XX:CompilerDirectivesFile=]file ✨

Adds directives from a file to the directives stack when a program starts. See Compiler Control (https://docs.oracle.com/en/java/javase/12/vm/compiler-control1.html#GUID-94AD8194-786A-4F19-BFFF-278F8E237F3A).

The -XX:CompilerDirectivesFile option has to be used together with the -XX:UnlockDiagnosticVMOptions option, which is the one that unlocks diagnostic JVM options. iykyk 🔓

---

[-XX:+CompilerDirectivesPrint] 🖨️

Prints the directives stack when the program starts, or when a new directive is added.

The -XX:+CompilerDirectivesPrint option has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.

---

[-XX:CompileOnly=]methods 🎯

Sets the list of methods (separated by commas) to which compilation should be restricted. Only the specified methods are compiled.

-XX:CompileOnly=method1,method2,...,methodN is an alias for:

-XX:CompileCommand=compileonly,method1
-XX:CompileCommand=compileonly,method2
...
-XX:CompileCommand=compileonly,methodN

---

[-XX:CompileThresholdScaling=]scale 📏

Provides unified control of first compilation. This option controls when methods are first compiled for both the tiered and the nontiered modes of operation. The CompileThresholdScaling option has a floating point value between 0 and +Inf, and it scales the thresholds corresponding to the current mode of operation (both tiered and nontiered). Setting CompileThresholdScaling to a value less than 1.0 results in earlier compilation, while values greater than 1.0 delay compilation. Setting CompileThresholdScaling to 0 is equivalent to disabling compilation.

---

[-XX:+DoEscapeAnalysis] 🏃‍♀️💨

Enables the use of escape analysis. This option is enabled by default. To disable the use of escape analysis, specify -XX:-DoEscapeAnalysis.

---

[-XX:InitialCodeCacheSize=]size 🧠

Sets the initial code cache size (in bytes). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value depends on the platform. The initial code cache size shouldn't be less than the system's minimal memory page size. The following example shows how to set the initial code cache size to 32 KB:

-XX:InitialCodeCacheSize=32k

---

[-XX:+Inline] 🫂

Enables method inlining. This option is enabled by default to increase performance. To disable method inlining, specify -XX:-Inline.

---

[-XX:InlineSmallCode=]size 📦

Sets the maximum code size (in bytes) for already compiled methods that may be inlined. This flag only applies to the C2 compiler. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value depends on the platform and on whether tiered compilation is enabled. In the following example it is set to 1000 bytes:

-XX:InlineSmallCode=1000

---

[-XX:+LogCompilation] 📝

Enables logging of compilation activity to a file named hotspot.log in the current working directory. You can specify a different log file path and name using the -XX:LogFile option.

By default, this option is disabled and compilation activity isn't logged. The -XX:+LogCompilation option has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.

You can enable verbose diagnostic output — with a message printed to the console every time a method is compiled — by using the -XX:+PrintCompilation option. 👀

---

[-XX:FreqInlineSize=]size 🔥

Sets the maximum bytecode size (in bytes) of a hot method to be inlined. This flag only applies to the C2 compiler. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value depends on the platform. In the following example it is set to 325 bytes:

-XX:FreqInlineSize=325

---

[-XX:MaxInlineSize=]size ❄️

Sets the maximum bytecode size (in bytes) of a cold method to be inlined. This flag only applies to the C2 compiler. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. By default, the maximum bytecode size is set to 35 bytes:

-XX:MaxInlineSize=35

---

[-XX:C1MaxInlineSize=]size ❄️✨

Sets the maximum bytecode size (in bytes) of a cold method to be inlined. This flag only applies to the C1 compiler. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. By default, the maximum bytecode size is set to 35 bytes:

-XX:MaxInlineSize=35

---

[-XX:MaxTrivialSize=]size 🍓

Sets the maximum bytecode size (in bytes) of a trivial method to be inlined. This flag only applies to the C2 compiler. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. By default, the maximum bytecode size of a trivial method is set to 6 bytes:

-XX:MaxTrivialSize=6

• -XX:C1MaxTrivialSize=_size_ — sets the max bytecode size (in bytes) a method can have and still count as "trivial" enough to get inlined. this one's C1-compiler-only bestie, no cap. append k or K for kilobytes, m or M for megabytes, g or G for gigabytes. default is 6 bytes:

-XX:MaxTrivialSize=6

• -XX:MaxNodeLimit=_nodes_ — caps how many nodes can be used while compiling a single method. the default depends on which features are enabled. example where it's set to 100,000:

-XX:MaxNodeLimit=100000

• -XX:NonNMethodCodeHeapSize=_size_ — sets the size in bytes of the code segment that holds nonmethod code.

so like, "nonmethod code" is the segment full of things that aren't methods — compiler buffers, the bytecode interpreter, that whole vibe. this code type lives in the code cache forever 💀. only used when -XX:SegmentedCodeCache is on.

• -XX:NonProfiledCodeHeapSize=_size_ — sets the size in bytes of the code segment holding nonprofiled methods. again, only applies when -XX:SegmentedCodeCache is enabled.
• -XX:+OptimizeStringConcat — turns on the optimization of String concatenation operations. on by default ✨. want it gone? pass -XX:-OptimizeStringConcat.
• -XX:+PrintAssembly — lets you print assembly code for bytecoded and native methods, using the external hsdis-<arch>.so or .dll library. for 64-bit VM on Windows it's hsdis-amd64.dll. this is the one that lets you actually see the generated code, which can help you diagnose performance issues 👑

off by default, so no assembly gets printed unless you ask. and heads up: -XX:+PrintAssembly has to be used together with -XX:UnlockDiagnosticVMOptions, which is the option that unlocks diagnostic JVM options 🔓

• -XX:ProfiledCodeHeapSize=_size_ — sets the size in bytes of the code segment containing profiled methods. only used if -XX:SegmentedCodeCache is enabled.
• -XX:+PrintCompilation — gives you verbose diagnostic output from the JVM, printing a message to the console every time a method gets compiled. you get to see which methods actually get compiled, iykyk. off by default, so no diagnostic output unless you turn it on.

you can also log compilation activity to a file with -XX:+LogCompilation 🫶

• -XX:+PrintInlining — prints out the inlining decisions so you can see which methods are getting inlined. disabled by default — no inlining info for you unless you ask. and it has to be used together with -XX:+UnlockDiagnosticVMOptions, the option that unlocks diagnostic JVM options.
• -XX:ReservedCodeCacheSize=_size_ — sets the max code cache size (in bytes) for JIT-compiled code. append k or K for kilobytes, m or M for megabytes, g or G for gigabytes. default max is 240 MB — but if you disable tiered compilation with -XX:-TieredCompilation, the default becomes 48 MB. this option has a limit of 2 GB; go over and an error gets generated 💀. also, the max code cache size shouldn't be less than the initial code cache size — peep -XX:InitialCodeCacheSize.
• -XX:+SegmentedCodeCache — enables segmentation of the code cache. without it, the code cache is just one big segment. with -XX:+SegmentedCodeCache, you get separate segments for non-method code, profiled method code, and non-profiled method code. the segments are not resized at runtime, periodt.

the perks: better control of the memory footprint, reduced code fragmentation, and better CPU iTLB (instruction translation lookaside buffer) + instruction cache behavior thanks to improved locality. iconic.

it's enabled by default if tiered compilation is enabled (-XX:+TieredCompilation) and the reserved code cache size (-XX:ReservedCodeCacheSize) is at least 240 MB.

• -XX:StartAggressiveSweepingAt=_percent_ — forces stack scanning of active methods to aggressively remove unused code once only that given percentage of the code cache is free. default value is 10%.
• -XX:-TieredCompilation — disables tiered compilation. (yes, it's enabled by default — this flag is the off-switch 🦋)
• -XX:UseSSE=_version_ — enables the SSE instruction set at the version you specify. set by default to the highest supported version available (x86 only).
• -XX:UseAVX=_version_ — enables the AVX instruction set at the version you specify. set by default to the highest supported version available (x86 only).
• -XX:+UseAES — enables hardware-based AES intrinsics on hardware that supports it. on by default on hardware with the necessary instructions. -XX:+UseAES is used in conjunction with UseAESIntrinsics. heads up: flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseAESIntrinsics — enables AES intrinsics. specifying -XX:+UseAESIntrinsics is equivalent to also enabling -XX:+UseAES. want hardware-based AES intrinsics off? specify -XX:-UseAES -XX:-UseAESIntrinsics. for example, to enable hardware AES use these flags:

-XX:+UseAES -XX:+UseAESIntrinsics

flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.

• -XX:+UseAESCTRIntrinsics — analogous to -XX:+UseAESIntrinsics, it enables AES/CTR intrinsics.
• -XX:+UseGHASHIntrinsics — controls the use of GHASH intrinsics. enabled by default on platforms that support the corresponding instructions. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseChaCha20Intrinsics — enable ChaCha20 intrinsics. on by default for supported platforms. to disable them, specify -XX:-UseChaCha20Intrinsics. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UsePoly1305Intrinsics — enable Poly1305 intrinsics. on by default for supported platforms. to disable, specify -XX:-UsePoly1305Intrinsics. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseBASE64Intrinsics — controls the use of accelerated BASE64 encoding routines for java.util.Base64. enabled by default on platforms that support it. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseAdler32Intrinsics — controls the use of the Adler32 checksum algorithm intrinsic for java.util.zip.Adler32. enabled by default on platforms that support it. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseCRC32Intrinsics — controls the use of CRC32 intrinsics for java.util.zip.CRC32. enabled by default on platforms that support it. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseCRC32CIntrinsics — controls the use of CRC32C intrinsics for java.util.zip.CRC32C. enabled by default on platforms that support it. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseSHA — enables hardware-based intrinsics for SHA crypto hash functions on some hardware. the UseSHA option is used in conjunction with the UseSHA1Intrinsics, UseSHA256Intrinsics, and UseSHA512Intrinsics options.

the UseSHA and UseSHA*Intrinsics flags are enabled by default on machines that support the corresponding instructions.

this feature is applicable only when you're using the sun.security.provider.Sun provider for SHA operations. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.

to disable all hardware-based SHA intrinsics, specify -XX:-UseSHA. to disable just one particular SHA intrinsic, use the appropriate corresponding option. for example: -XX:-UseSHA256Intrinsics.

• -XX:+UseSHA1Intrinsics — enables intrinsics for the SHA-1 crypto hash function. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseSHA256Intrinsics — enables intrinsics for the SHA-224 and SHA-256 crypto hash functions. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseSHA512Intrinsics — enables intrinsics for the SHA-384 and SHA-512 crypto hash functions. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseMathExactIntrinsics — enables intrinsification of the various java.lang.Math.*Exact() functions. enabled by default. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseMultiplyToLenIntrinsic — enables intrinsification of BigInteger.multiplyToLen(). enabled by default on platforms that support it. flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseSquareToLenIntrinsic — turns on intrinsification of BigInteger.squareToLen(). ✨ It’s enabled by default on platforms that support it. Heads up: flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions. No cap.
• -XX:+UseMulAddIntrinsic — enables intrinsification of BigInteger.mulAdd(). 💅 Enabled by default on platforms that support it. And yeah, flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseMontgomeryMultiplyIntrinsic — enables intrinsification of BigInteger.montgomeryMultiply(). 🦋 Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseMontgomerySquareIntrinsic — enables intrinsification of BigInteger.montgomerySquare(). 🌸 Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseCMoveUnconditionally — generates CMove (scalar and vector) instructions no matter what profitability analysis says. It’s giving “we’re not doing the math, we’re just doing it.” 💀
• -XX:+UseCodeCacheFlushing — enables flushing of the code cache before shutting down the compiler. 🔥 This option is enabled by default. To disable flushing of the code cache before shutting down the compiler, specify -XX:-UseCodeCacheFlushing.
• -XX:+UseCondCardMark — enables checking if the card is already marked before updating the card table. 🎀 This option is disabled by default. It should be used only on machines with multiple sockets, where it increases the performance of Java applications that rely on concurrent operations.
• -XX:+UseCountedLoopSafepoints — keeps safepoints in counted loops. ⭐ Its default value depends on whether the selected garbage collector requires low latency safepoints.
• -XX:LoopStripMiningIter=_number_of_iterations_ — controls the number of iterations in the inner strip mined loop. Strip mining transforms counted loops into two level nested loops. Safepoints are kept in the outer loop while the inner loop can execute at full speed. This option controls the maximum number of iterations in the inner loop. The default value is 1,000.
• -XX:LoopStripMiningIterShortLoop=_number_of_iterations_ — controls loop strip mining optimization. Loops with the number of iterations less than specified will not have safepoints in them. Default value is 1/10th of -XX:LoopStripMiningIter.
• -XX:+UseFMA — enables hardware-based FMA intrinsics for hardware where FMA instructions are available (such as, Intel and ARM64). 🌈 FMA intrinsics are generated for the java.lang.Math.fma(a, b, c) methods that calculate the value of ( a * b + c ) expressions.
• -XX:+UseSuperWord — enables the transformation of scalar operations into superword operations. Superword is a vectorization optimization. 👑 This option is enabled by default. To disable the transformation of scalar operations into superword operations, specify -XX:-UseSuperWord.

🛠️ ADVANCED SERVICEABILITY OPTIONS FOR JAVA

These alt-java options are the “gather system information and perform extensive debugging” squad. 🔍

• -XX:+DisableAttachMechanism — disables the mechanism that lets tools attach to the JVM. By default, this option is disabled, meaning that the attach mechanism is enabled and you can use diagnostics and troubleshooting tools such as jcmd, jstack, jmap, and jinfo. 🫶

Note: The tools such as jcmd, jinfo, jmap, and jstack shipped with the JDK aren’t supported when using the tools from one JDK version to troubleshoot a different JDK version. 💀

• -XX:+DTraceAllocProbes — Linux and macOS: enable dtrace tool probes for object allocation. 🍓
• -XX:+DTraceMethodProbes — Linux and macOS: enable dtrace tool probes for method-entry and method-exit. ✨
• -XX:+DTraceMonitorProbes — Linux and macOS: enable dtrace tool probes for monitor events. 💅
• -XX:+HeapDumpOnOutOfMemoryError — enables the dumping of the Java heap to a file in the current directory by using the heap profiler (HPROF) when a java.lang.OutOfMemoryError exception is thrown by the JVM. 😭 You can explicitly set the heap dump file path and name using the -XX:HeapDumpPath option. By default, this option is disabled and the heap isn’t dumped when an OutOfMemoryError exception is thrown. This applies only to OutOfMemoryError exceptions caused by Java Heap exhaustion; it does not apply to OutOfMemoryError exceptions thrown directly from Java code, nor by the JVM for other types of resource exhaustion (such as native thread creation errors).
• -XX:HeapDumpPath=_path_ — sets the path and file name for writing the heap dump provided by the heap profiler (HPROF) when the -XX:+HeapDumpOnOutOfMemoryError option is set. 🎀 By default, the file is created in the current working directory, and it’s named alt-java_pid<pid>.hprof where <pid> is the identifier of the process that caused the error. The following example shows how to set the default file explicitly (%p represents the current process identifier):

-XX:HeapDumpPath=./java_pid%p.hprof

• Non-Windows: the following example shows how to set the heap dump file to /var/log/java/java_heapdump.hprof:

-XX:HeapDumpPath=/var/log/java/java_heapdump.hprof

• Windows: the following example shows how to set the heap dump file to C:/log/java/java_heapdump.log:

-XX:HeapDumpPath=C:/log/java/java_heapdump.log

• -XX:LogFile=_path_ — sets the path and file name to where log data is written. 🌸 By default, the file is created in the current working directory, and it’s named hotspot.log.
  • Non-Windows: the following example shows how to set the log file to /var/log/java/hotspot.log:

-XX:LogFile=/var/log/java/hotspot.log

• Windows: the following example shows how to set the log file to C:/log/java/hotspot.log:

-XX:LogFile=C:/log/java/hotspot.log

• -XX:+PrintClassHistogram — enables printing of a class instance histogram after one of the following events:
  • Non-Windows: Control+</kbd> (SIGQUIT)
  • Windows: Control+C (SIGTERM)

By default, this option is disabled.

Setting this option is equivalent to running the jmap -histo command, or the jcmd pid GC.class_histogram command, where pid is the current Java process identifier. 🔥

• -XX:+PrintConcurrentLocks — enables printing of java.util.concurrent locks after one of the following events:
  • Non-Windows: Control+</kbd> (SIGQUIT)
  • Windows: Control+C (SIGTERM)

By default, this option is disabled.

Setting this option is equivalent to running the jstack -l command or the jcmd pid Thread.print -l command, where pid is the current Java process identifier. 💅

• -XX:+PrintFlagsRanges — prints the range specified and allows automatic testing of the values. See Validate Java Virtual Machine Flag Arguments. ⭐
• -XX:+PerfDataSaveToFile — if enabled, saves jstat binary data when the Java application exits. 🦋 This binary data is saved in a file named hsperfdata_pid, where pid is the process identifier of the Java application that you ran. Use the jstat command to display the performance data contained in this file as follows:

jstat -class file:///path/hsperfdata_pid
jstat -gc file:///path/hsperfdata_pid

• -XX:+UsePerfData — enables the perfdata feature. 👑 This option is enabled by default to allow JVM monitoring and performance testing. Disabling it suppresses the creation of the hsperfdata_userid directories. To disable the perfdata feature, specify -XX:-UsePerfData.

🗑️ ADVANCED GARBAGE COLLECTION OPTIONS FOR JAVA

These alt-java options control how garbage collection (GC) is performed by the Java HotSpot VM. 🧹✨

• -XX:+AlwaysPreTouch — requests the VM to touch every page on the Java heap after requesting it from the operating system and before handing memory out to the application. 🫶 By default, this option is disabled and all pages are committed as the application uses the heap space.
• -XX:ConcGCThreads=_threads_ — sets the number of threads used for concurrent GC. Sets threads to approximately 1/4 of the number of parallel garbage collection threads. The default value depends on the number of CPUs available to the JVM. 🔥

For example, to set the number of threads for concurrent GC to 2, specify the following option:

-XX:ConcGCThreads=2

• -XX:+DisableExplicitGC — enables the option that disables processing of calls to the System.gc() method. 💀 This option is disabled by default, meaning that calls to System.gc() are processed. If processing of calls to System.gc() is disabled, then the JVM still performs GC when necessary.
• -XX:+ExplicitGCInvokesConcurrent — enables invoking of concurrent GC by using the System.gc() request. 🌈 This option is disabled by default and can be enabled only with the -XX:+UseG1GC option.
• -XX:G1AdaptiveIHOPNumInitialSamples= — okay so when -XX:UseAdaptiveIHOP is enabled, this one sets the number of completed concurrent cycles used to gather samples until G1 adaptively determines the optimum value of -XX:G1IHOP 💅 before that, G1 just uses the value of -XX:G1IHOP directly for this purpose. The default value is 3. tiny number, big adaptive energy ✨
• -XX:G1HeapRegionSize= — sets the size of the regions into which the Java heap is subdivided when using the garbage-first (G1) collector 🦋 the value is a power of 2 and can range from 1 MB to 32 MB. The default region size is determined ergonomically based on the heap size with a goal of approximately 2048 regions. The following example sets the size of the subdivisions to 16 MB:

-XX:G1HeapRegionSize=16m

• -XX:G1HeapWastePercent= — sets the percentage of heap that you're willing to waste 🎀 the Java HotSpot VM doesn't initiate the mixed garbage collection cycle when the reclaimable percentage is less than the heap waste percentage. The default is 5 percent. waste not, want not, etc.
• -XX:G1MaxNewSizePercent= — sets the percentage of the heap size to use as the maximum for the young generation size 🌸 the default value is 60 percent of your Java heap. This is an experimental flag. This setting replaces the -XX:DefaultMaxNewGenPercent setting. experimental flag bestie, know before you go 🧪
• -XX:G1MixedGCCountTarget= — sets the target number of mixed garbage collections after a marking cycle to collect old regions with at most G1MixedGCLiveThresholdPercent live data 🔥 the default is 8 mixed garbage collections. The goal for mixed collections is to be within this target number. target acquired, slay
• -XX:G1MixedGCLiveThresholdPercent= — sets the occupancy threshold for an old region to be included in a mixed garbage collection cycle 🌈 the default occupancy is 85 percent. This is an experimental flag. This setting replaces the -XX:G1OldCSetRegionLiveThresholdPercent setting. another experimental one, iykyk 🧪
• -XX:G1NewSizePercent= — sets the percentage of the heap to use as the minimum for the young generation size 💖 the default value is 5 percent of your Java heap. This is an experimental flag. This setting replaces the -XX:DefaultMinNewGenPercent setting. experimental flag, no cap 🧪
• -XX:G1OldCSetRegionThresholdPercent= — sets an upper limit on the number of old regions to be collected during a mixed garbage collection cycle 👑 the default is 10 percent of the Java heap. old regions getting a whole limit, iconic
• -XX:G1ReservePercent= — sets the percentage of the heap (0 to 50) that's reserved as a false ceiling to reduce the possibility of promotion failure for the G1 collector 🫶 when you increase or decrease the percentage, ensure that you adjust the total Java heap by the same amount. By default, this option is set to 10%. The following example sets the reserved heap to 20%:

-XX:G1ReservePercent=20

• -XX:+G1UseAdaptiveIHOP — controls adaptive calculation of the old generation occupancy to start background work preparing for an old generation collection ⭐ if enabled, G1 uses -XX:G1IHOP for the first few times as specified by the value of -XX:G1AdaptiveIHOPNumInitialSamples, and after that adaptively calculates a new optimum value for the initiating occupancy automatically. Otherwise, the old generation collection process always starts at the old generation occupancy determined by -XX:G1IHOP. The default is enabled. adaptive and thriving
• -XX:InitialHeapSize= — sets the initial size (in bytes) of the memory allocation pool 💜 this value must be either 0, or a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value is selected at run time based on the system configuration. The following examples show how to set the size of allocated memory to 6 MB using various units:

-XX:InitialHeapSize=6291456
-XX:InitialHeapSize=6144k
-XX:InitialHeapSize=6m

If you set this option to 0, then the initial size is set as the sum of the sizes allocated for the old generation and the young generation 🧚 the size of the heap for the young generation can be set using the -XX:NewSize option. Note that the -Xms option sets both the minimum and the initial heap size of the heap. If -Xms appears after -XX:InitialHeapSize on the command line, then the initial heap size gets set to the value specified with -Xms.

• -XX:InitialRAMPercentage= — sets the initial amount of memory that the JVM will use for the Java heap before applying ergonomics heuristics as a percentage of the available memory to the JVM process 🌸 the following example shows how to set the percentage of the initial amount of memory used for the Java heap:

-XX:InitialRAMPercentage=5

• -XX:InitialSurvivorRatio= — sets the initial survivor space ratio used by the throughput garbage collector (which is enabled by the -XX:+UseParallelGC option) 🦋 adaptive sizing is enabled by default with the throughput garbage collector by using the -XX:+UseParallelGC option, and the survivor space is resized according to the application behavior, starting with the initial value. If adaptive sizing is disabled (using the -XX:-UseAdaptiveSizePolicy option), then the -XX:SurvivorRatio option should be used to set the size of the survivor space for the entire execution of the application. The following formula can be used to calculate the initial size of survivor space (S) based on the size of the young generation (Y), and the initial survivor space ratio (R):

S=Y/(R+2)

The 2 in the equation denotes two survivor spaces 💖 the larger the value specified as the initial survivor space ratio, the smaller the initial survivor space size. By default, the initial survivor space ratio is set to 8. If the default value for the young generation space size is used (2 MB), then the initial size of the survivor space is 0.2 MB. The following example shows how to set the initial survivor space ratio to 4:

-XX:InitialSurvivorRatio=4

• -XX:G1IHOP= — sets the percentage of the old generation occupancy (0 to 100) at which to start the first few concurrent marking cycles for the G1 garbage collector 🔥 by default, the initiating value is set to 45%. A value of 0 implies nonstop concurrent GC cycles from the beginning until G1 adaptively sets this value. See also the -XX:G1UseAdaptiveIHOP and -XX:G1AdaptiveIHOPNumInitialSamples options. The following example shows how to set the initiating heap occupancy to 75%:

-XX:G1IHOP=75

• -XX:MaxGCPauseMillis= — sets a target for the maximum GC pause time (in milliseconds) 🎀 this is a soft goal, and the JVM will make its best effort to achieve it. Only G1 and Parallel support a maximum GC pause time target. For G1, the default maximum pause time target is 200 milliseconds. The following example shows how to set the maximum target pause time to 500 ms:

-XX:MaxGCPauseMillis=500

• -XX:MaxHeapSize= — sets the maximum size (in byes) of the memory allocation pool 💜 this value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value is selected at run time based on the system configuration. For server deployments, the options -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value. The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:

-XX:MaxHeapSize=83886080
-XX:MaxHeapSize=81920k
-XX:MaxHeapSize=80m

The -XX:MaxHeapSize option is equivalent to -Xmx. same vibe, different spelling

• -XX:MaxHeapFreeRatio= — sets the maximum allowed percentage of free heap space (0 to 100) after a GC event 🌈 if free heap space expands above this value, then the heap is shrunk. By default, this value is set to 70%. Minimize the Java heap size by lowering the values of the parameters MaxHeapFreeRatio (default value is 70%) and MinHeapFreeRatio (default value is 40%) with the command-line options -XX:MaxHeapFreeRatio and -XX:MinHeapFreeRatio. Lowering MaxHeapFreeRatio to as low as 10% and MinHeapFreeRatio to 5% has successfully reduced the heap size without too much performance regression; however, results may vary greatly depending on your application. Try different values for these parameters until they're as low as possible yet still retain acceptable performance.

-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5

Customers trying to keep the heap small should also add the option -XX:-ShrinkHeapInSteps. See Performance Tuning Examples for a description of using this option to keep the Java heap small by reducing the dynamic footprint for embedded applications. small heap, big brain energy

• -XX:MaxMetaspaceSize= — sets the maximum amount of native memory that can be allocated for class metadata 🦋 by default, the size isn't limited. The amount of metadata for an application depends on the application itself, other running applications, and the amount of memory available on the system. The following example shows how to set the maximum class metadata size to 256 MB:

-XX:MaxMetaspaceSize=256m

• -XX:MaxNewSize= size — sets the maximum size in bytes of the young-gen heap (the nursery, bestie 🌱). Default value is set ergonomically, iykyk.
• -XX:MaxRAMPercentage= percent — sets the max memory the JVM may use for the Java heap before ergonomics heuristics kick in, as a percentage of the available memory to the JVM process. Default is 25 percent.

Heads up bestie 💅: specifying this option disables automatic use of compressed oops if the combined result of this and other options influencing the maximum amount of memory is larger than the range of memory addressable by compressed oops. See -XX:UseCompressedOops for further info about compressed oops.

Here's how you set the percentage of the maximum amount of memory used for the Java heap:

-XX:MaxRAMPercentage=75

• -XX:MinRAMPercentage= percent — same energy as above, but for small heaps — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a percentage of the available memory to the JVM process for small heaps. A small heap is a heap of approximately 125 MB. Default value is 50 percent.

Example, setting that percentage for small heaps:

-XX:MinRAMPercentage=75

• -XX:MaxTenuringThreshold= threshold — sets the maximum tenuring threshold for use in adaptive GC sizing. The largest value is 15. Default value is 15 for the parallel (throughput) collector.

Setting it to 10, iconically:

-XX:MaxTenuringThreshold=10

• -XX:MetaspaceSize= size — sets the size of the allocated class metadata space that triggers a garbage collection the first time it's exceeded. This threshold for a GC is increased or decreased depending on the amount of metadata used. The default size depends on the platform 🫶.
• -XX:MinHeapFreeRatio= percent — sets the minimum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space falls below this value, then the heap is expanded. By default this value is set to 40%.

Wanna minimize Java heap size? Lower the values of the parameters MaxHeapFreeRatio (default is 70%) and MinHeapFreeRatio (default is 40%) with the command-line options -XX:MaxHeapFreeRatio and -XX:MinHeapFreeRatio. Lowering MaxHeapFreeRatio to as low as 10% and MinHeapFreeRatio to 5% has successfully reduced the heap size without too much performance regression — however, results may vary greatly depending on your application 😭. Try different values for these parameters until they're as low as possible, yet still retain acceptable performance.

-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5

Customers trying to keep the heap small should also add the option -XX:-ShrinkHeapInSteps. See Performance Tuning Examples for a description of using this option to keep the Java heap small by reducing the dynamic footprint for embedded applications.

• -XX:MinHeapSize= size — sets the minimum size in bytes of the memory allocation pool. This value must be either 0, or a multiple of 1024 and greater than 1 MB. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. The default value is selected at run time based on the system configuration.

Setting the minimum size of allocated memory to 6 MB, three ways to slay ✨:

-XX:MinHeapSize=6291456
-XX:MinHeapSize=6144k
-XX:MinHeapSize=6m

If you set this option to 0, then the minimum size is set to the same value as the initial size.

• -XX:NewRatio= ratio — sets the ratio between young and old generation sizes. By default this option is 2. Setting the young-to-old ratio to 1:

-XX:NewRatio=1

• -XX:NewSize= size — sets the initial size in bytes of the heap for the young generation (nursery). Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes.

The young generation region of the heap is where new objects hang out 🐣. GC happens in this region more often than in other regions. If the young gen size is too low, you get a large number of minor GCs. If it's too high, you get only full GCs, which can take a long time to complete. Recommendation: keep the young gen size greater than 25% and less than 50% of the overall heap size.

Setting the initial young gen size to 256 MB, three ways:

-XX:NewSize=256m
-XX:NewSize=262144k
-XX:NewSize=268435456

The -XX:NewSize option is equivalent to -Xmn. Periodt.

• -XX:ParallelGCThreads= threads — sets the number of the stop-the-world (STW) worker threads 💀. Default depends on the number of CPUs available to the JVM and the garbage collector selected.

Setting the number of threads for G1 GC to 2:

-XX:ParallelGCThreads=2

• -XX:+PrintAdaptiveSizePolicy — enables printing of info about adaptive-generation sizing. By default this option is disabled.
• -XX:SoftRefLRUPolicyMSPerMB= time — sets the amount of time in milliseconds a softly reachable object is kept active on the heap after the last time it was referenced. Default is one second of lifetime per free megabyte in the heap.

The -XX:SoftRefLRUPolicyMSPerMB option accepts integer values representing milliseconds per one megabyte of the current heap size (for Java HotSpot Client VM) or the maximum possible heap size (for Java HotSpot Server VM). This difference means the Client VM tends to flush soft references rather than grow the heap, whereas the Server VM tends to grow the heap rather than flush soft references 👀. In the latter case, the value of the -Xmx option has a significant effect on how quickly soft references get garbage collected.

Setting the value to 2.5 seconds:

-XX:SoftRefLRUPolicyMSPerMB=2500

• -XX:-ShrinkHeapInSteps — incrementally reduces the Java heap to the target size specified by the option -XX:MaxHeapFreeRatio. This option is enabled by default. If disabled, then it immediately reduces the Java heap to the target size instead of requiring multiple garbage collection cycles. Disable this option if you want to minimize the Java heap size — you will likely encounter performance degradation when this option is disabled 😬.

See Performance Tuning Examples for a description of using the MaxHeapFreeRatio option to keep the Java heap small by reducing the dynamic footprint for embedded applications.

• -XX:StringDeduplicationAgeThreshold= threshold — identifies String objects reaching the specified age that are considered candidates for deduplication. An object's age is a measure of how many times it has survived garbage collection — sometimes called tenuring.

Note: String objects that are promoted to an old heap region before this age has been reached are always considered candidates for deduplication. The default value for this option is 3. See the -XX:+UseStringDeduplication option.

• -XX:SurvivorRatio= ratio — sets the ratio between eden space size and survivor space size. By default this option is 8. Setting the eden/survivor space ratio to 4:

-XX:SurvivorRatio=4

• -XX:TargetSurvivorRatio= percent — sets the desired percentage of survivor space (0 to 100) used after young garbage collection. By default this option is 50%.

Setting the target survivor space ratio to 30%:

-XX:TargetSurvivorRatio=30

• -XX:TLABSize= size — sets the initial size in bytes of a thread-local allocation buffer (TLAB) 🧵. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. If this option is set to 0, then the JVM selects the initial size automatically.

Setting the initial TLAB size to 512 KB:

-XX:TLABSize=512k

• -XX:+UseAdaptiveSizePolicy — enables the use of adaptive generation sizing. This option is enabled by default. To disable adaptive generation sizing, specify -XX:-UseAdaptiveSizePolicy and set the size of the memory allocation pool explicitly. See the -XX:SurvivorRatio option.
• -XX:+UseG1GC — enables the use of the garbage-first (G1) garbage collector 👑. It's a server-style garbage collector, targeted for multiprocessor machines with a large amount of RAM. This option meets GC pause time goals with high probability, while maintaining good throughput. The G1 collector is recommended for applications requiring large heaps (sizes of around 6 GB or larger) with limited GC latency requirements (a stable and predictable pause time below 0.5 seconds). By default, this option is enabled and G1 is used as the default garbage collector 🔥.

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• -XX:+UseGCOverheadLimit — turns on the policy that caps how much of the JVM's time can go to GC before it just throws an OutOfMemoryError 🫠. This one's on by default, and the parallel GC will throw an OutOfMemoryError if more than 98% of the total time is spent on garbage collection and less than 2% of the heap is recovered. When the heap is small, this feature can be used to prevent applications from running for long periods of time with little or no progress. To disable this option, specify the option -XX:-UseGCOverheadLimit.
• -XX:+UseNUMA — enables performance optimization of an application on a machine with nonuniform memory architecture (NUMA) by increasing the application's use of lower latency memory ✨. The default value for this option depends on the garbage collector.
• -XX:+UseParallelGC — enables the use of the parallel scavenge garbage collector (also known as the throughput collector) to improve the performance of your application by leveraging multiple processors 💪.

By default, this option is disabled and the default collector is used.

• -XX:+UseSerialGC — enables the use of the serial garbage collector. This is generally the best choice for small and simple applications that don't require any special functionality from garbage collection 🌸. By default, this option is disabled and the default collector is used.
• -XX:+UseStringDeduplication — enables string deduplication 🧵. By default, this option is disabled. To use this option, you must enable the garbage-first (G1) garbage collector.

String deduplication reduces the memory footprint of String objects on the Java heap by taking advantage of the fact that many String objects are identical. Instead of each String object pointing to its own character array, identical String objects can point to and share the same character array. iconic behaviour tbh 💅

• -XX:+UseTLAB — enables the use of thread-local allocation blocks (TLABs) in the young generation space 🎀. This option is enabled by default. To disable the use of TLABs, specify the option -XX:-UseTLAB.
• -XX:+UseZGC — enables the use of the Z garbage collector (ZGC) 👑. This is a low latency garbage collector, providing max pause times of a few milliseconds, at some throughput cost. Pause times are independent of what heap size is used. Supports heap sizes from 8MB to 16TB. no cap, 16TB 😭
• -XX:ZAllocationSpikeTolerance=factor — sets the allocation spike tolerance for ZGC. By default, this option is set to 2.0. This factor describes the level of allocation spikes to expect. For example, using a factor of 3.0 means the current allocation rate can be expected to triple at any time 📈.
• -XX:ZCollectionInterval=seconds — sets the maximum interval (in seconds) between two GC cycles when using ZGC. By default, this option is set to 0 (disabled).
• -XX:ZFragmentationLimit=percent — sets the maximum acceptable heap fragmentation (in percent) for ZGC. By default, this option is set to 25. Using a lower value will cause the heap to be compacted more aggressively, to reclaim more memory at the cost of using more CPU time.
• -XX:+ZProactive — enables proactive GC cycles when using ZGC. By default, this option is enabled. ZGC will start a proactive GC cycle if doing so is expected to have minimal impact on the running application 🦋. This is useful if the application is mostly idle or allocates very few objects, but you still want to keep the heap size down and allow reference processing to happen even when there are a lot of free space on the heap.
• -XX:+ZUncommit — enables uncommitting of unused heap memory when using ZGC. By default, this option is enabled. Uncommitting unused heap memory will lower the memory footprint of the JVM, and make that memory available for other processes to use 🤝.
• -XX:ZUncommitDelay=seconds — sets the amount of time (in seconds) that heap memory must have been unused before being uncommitted. By default, this option is set to 300 (5 minutes) ⏰. Committing and uncommitting memory are relatively expensive operations. Using a lower value will cause heap memory to be uncommitted earlier, at the risk of soon having to commit it again.
• -XX:+UseShenandoahGC — enables the use of the Shenandoah garbage collector. This is a low pause time, concurrent garbage collector. Its pause times are not proportional to the size of the heap 💜. Shenandoah garbage collector can work with compressed pointers. See -XX:UseCompressedOops for further information about compressed pointers.
• -XX:ShenandoahGCMode=mode — sets the GC mode for Shenandoah GC to use. By default, this option is set to satb. Among other things, this defines which barriers are in use. Possible mode values include the following:
  • satb — Snapshot-at-the-beginning concurrent GC (three pass mark-evac-update). It is a single generation GC.
  • generational — It is also a snapshot-at-the-beginning and concurrent GC, but it is generational. Please see JEP 404 [https://openjdk.org/jeps/404] and JEP 521 [https://openjdk.org/jeps/521] for its advantages and risks.
• -XX:ShenandoahGCHeuristics=heuristics — sets the heuristics for Shenandoah GC to use. By default, this option is set to adaptive. This fine-tunes the GC mode selected, by choosing when to start the GC, how much to process on each cycle, and what other features to automatically enable. When -XX:ShenandoahGCMode is generational, the only supported option is the default, adaptive.

Possible heuristics are the following:

• adaptive — To maintain the given amount of free heap at all times, even during the GC cycle.
• static — Trigger GC when free heap falls below a specified threshold.
• compact — Run GC more frequently and with deeper targets to free up more memory.

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🥀 DEPRECATED JAVA OPTIONS 🥀

These alt-java options are deprecated and might be removed in a future JDK release 😬. They're still accepted and acted upon, but a warning is issued when they're used. so like, they still slay but the clock is ticking bestie ⏳

• -Xloggc:filename — sets the file to which verbose GC events information should be redirected for logging. The -Xloggc option overrides -verbose:gc if both are given with the same alt-java command. -Xloggc:filename is replaced by -Xlog:gc:filename. See Enable Logging with the JVM Unified Logging Framework.

Example:

-Xlog:gc:garbage-collection.log

• -XX:+FlightRecorder — enables the use of Java Flight Recorder (JFR) during the runtime of the application ✈️. Since JDK 8u40 this option has not been required to use JFR.
• -XX:+ParallelRefProcEnabled — enables parallel reference processing. By default, collectors employing multiple threads perform parallel reference processing if the number of parallel threads to use is larger than one. The option is available only when the throughput or G1 garbage collector is used (-XX:+UseParallelGC or -XX:+UseG1GC). Other collectors employing multiple threads always perform reference processing in parallel 🙌.

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🦕 OBSOLETE JAVA OPTIONS 🦕

These alt-java options are still accepted but ignored, and a warning is issued when they're used. like they're literally just vibing there, doing nothing 💀

• --illegal-access=parameter — controlled relaxed strong encapsulation, as defined in JEP 261 [https://openjdk.org/jeps/261#Relaxed-strong-encapsulation]. This option was deprecated in JDK 16 by JEP 396 [https://openjdk.org/jeps/396] and made obsolete in JDK 17 by JEP 403 [https://openjdk.org/jeps/403].
• -XX:+NeverActAsServerClassMachine — enabled the "Client VM emulation" mode, which used only the C1 JIT compiler, a 32Mb CodeCache, and the Serial GC 🧚. The maximum amount of memory that the JVM could use was set to 1GB by default. The string "emulated-client" was added to the JVM version string.

By default the flag was set to true only on Windows in 32-bit mode and false in all other cases.

The "Client VM emulation" mode was not enabled if any of the following flags were used on the command line:

-XX:{+|-}TieredCompilation
-XX:CompilationMode=mode
-XX:TieredStopAtLevel=n
-XX:{+|-}EnableJVMCI
-XX:{+|-}UseJVMCICompiler

• -XX:+AggressiveHeap — enabled Java heap optimization 🔥. This set various parameters to be optimal for long-running jobs with intensive memory allocation, based on the configuration of the computer (RAM and CPU). By default, the option was disabled and the heap sizes configured less aggressively.

💅 REMOVED JAVA OPTIONS

Okay so bestie, here's the tea: no documented alt-java options have been removed in JDK 27. Zero. Nothing. The list is empty, the vibe is immaculate, nobody got cut this cycle. 🎉

But if you're the kind of girl who needs the receipts from past seasons, the full lists and descriptions of options removed in previous releases live in the Removed Java Options section of these:

• The alt-java Command, Release 26 — [https://docs.oracle.com/en/java/javase/26/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/26/docs/specs/man/java.html)
• The alt-java Command, Release 25 — [https://docs.oracle.com/en/java/javase/25/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/25/docs/specs/man/java.html)
• The alt-java Command, Release 24 — [https://docs.oracle.com/en/java/javase/24/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/24/docs/specs/man/java.html)
• The alt-java Command, Release 23 — [https://docs.oracle.com/en/java/javase/23/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/23/docs/specs/man/java.html)
• The alt-java Command, Release 22 — [https://docs.oracle.com/en/java/javase/22/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/22/docs/specs/man/java.html)
• The alt-java Command, Release 21 — [https://docs.oracle.com/en/java/javase/21/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/21/docs/specs/man/java.html)
• The alt-java Command, Release 20 — [https://docs.oracle.com/en/java/javase/20/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/20/docs/specs/man/java.html)
• The alt-java Command, Release 19 — [https://docs.oracle.com/en/java/javase/19/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/19/docs/specs/man/java.html)
• The alt-java Command, Release 18 — [https://docs.oracle.com/en/java/javase/18/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/18/docs/specs/man/java.html)
• The alt-java Command, Release 17 — [https://docs.oracle.com/en/java/javase/17/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/17/docs/specs/man/java.html)
• The alt-java Command, Release 16 — [https://docs.oracle.com/en/java/javase/16/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/16/docs/specs/man/java.html)
• The alt-java Command, Release 15 — [https://docs.oracle.com/en/java/javase/15/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/15/docs/specs/man/java.html)
• The alt-java Command, Release 14 — [https://docs.oracle.com/en/java/javase/14/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/14/docs/specs/man/java.html)
• The alt-java Command, Release 13 — [https://docs.oracle.com/en/java/javase/13/docs/specs/man/java.html](https://docs.oracle.com/en/java/javase/13/docs/specs/man/java.html)
• Java Platform, Standard Edition Tools Reference, Release 12 — [https://docs.oracle.com/en/java/javase/12/tools/java.html#GUID-3B1CE181-CD30-4178-9602-230B800D4FAE](https://docs.oracle.com/en/java/javase/12/tools/java.html#GUID-3B1CE181-CD30-4178-9602-230B800D4FAE)
• Java Platform, Standard Edition Tools Reference, Release 11 — [https://docs.oracle.com/en/java/javase/11/tools/java.html#GUID-741FC470-AA3E-494A-8D2B-1B1FE4A990D1](https://docs.oracle.com/en/java/javase/11/tools/java.html#GUID-741FC470-AA3E-494A-8D2B-1B1FE4A990D1)
• Java Platform, Standard Edition Tools Reference, Release 10 — [https://docs.oracle.com/javase/10/tools/java.htm#JSWOR624](https://docs.oracle.com/javase/10/tools/java.htm#JSWOR624)
• Java Platform, Standard Edition Tools Reference, Release 9 — [https://docs.oracle.com/javase/9/tools/java.htm#JSWOR624](https://docs.oracle.com/javase/9/tools/java.htm#JSWOR624)
• Java Platform, Standard Edition Tools Reference, Release 8 for Oracle JDK on Windows — [https://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html#BGBCIEFC](https://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html#BGBCIEFC)
• Java Platform, Standard Edition Tools Reference, Release 8 for Oracle JDK on Solaris, Linux, and macOS — [https://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html#BGBCIEFC](https://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html#BGBCIEFC)

---

🎀 ALT-JAVA COMMAND-LINE ARGUMENT FILES

Okay so real talk — sometimes your alt-java command gets so long it's giving unreadable. Bestie, that's what @ argument files are for. You point at one or more text files that hold arguments (options, class names, whatever) and they get passed to the alt-java command for you. This lets you create alt-java commands of any length on any operating system. Iconic. 🦋

Here's the deal: on the command line you use the at sign (@) prefix to identify an argument file that contains alt-java options and class names. When the alt-java command bumps into a file beginning with the at sign (@), it expands the contents of that file into an argument list just as they would be specified on the command line. Like magic but it's just parsing. ✨

The alt-java launcher keeps expanding argument file contents until it encounters the --disable-@files option. And plot twist — you can use --disable-@files anywhere on the command line, including inside an argument file, to stop @ argument file expansion. iykyk.

🌸 The Syntax Rules (aka the vibe check for argument files)

• The argument file must contain only ASCII characters or characters in system default encoding that's ASCII friendly, such as UTF-8.
• The argument file size must not exceed MAXINT (2,147,483,647) bytes.
• The launcher doesn't expand wildcards that are present within an argument file. That means an asterisk (*) is passed on as-is to the starting VM. For example *.java stays *.java and is not expanded to Foo.java Bar.java ..., as would happen with some command line shells. No cap.
• Use white space or new line characters to separate arguments included in the file.
• White space includes a white space character, \t, \n, \r, and \f.

For example, it is possible to have a path with a space, such as c:\Program Files that can be specified as either "c:\\Program Files" or, to avoid an escape, c:\Program" "Files.

• Any option that contains spaces, such as a path component, must be within quotation marks using quotation ('"') characters in its entirety.
• A string within quotation marks may contain the characters \n, \r, \t, and \f. They are converted to their respective ASCII codes.
• If a file name contains embedded spaces, then put the whole file name in double quotation marks.
• File names in an argument file are relative to the current directory, not to the location of the argument file. (This one trips people up literally every time, so like… note it. 🫶)
• Use the number sign # in the argument file to identify comments. All characters following the # are ignored until the end of line.
• Additional at sign @ prefixes to @ prefixed options act as an escape, (the first @ is removed and the rest of the arguments are presented to the launcher literally).
• Lines may be continued using the continuation character (\) at the end-of-line. The two lines are concatenated with the leading white spaces trimmed. To prevent trimming the leading white spaces, a continuation character (\) may be placed at the first column.
• Because backslash (\) is an escape character, a backslash character must be escaped with another backslash character.
• Partial quote is allowed and is closed by an end-of-file.
• An open quote stops at end-of-line unless \ is the last character, which then joins the next line by removing all leading white space characters.
• Use of the at sign (@) to recursively interpret files isn't supported.

🧚 Example of Open or Partial Quotes in an Argument File

In the argument file,

-cp "lib/
cool/
app/
jars

this is interpreted as:

-cp lib/cool/app/jars

🧚 Example of a Backslash Character Escaped with Another Backslash Character in an Argument File

To output the following:

-cp c:\Program Files (x86)\Java\jre\lib\ext;c:\Program Files\Java\jre9\lib\ext

The backslash character must be specified in the argument file as:

-cp "c:\\Program Files (x86)\\Java\\jre\\lib\\ext;c:\\Program Files\\Java\\jre9\\lib\\ext"

🧚 Example of an EOL Escape Used to Force Concatenation of Lines in an Argument File

In the argument file,

-cp "/lib/cool app/jars:\
    /lib/another app/jars"

This is interpreted as:

-cp /lib/cool app/jars:/lib/another app/jars

🧚 Example of Line Continuation with Leading Spaces in an Argument File

In the argument file,

-cp "/lib/cool\
\app/jars"

This is interpreted as:

-cp /lib/cool app/jars

🌈 Examples of Using Single Argument File

You can use a single argument file, such as myargumentfile in the following example, to hold all required alt-java arguments:

alt-java @myargumentfile

🌈 Examples of Using Argument Files with Paths

You can include relative paths in argument files; however, they're relative to the current working directory and not to the paths of the argument files themselves. In the following example, path1/options and path2/options represent argument files with different paths. Any relative paths that they contain are relative to the current working directory and not to the argument files:

alt-java @path1/options @path2/classes

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🔥 CODE HEAP STATE ANALYTICS

👑 Overview

There are occasions when having insight into the current state of the JVM code heap would be helpful to answer questions such as:

• Why was the JIT turned off and then on again and again?
• Where has all the code heap space gone?
• Why is the method sweeper not working effectively?

To provide this insight, a code heap state analytics feature has been implemented that enables on-the-fly analysis of the code heap. The analytics process is divided into two parts. The first part examines the entire code heap and aggregates all information that is believed to be useful or important. The second part consists of several independent steps that print the collected information with an emphasis on different aspects of the data. Data collection and printing are done on an "on request" basis. So it's not doing all this work behind your back, it's very respectful of your time. 💅

👑 Syntax

Requests for real-time, on-the-fly analysis can be issued with the following command:

jcmd pid Compiler.CodeHeap_Analytics [function] [granularity]

If you are only interested in how the code heap looks like after running a sample workload, you can use the command line option:

-Xlog:codecache=Trace

To see the code heap state when a "CodeCache full" condition exists, start the VM with the command line option:

-Xlog:codecache=Debug

See CodeHeap State Analytics (OpenJDK) — [https://bugs.openjdk.org/secure/attachment/75649/JVM_CodeHeap_StateAnalytics_V2.pdf](https://bugs.openjdk.org/secure/attachment/75649/JVM_CodeHeap_StateAnalytics_V2.pdf) — for a detailed description of the code heap state analytics feature, the supported functions, and the granularity options. Fr fr, that PDF is the deep lore.

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✨ ENABLE LOGGING WITH THE JVM UNIFIED LOGGING FRAMEWORK

You use the -Xlog option to configure or enable logging with the Java Virtual Machine (JVM) unified logging framework. That's the whole assignment. 🎀

💜 Synopsis

-Xlog[:[what][:[output][:[decorators][:output-options[,...]]]]]

-Xlog:directive

💜 The Pieces, Explained

• what — Specifies a combination of tags and levels of the form tag1[+tag2...][*][=level][,...]. Unless the wildcard (*) is specified, only log messages tagged with exactly the tags specified are matched. See -Xlog Tags and Levels. (Yes, "exactly" — she's picky and we respect it.)
• output — Sets the type of output. Omitting the output type defaults to stdout. See -Xlog Output.
• decorators — Configures the output to use a custom set of decorators. Omitting decorators defaults to uptime, level, and tags. See Decorations.
• output-options — Sets the -Xlog logging output options.
• directive — A global option or subcommand: help, disable, async

📝 Description

okay so bestie, here’s the tea ☕: the Java Virtual Machine (JVM) unified logging framework gives you one common logging system for all components of the JVM. GC logging for the JVM has been changed to use this new logging framework. The mapping of old GC flags to the corresponding new Xlog configuration is described in Convert GC Logging Flags to Xlog. Runtime logging also got switched over to the JVM unified logging framework, and the mapping of legacy runtime logging flags to the corresponding new Xlog configuration is described in Convert Runtime Logging Flags to Xlog.

Here’s your quick-reference moment for the -Xlog command and its syntax, because we love a cheat sheet 💅:

• -Xlog — enables JVM logging on an info level.
• -Xlog:help — prints -Xlog usage syntax and available tags, levels, and decorators, along with example command lines with explanations. iconic behavior, honestly ✨
• -Xlog:disable — turns off all logging and clears all configuration of the logging framework, including the default configuration for warnings and errors.
• -Xlog[:option] — applies multiple arguments in the order that they appear on the command line. Multiple -Xlog arguments for the same output override each other in their given order.

The option is set like this:

[tag-selection][:[output][:[decorators][:output-options]]]

Omitting the tag-selection defaults to a tag-set of all and a level of info.

tag[+...] all

The all tag is a meta tag consisting of all tag-sets available. The asterisk * in a tag set definition denotes a wildcard tag match. Matching with a wildcard selects all tag sets that contain at least the specified tags. Without the wildcard, only exact matches of the specified tag sets are selected.

output-options is:

filecount=file-count filesize=file size with optional K, M or G suffix foldmultilines=<true|false>

When foldmultilines is true, a log event that consists of multiple lines will be folded into a single line by replacing newline characters with the sequence '\' and 'n' in the output. Existing single backslash characters will also be replaced with a sequence of two backslashes so that the conversion can be reversed. This option is safe to use with UTF-8 character encodings, but other encodings may not work. For example, it may incorrectly convert multi-byte sequences in Shift JIS and BIG5. 🫶

---

⚙️ Default Configuration

When the -Xlog option and nothing else is specified on the command line, the default configuration is used. The default configuration logs all messages with a level that matches either warning or error, regardless of what tags the message is associated with.

It is equivalent to entering this on the command line:

-Xlog:all=warning:stdout:uptime,level,tags

no cap, that’s the whole default vibe 🎀

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🕹️ Controlling Logging at Runtime

Logging can also be controlled at run time through Diagnostic Commands (with the jcmd utility). Everything that can be specified on the command line can also be specified dynamically with the VM.log command. And because diagnostic commands are automatically exposed as MBeans, you can use JMX to change logging configuration at run time. runtime glow-up, periodt 💅

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🏷️ -Xlog Tags and Levels

Each log message has a level and a tag set associated with it. The level of the message corresponds to its details, and the tag set corresponds to what the message contains or which JVM component it involves (such as, gc, jit, or os). Mapping GC flags to the Xlog configuration is described in Convert GC Logging Flags to Xlog. Mapping legacy runtime logging flags to the corresponding Xlog configuration is described in Convert Runtime Logging Flags to Xlog.

Available log levels:

• off
• trace
• debug
• info
• warning
• error

Available log tags:

There are literally dozens of log tags, which in the right combinations will enable a range of logging output. The full set of available log tags can be seen using -Xlog:help. Specifying all instead of a tag combination matches all tag combinations. 💖

---

📤 -Xlog Output

The -Xlog option supports the following types of outputs:

• stdout — Sends output to stdout
• stderr — Sends output to stderr
• file=filename — Sends output to text file(s).

When using file=filename, specifying %p, %t and/or %hn in the file name expands to the JVM's PID, startup timestamp and host name, respectively. You can also configure text files to handle file rotation based on file size and a number of files to rotate. For example, to rotate the log file every 10 MB and keep 5 files in rotation, specify the options filesize=10M, filecount=5.

The target size of the files isn't guaranteed to be exact — it's just an approximate value, bestie. 🍓

Files are rotated by default with up to 5 rotated files of target size 20 MB, unless configured otherwise. Specifying filecount=0 means that the log file shouldn't be rotated. There’s a possibility of the pre-existing log file getting overwritten. 👀

---

🔄 -Xlog Output Mode

By default, logging messages are output synchronously — each log message is written to the designated output when the logging call is made. You can instead use asynchronous logging mode by specifying:

-Xlog:async[:[stall|drop]]

Write all logging asynchronously.

In asynchronous logging mode, log sites enqueue all logging messages to an intermediate buffer, and a standalone thread is responsible for flushing them to the corresponding outputs. The intermediate buffer is bounded. On buffer exhaustion, the enqueuing message is either discarded (async:drop), or logging threads are stalled until the flushing thread catches up (async:stall). If no specific mode is chosen, then async:drop is chosen by default. Log entry write operations are guaranteed to be non-blocking in the async:drop case.

The option -XX:AsyncLogBufferSize=N specifies the memory budget in bytes for the intermediate buffer. The default value should be big enough to cater for most cases. Users can provide a custom value to trade memory overhead for log accuracy if they need to. Trade-offs, but make it fashion 💅

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🎀 Decorations

Logging messages are decorated with information about the message. You can configure each output to use a custom set of decorators. The order of the output is always the same as listed in the list below. You can configure the decorations to be used at run time. Decorations are prepended to the log message. For example:

[6.567s][info][gc,old] Old collection complete

Omitting decorators defaults to uptime, level, and tags. The none decorator is special and is used to turn off all decorations.

time (t), utctime (utc), uptime (u), timemillis (tm), uptimemillis (um), timenanos (tn), uptimenanos (un), hostname (hn), pid (p), tid (ti), level (l), tags (tg) decorators can also be specified as none for no decoration.

🌸 Logging Messages Decorations

• time or t — Current time and date in ISO-8601 format.
• utctime or utc — Universal Time Coordinated or Coordinated Universal Time.
• uptime or u — Time since the start of the JVM in seconds and milliseconds. For example, 6.567s.
• timemillis or tm — The same value as generated by System.currentTimeMillis()
• uptimemillis or um — Milliseconds since the JVM started.
• timenanos or tn — The same value generated by System.nanoTime().
• uptimenanos or un — Nanoseconds since the JVM started.
• hostname or hn — The host name.
• pid or p — The process identifier.
• tid or ti — The thread identifier.
• level or l — The level associated with the log message.
• tags or tg — The tag-set associated with the log message.

Convert GC Logging Flags to Xlog

Okay bestie, this is the glow-up era for GC logging 💅 The old legacy flags are out here getting mapped onto their -Xlog equivalents, and honestly? Some of them didn't even make the cut. Let's go row by row, no skipping, no cap.

Legacy GC Logging Flags to Xlog Configuration Mapping

• G1PrintHeapRegions → -Xlog:gc+region=trace — Not Applicable
• GCLogFileSize → No configuration available — Log rotation is handled by the framework.
• NumberOfGCLogFiles → Not Applicable — Log rotation is handled by the framework.
• PrintAdaptiveSizePolicy → -Xlog:gc+ergo*=level — Use a level of debug for most of the information, or a level of trace for all of what was logged for PrintAdaptiveSizePolicy.
• PrintGC → -Xlog:gc — Not Applicable
• PrintGCApplicationConcurrentTime → -Xlog:safepoint — Note that PrintGCApplicationConcurrentTime and PrintGCApplicationStoppedTime are logged on the same tag and aren't separated in the new logging.
• PrintGCApplicationStoppedTime → -Xlog:safepoint — Note that PrintGCApplicationConcurrentTime and PrintGCApplicationStoppedTime are logged on the same tag and not separated in the new logging.
• PrintGCCause → Not Applicable — GC cause is now always logged.
• PrintGCDateStamps → Not Applicable — Date stamps are logged by the framework.
• PrintGCDetails → -Xlog:gc* — Not Applicable
• PrintGCID → Not Applicable — GC ID is now always logged.
• PrintGCTaskTimeStamps → -Xlog:gc+task*=debug — Not Applicable
• PrintGCTimeStamps → Not Applicable — Time stamps are logged by the framework.
• PrintHeapAtGC → -Xlog:gc+heap=trace — Not Applicable
• PrintReferenceGC → -Xlog:gc+ref*=debug — Note that in the old logging, PrintReferenceGC had an effect only if PrintGCDetails was also enabled.
• PrintStringDeduplicationStatistics → -Xlog:gc+stringdedup*=debug — Not Applicable
• PrintTenuringDistribution → -Xlog:gc+age*=level — Use a level of debug for the most relevant information, or a level of trace for all of what was logged for PrintTenuringDistribution.
• UseGCLogFileRotation → Not Applicable — What was logged for PrintTenuringDistribution.

---

Convert Runtime Logging Flags to Xlog

These legacy flags are no longer recognized and will cause an error if used directly. Use their unified logging equivalent instead. Like, do not try to sneak them past the JVM, she will notice 👀

Runtime Logging Flags to Xlog Configuration Mapping

• TraceExceptions → -Xlog:exceptions=info — Not Applicable
• TraceClassLoading → -Xlog:class+load=level — Use level=info for regular information, or level=debug for additional information. In Unified Logging syntax, -verbose:class equals -Xlog:class+load=info,class+unload=info.
• TraceClassLoadingPreorder → -Xlog:class+preorder=debug — Not Applicable
• TraceClassUnloading → -Xlog:class+unload=level — Use level=info for regular information, or level=trace for additional information. In Unified Logging syntax, -verbose:class equals -Xlog:class+load=info,class+unload=info.
• VerboseVerification → -Xlog:verification=info — Not Applicable
• TraceClassPaths → -Xlog:class+path=info — Not Applicable
• TraceClassResolution → -Xlog:class+resolve=debug — Not Applicable
• TraceClassInitialization → -Xlog:class+init=info — Not Applicable
• TraceLoaderConstraints → -Xlog:class+loader+constraints=info — Not Applicable
• TraceClassLoaderData → -Xlog:class+loader+data=level — Use level=debug for regular information or level=trace for additional information.
• TraceSafepointCleanupTime → -Xlog:safepoint+cleanup=info — Not Applicable
• TraceSafepoint → -Xlog:safepoint=debug — Not Applicable
• TraceMonitorInflation → -Xlog:monitorinflation=debug — Not Applicable
• TraceRedefineClasses → -Xlog:redefine+class*=level — level=info, debug, and trace provide increasing amounts of information.

---

-Xlog Usage Examples

The following are -Xlog examples. Study them, screenshot them, send them to the group chat 🫶

• [-Xlog]{#-Xlog} — Logs all messages by using the info level to stdout with uptime, levels, and tags decorations. This is equivalent to using:

-Xlog:all=info:stdout:uptime,levels,tags

• -Xlog:gc — Logs messages tagged with the gc tag using info level to stdout. The default configuration for all other messages at level warning is in effect.
• -Xlog:gc,safepoint — Logs messages tagged either with the gc or safepoint tags, both using the info level, to stdout, with default decorations. Messages tagged with both gc and safepoint won't be logged.
• -Xlog:gc+ref=debug — Logs messages tagged with both gc and ref tags, using the debug level to stdout, with default decorations. Messages tagged only with one of the two tags won't be logged.
• -Xlog:gc=debug:file=gc.txt:none — Logs messages tagged with the gc tag using the debug level to a file called gc.txt with no decorations. The default configuration for all other messages at level warning is still in effect.
• -Xlog:gc=trace:file=gctrace.txt:uptimemillis,pid:filecount=5,filesize=1024 — Logs messages tagged with the gc tag using the trace level to a rotating file set with 5 files with size 1 MB with the base name gctrace.txt and uses decorations uptimemillis and pid.

The default configuration for all other messages at level warning is still in effect.

• -Xlog:gc::uptime,tid — Logs messages tagged with the gc tag using the default 'info' level to default the output stdout and uses decorations uptime and tid. The default configuration for all other messages at level warning is still in effect.
• -Xlog:gc*=info,safepoint*=off — Logs messages tagged with at least gc using the info level, but turns off logging of messages tagged with safepoint. Messages tagged with both gc and safepoint won't be logged.
• -Xlog:disable -Xlog:safepoint=trace:safepointtrace.txt — Turns off all logging, including warnings and errors, and then enables messages tagged with safepoint using trace level to the file safepointtrace.txt. The default configuration doesn't apply, because the command line started with -Xlog:disable.

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Complex -Xlog Usage Examples

The following describes a few complex examples of using the -Xlog option. Buckle up, this is where the tag wildcards start doing the absolute most 🌀

• -Xlog:gc+class*=debug — Logs messages tagged with at least gc and class tags using the debug level to stdout. The default configuration for all other messages at the level warning is still in effect
• -Xlog:gc+meta*=trace,class*=off:file=gcmetatrace.txt — Logs messages tagged with at least the gc and meta tags using the trace level to the file metatrace.txt but turns off all messages tagged with class. Messages tagged with gc, meta, and class aren't be logged as class* is set to off. The default configuration for all other messages at level warning is in effect except for those that include class.
• -Xlog:gc+meta=trace — Logs messages tagged with exactly the gc and meta tags using the trace level to stdout. The default configuration for all other messages at level warning is still be in effect.
• -Xlog:gc+class+heap*=debug,meta*=warning,threads*=off — Logs messages tagged with at least gc, class, and heap tags using the trace level to stdout but only log messages tagged with meta with level. The default configuration for all other messages at the level warning is in effect except for those that include threads.

✨ VALIDATE JAVA VIRTUAL MACHINE FLAG ARGUMENTS ✨

Okay bestie, so every single value you hand to all Java Virtual Machine (JVM) command-line flags gets validated — and if your value is invalid or out-of-range, the JVM hits you with an appropriate error message. No silent chaos. We check. 💅

And it doesn't matter how those values got there — set ergonomically, typed on a command line, shoved in by an input tool, or passed through the APIs (for example, classes contained in the package java.lang.management) — the values provided to all Java Virtual Machine (JVM) command-line flags are validated. Ergonomics are described in Java Platform, Standard Edition HotSpot Virtual Machine Garbage Collection Tuning Guide. iykyk 📚

Range and constraints are validated either when all flags have their values set during JVM initialization, or a flag's value is changed during runtime (for example using the jcmd tool). And if a value violates either the range or constraint check? The JVM is terminated, and an appropriate error message is printed on the error stream. Not dramatic of us at all. 💀

For example, if a flag violates a range or a constraint check, then the JVM exits with an error:

java -XX:AllocatePrefetchStyle=5 -version
intx AllocatePrefetchStyle=5 is outside the allowed range [ 0 ... 3 ]
Improperly specified VM option 'AllocatePrefetchStyle=5'
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

The flag -XX:+PrintFlagsRanges prints the range of all the flags. This flag allows automatic testing of the flags by the values provided by the ranges. For the flags that have the ranges specified, the type, name, and the actual range is printed in the output.

For example,

intx   ThreadStackSize [ 0 ... 9007199254740987 ] {pd product}

For the flags that don't have the range specified, the values aren't displayed in the print out. For example:

size_t NewSize         [   ...                  ] {product}

This helps to identify the flags that need to be implemented. The automatic testing framework can skip those flags that don't have values and aren't implemented. Efficiency queen behavior. 👑

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🦋 LARGE PAGES 🦋

So you use large pages, also known as huge pages, as memory pages that are significantly larger than the standard memory page size (which varies depending on the processor and operating system). Large pages optimize processor Translation-Lookaside Buffers. Fancy, right? ✨

A Translation-Lookaside Buffer (TLB) is a page translation cache that holds the most-recently used virtual-to-physical address translations. A TLB is a scarce system resource. A TLB miss can be costly because the processor must then read from the hierarchical page table, which may require multiple memory accesses. By using a larger memory page size, a single TLB entry can represent a larger memory range. This results in less pressure on a TLB, and memory-intensive applications may have better performance. Slay, less pressure. 🫶

⚠️ But listen — using large pages can negatively affect system performance. For example, when a large amount of memory is pinned by an application, it may create a shortage of regular memory and cause excessive paging in other applications and slow down the entire system. Also, a system that has been up for a long time could produce excessive fragmentation, which could make it impossible to reserve enough large page memory. When this happens, either the OS or JVM reverts to using regular pages. Not the vibe, but good to know. 🙈

Linux and Windows support large pages. Everyone else, sorry bestie. 😭

🌸 Large Pages Support for Linux 🌸

Linux supports large pages since version 2.6. To check if your environment supports large pages, try the following:

# cat /proc/meminfo | grep Huge
HugePages_Total: 0
HugePages_Free: 0
...
Hugepagesize: 2048 kB

If the output contains items prefixed with "Huge", then your system supports large pages. The values may vary depending on environment. The Hugepagesize field shows the default large page size in your environment, and the other fields show details for large pages of this size. Newer kernels have support for multiple large page sizes. To list the supported page sizes, run this:

# ls /sys/kernel/mm/hugepages/
hugepages-1048576kB  hugepages-2048kB

The above environment supports 2 MB and 1 GB large pages, but they need to be configured so that the JVM can use them. When using large pages and not enabling transparent huge pages (option -XX:+UseTransparentHugePages), the number of large pages must be pre-allocated. For example, to enable 8 GB of memory to be backed by 2 MB large pages, login as root and run:

# echo 4096 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

It is always recommended to check the value of nr_hugepages after the request to make sure the kernel was able to allocate the requested number of large pages. Trust but verify, bestie. 💜

📌 Note: The values contained in /proc and /sys reset after you reboot your system, so may want to set them in an initialization script (for example, rc.local or sysctl.conf).

If you configure the OS kernel parameters to enable use of large pages, the Java processes may allocate large pages for the Java heap as well as other internal areas, for example:

• Code cache
• Marking bitmaps

Consequently, if you configure the nr_hugepages parameter to the size of the Java heap, then the JVM can still fail to allocate the heap using large pages because other areas such as the code cache might already have used some of the configured large pages. Sneaky lil code cache. 😤

🎀 Large Pages Support for Windows 🎀

To use large pages support on Windows, the administrator must first assign additional privileges to the user who is running the application:

1. Select Control Panel, Administrative Tools, and then Local Security Policy.
2. Select Local Policies and then User Rights Assignment.
3. Double-click Lock pages in memory, then add users and/or groups.
4. Reboot your system.

Note that these steps are required even if it's the administrator who's running the application, because administrators by default don't have the privilege to lock pages in memory. Yeah. Even admin. Wild, I know. 💀

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🌈 APPLICATION CLASS DATA SHARING 🌈

Application Class Data Sharing (AppCDS) stores classes used by your applications in an archive file. Since these classes are stored in a format that can be loaded very quickly (compared to classes stored in a JAR file), AppCDS can improve the start-up time of your applications. In addition, AppCDS can reduce the runtime memory footprint by sharing parts of these classes across multiple processes. Faster startup AND less memory? Obsessed. 🔥

⚠️ Real talk though: classes in the CDS archive are stored in an optimized format that's about 2 to 5 times larger than classes stored in JAR files or the JDK runtime image. Therefore, it's a good idea to archive only those classes that are actually used by your application. These usually are just a small portion of all available classes. For example, your application may use only a few APIs provided by a large library. Don't archive the whole world, bestie. 🍓

🧚 Using CDS Archives 🧚

By default, in most JDK distributions, unless -Xshare:off is specified, the JVM starts up with a default CDS archive, which is usually located in JAVA_HOME/lib/server/classes.jsa (or JAVA_HOME\bin\server\classes.jsa on Windows). This archive contains about 1300 core library classes that are used by most applications. Free real estate, literally. ✨

To use CDS for the exact set of classes used by your application, you can use the -XX:SharedArchiveFile option, which has the general form:

-XX:SharedArchiveFile=<static_archive>:<dynamic_archive>

• The <static_archive> overrides the default CDS archive.
• The <dynamic_archive> provides additional classes that can be loaded on top of those in the <static_archive>.
• On Windows, the above path delimiter : should be replaced with ;

The names "static" and "dynamic" are used for historical reasons. The dynamic archive, while still useful, supports fewer optimizations than available for the static CDS archive. If the full set of CDS/AOT optimizations are desired, consider using the AOT cache described below.

The JVM can use up to two archives. To use only a single <static_archive>, you can omit the <dynamic_archive> portion:

-XX:SharedArchiveFile=<static_archive>

For convenience, the <dynamic_archive> records the location of the <static_archive>. Therefore, you can omit the <static_archive> by saying only:

-XX:SharedArchiveFile=<dynamic_archive>

Look at that, it remembers for you. Main character energy. 👑

💅 Manually Creating CDS Archives 💅

CDS archives can be created manually using several methods:

• -Xshare:dump
• -XX:ArchiveClassesAtExit
• jcmd VM.cds

One common operation in all these methods is a "trial run", where you run the application once to determine the classes that should be stored in the archive. Little recon mission first. 🕵️

⭐ Creating a Static CDS Archive File with -Xshare:dump ⭐

The following steps create a static CDS archive file that contains all the classes used by the test.Hello application.

1. Create a list of all classes used by the test.Hello application. The following command creates a file named hello.classlist that contains a list of all classes used by this application:

alt-java -Xshare:off -XX:DumpLoadedClassList=hello.classlist -cp hello.jar test.Hello

The classpath specified by the -cp parameter must contain only JAR files. No loose class dirs, bestie. 🚫

2. Create a static archive, named hello.jsa, that contains all the classes in hello.classlist:

alt-java -Xshare:dump -XX:SharedArchiveFile=hello.jsa -XX:SharedClassListFile=hello.classlist -cp hello.jar

3. Run the application test.Hello with the archive hello.jsa:

alt-java -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello

4. Optional — Verify that the test.Hello application is using the class contained in the hello.jsa shared archive:

alt-java -XX:SharedArchiveFile=hello.jsa -cp hello.jar -Xlog:class+load test.Hello

The output of this command should contain the following text:

[info][class,load] test.Hello source: shared objects file

By default, when the -Xshare:dump option is used, the JVM runs in interpreter-only mode (as if the -Xint option were specified). This is required for generating deterministic output in the shared archive file. I.e., the exact same archive will be generated, bit-for-bit, every time you dump it. However, if deterministic output is not needed, and you have a large classlist, you can explicitly add -Xmixed to the command-line to enable the JIT compiler. This will speed up the archive creation. Bit-for-bit identical every time, that's commitment. 🎀

🌸 Creating a Dynamic CDS Archive File with -XX:ArchiveClassesAtExit 🌸

Advantages of dynamic CDS archives are:

• They usually use less disk space, since they don't need to store the classes that are already in the static archive.
• 🎀 They are created with one fewer step than the comparable static archive.

Okay bestie, here’s the little recipe for a dynamic CDS archive file that contains the classes used by the test.Hello application, excluding the ones already chilling in the default CDS archive. ✨

1. Create a dynamic CDS archive named hello.jsa that contains all the classes in hello.jar loaded by the application test.Hello:

alt-java -XX:ArchiveClassesAtExit=hello.jsa -cp hello.jar Hello

2. Run the application test.Hello with the shared archive hello.jsa:

alt-java -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello

3. Optional Repeat step 4 of the previous section to verify that the test.Hello application is using the class contained in the hello.jsa shared archive.

It’s also possible to create a dynamic CDS archive with a non-default static CDS archive. E.g., 👇

alt-java -XX:SharedArchiveFile=base.jsa -XX:ArchiveClassesAtExit=hello.jsa -cp hello.jar Hello

To run the application using this dynamic CDS archive:

alt-java -XX:SharedArchiveFile=base.jsa:hello.jsa -cp hello.jar Hello

On Windows, the above path delimiter : should be replaced with ; 🪟

As mentioned above, the name of the static archive can be skipped:

alt-java -XX:SharedArchiveFile=hello.jsa -cp hello.jar Hello

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🧰 Creating CDS Archive Files with jcmd 🧰

The previous two sections require you to modify the application’s start-up script in order to create a CDS archive. Sometimes this could be difficult, for example, if the application’s class path is set up by complex routines. 😭

The jcmd VM.cds command provides a less intrusive way for creating a CDS archive by connecting to a running JVM process. You can create either a static:

jcmd <pid> VM.cds static_dump my_static_archive.jsa

or a dynamic archive:

jcmd <pid> VM.cds dynamic_dump my_dynamic_archive.jsa

To use the resulting archive file in a subsequent run of the application without modifying the application’s start-up script, you can use the following technique:

env JAVA_TOOL_OPTIONS=-XX:SharedArchiveFile=my_static_archive.jsa bash app_start.sh

Note: to use jcmd <pid> VM.cds dynamic_dump, the JVM process identified by <pid> must be started with -XX:+RecordDynamicDumpInfo, which can also be passed to the application start-up script with the same technique:

env JAVA_TOOL_OPTIONS=-XX:+RecordDynamicDumpInfo bash app_start.sh

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✨ Creating Dynamic CDS Archive File with -XX:+AutoCreateSharedArchive ✨

-XX:+AutoCreateSharedArchive is a more convenient way of creating/using CDS archives. Unlike the methods of manual CDS archive creation described in the previous section, with -XX:+AutoCreateSharedArchive, it’s no longer necessary to have a separate trial run. Instead, you can always run the application with the same command-line and enjoy the benefits of CDS automatically. 💅

alt-java -XX:+AutoCreateSharedArchive -XX:SharedArchiveFile=hello.jsa -cp hello.jar Hello

If the specified archive file exists and was created by the same version of the JDK, then it will be loaded as a dynamic archive; otherwise it is ignored at VM startup.

At VM exit, if the specified archive file does not exist, it will be created. If it exists but was created with a different (but post JDK 19) version of the JDK, then it will be replaced. In both cases the archive will be ready to be loaded the next time the JVM is launched with the same command line. 🔁

If the specified archive file exists but was created by a JDK version prior to JDK 19, then it will be ignored: neither loaded at startup, nor replaced at exit. 💀

Developers should note that the contents of the CDS archive file are specific to each build of the JDK. Therefore, if you switch to a different JDK build, -XX:+AutoCreateSharedArchive will automatically recreate the archive to match the JDK. If you intend to use this feature with an existing archive, you should make sure that the archive is created by at least version 19 of the JDK. 🫶

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🚧 Restrictions on Class Path and Module Path 🚧

• Neither the class path (-classpath and -Xbootclasspath/a) nor the module path (--module-path) can contain non-empty directories.
• Only modular JAR files are supported in --module-path. Exploded modules are not supported.
• The class path used at archive creation time must be the same as (or a prefix of) the class path used at run time. (There’s no such requirement for the module path.)
• The CDS archive cannot be loaded if any JAR files in the class path or module path are modified after the archive is generated.

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🧩 Module related options 🧩

The following module related options are supported by CDS: --module-path, --module, --add-modules, and --enable-native-access.

The values for these options (if specified), should be identical when creating and using the CDS archive. Otherwise, if there is a mismatch of any of these options, the CDS archive may be partially or completely disabled, leading to lower performance. 😬

• If the AOTClassLinking option (see below) was enabled during CDS archive creation, the CDS archive cannot be used, and the following error message is printed:

CDS archive has aot-linked classes. It cannot be used when archived full module graph is not used

• If the AOTClassLinking option was not enabled during CDS archive creation, the CDS archive can be used, but the "archived module graph" feature will be disabled. This can lead to increased start-up time.

To diagnose problems with the AOT options, you can add -Xlog:aot to the application’s VM arguments. For example, if --add-modules jdk.jconsole was specified during archive creation and --add-modules jdk.incubator.vector is specified during runtime, the following messages will be logged:

Mismatched values for property jdk.module.addmods

runtime jdk.incubator.vector dump time jdk.jconsole

subgraph jdk.internal.module.ArchivedBootLayer cannot be used because full module graph is disabled

If any of the VM options --upgrade-module-path, --patch-module or --limit-modules are specified, CDS is disabled. This means that the JVM will execute without loading any CDS archives. In addition, if you try to create a CDS archive with any of these 3 options specified, the JVM will report an error. 🚨

🦋 AHEAD-OF-TIME CACHE 🦋

Okay bestie, so the JDK does this thing called ahead-of-time (AOT) optimizations that happen before your app even runs. Iconic behavior honestly. One example is Class Data Sharing (CDS), which we literally just talked about above — it parses classes ahead of time. And AOT optimizations can improve the start-up and warm-up performance of Java applications. That's the whole slay. 🔥

The Ahead-of-Time Cache (AOT cache) is a container introduced in JDK 24 for storing artifacts produced by AOT optimizations. The AOT cache currently contains Java classes and heap objects. In future JDK releases, the AOT cache may contain additional artifacts, such as execution profiles and compiled methods. Manifesting more stuff later, we love to see it. ✨

An AOT cache is specific to a combination of the following:

• A particular application (as expressed by -classpath, -jar, or --module-path.)
• A particular JDK release.
• A particular OS and CPU architecture.

If any of the above changes, you must recreate the AOT cache. No cap, it is not flexible about this. 💅

The deployment of the AOT cache is divided into three phases:

• Training: We execute the application with a representative work-load to gather statistical data that tell us what artifacts should be included into the AOT cache. The data are saved in an AOT Configuration file.
• Assembly: We use the AOT Configuration file to produce an AOT cache.
• Production: We execute the application with the AOT cache for better start-up and warm-up performance.

It's giving glow-up pipeline. 🎀

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The AOT cache can be used with the following command-line options:

• -XX:AOTCache= cachefile — Specifies the location of the AOT cache. The standard extension for cachefile is .aot. This option cannot be used together with -XX:AOTCacheOutput. It's compatible with AOTMode settings of on, create, or auto (the default). The cachefile is read in AOT modes on and auto, and is ignored by mode off. The cachefile is written by AOT mode create. In that case, this option is equivalent to -XX:AOTCacheOutput=cachefile. 🫶
• -XX:AOTCacheOutput= cachefile — Specifies the location of the AOT cache to write. The standard extension for cachefile is .aot. This option cannot be used together with -XX:AOTCache. It's compatible with AOTMode settings of record, create, or auto (the default). ⭐
• -XX:AOTConfiguration= configfile — Specifies the AOT Configuration file for the JVM to write to or read from. The standard extension for configfile is .aotconfig. This option is compatible with AOTMode settings of record, create, or auto (the default). The configfile is read by AOT mode create, and written by the other applicable modes. If the AOT mode is auto, then AOTCacheOutput must also be present. 💖
• -XX:AOTMode= mode — Specifies the AOT Mode for this run. mode must be one of the following: auto, off, record, create, on, or required.

Heads up bestie: on is an alias for required. All discussion below about required also applies to on. The use of on is discouraged. The support of on may be deprecated and removed in future releases. 👀

• auto: This AOT mode is the default, and takes effect if no -XX:AOTMode option is present. It automatically sets the AOT mode to record, required, or off, as follows:
  • If -XX:AOTCacheOutput=cachefile is specified, the AOT mode is changed to record (a training run, with a subsequent create operation).
  • Otherwise, if an AOT cache can be loaded, the AOT mode is changed to required (a production run).
  • Otherwise, the AOT mode is changed to off (a production run with no AOT cache).
• off: No AOT cache is used. Other AOT command line options are ignored. (Ghost mode. 💀)
• record: Execute the application in the training phase. At least one of -XX:AOTConfiguration=configfile and/or -XX:AOTCacheOutput=cachefile must be specified. If -XX:AOTConfiguration=configfile is specified, the JVM gathers statistical data and stores them into configfile. If -XX:AOTConfiguration=configfile is not specified, the JVM uses a temporary file name, which may be the string AOTCacheOutput+".config", or else a fresh implementation-dependent temporary file name. If -XX:AOTCacheOutput=cachefile is specified, a second JVM process is launched to perform the Assembly phase to write the optimization artifacts into cachefile.
  • Extra JVM options can be passed to the second JVM process using the environment variable JDK_AOT_VM_OPTIONS, with the same format as the environment variable JAVA_TOOL_OPTIONS, which is defined by JVMTI https://docs.oracle.com/en/java/javase/24/docs/specs/jvmti.html#tooloptions. 🧚
• create: Perform the Assembly phase. -XX:AOTConfiguration=configfile must be specified. The JVM reads history and statistics from configfile and writes the optimization artifacts into cachefile. Note that the application itself is not executed in this phase. 😴
• required: Execute the application in the Production phase. -XX:AOTCache=cachefile must be specified. The JVM tries to load cachefile as the AOT cache.

The loading of an AOT cache can fail for a number of reasons:

• You are trying to use the AOT cache with an incompatible application, JDK release, or OS/CPU.
• The specified cachefile does not exist or is not accessible.
• Incompatible JVM options are used (for example, certain JVMTI options).

Since the AOT cache is an optimization feature, there's no guarantee that it will be compatible with all possible JVM options. See JEP 483 https://openjdk.org/jeps/483, section Consistency of training and subsequent runs for a representative list of scenarios that may be incompatible with the AOT cache.

These scenarios usually involve arbitrary modification of classes for diagnostic purposes and are typically not relevant for production environments.

When the AOT cache fails to load:

• If AOTMode was originally auto, the JVM will continue execution without using the AOT cache. This is the recommended mode for production environments, especially when you may not have complete control of the command-line (e.g., your application's launch script may allow users to inject options to the command-line). This allows your application to function correctly, although sometimes it may not benefit from the AOT cache. 🫶
• If AOTMode was originally required, the JVM will print an error message and exit immediately. This mode should be used only as a "fail-fast" debugging aid to check if your command-line options are compatible with the AOT cache. An alternative is to run your application with -XX:AOTMode=auto -Xlog:aot to see if the AOT cache can be used or not.

In production environments, -XX:AOTMode=required should be used with care. Your application may fail to launch if an incompatible VM option is added without your knowledge. For example, a cloud provider might run your JVM with a JVMTI class-file load hook for monitoring purposes. 😭 Not the surprise plot twist you want.

• -XX:+AOTClassLinking — If this option is enabled, the JVM will perform more advanced optimizations (such as ahead-of-time resolution of invokedynamic instructions) when creating the AOT cache. As a result, the application will see further improvements in start-up and warm-up performance. However, an AOT cache created with this option cannot be used when certain command-line parameters are specified in the Production phase. Please see JEP 483 https://openjdk.org/jeps/483 for a detailed discussion of -XX:+AOTClassLinking and its restrictions. 👑

When -XX:AOTMode is used in the command-line, AOTClassLinking is automatically enabled. To disable it, you must explicitly pass the -XX:-AOTClassLinking option.

When -XX:AOTMode is not used in the command-line, AOTClassLinking is disabled by default to provide full compatibility with traditional CDS options such as -Xshare:dump.

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The first occurrence of the special sequence %p in configfile and cachefile is replaced with the process ID of the JVM process launched in the command-line, and likewise the first occurrence of %t is replaced by the JVM's startup timestamp. (After replacement there must be no further occurrences of %p or %t, to prevent problems with sub-processes.) For example:

alt-java -XX:AOTConfiguration=foo%p.aotconfig -XX:AOTCacheOutput=foo%p.aot -cp foo.jar Foo

will create two files: foopid123.aotconfig and foopid123.aot, where 123 is the process ID of the JVM that has executed the application Foo. Cute naming scheme fr fr. 🍓

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🌈 PERFORMANCE TUNING EXAMPLES 🌈

You can use the Java advanced runtime options to optimize the performance of your applications. Let's go, we're tuning. ✨

Tuning for Higher Throughput

Use the following commands and advanced options to achieve higher throughput performance for your application:

alt-java -server -XX:+UseParallelGC -XX:+UseLargePages -Xmn10g  -Xms26g -Xmx26g

Tuning for Lower Response Time

Use the following commands and advanced options to achieve lower response times for your application:

alt-java -XX:+UseG1GC -XX:MaxGCPauseMillis=100

Keeping the Java Heap Small and Reducing the Dynamic Footprint of Embedded Applications

Use the following advanced runtime options to keep the Java heap small and reduce the dynamic footprint of embedded applications:

-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5

Note: The defaults for these two options are 70% and 40% respectively. Because performance sacrifices can occur when using these small settings, you should optimize for a small footprint by reducing these settings as much as possible without introducing unacceptable performance degradation. Tiny heap, big brain energy — but don't tank your perf, okay? 💅

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👑 EXIT STATUS 👑

The following exit values are typically returned by the launcher when the launcher is called with the wrong arguments, serious errors, or exceptions thrown by the JVM. However, a Java application may choose to return any value by using the API call System.exit(exitValue). The values are:

• 0: Successful completion 🎉
• > 0: An error occurred 💀

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And that's a wrap on the whole page, bestie!! This one's from JDK 27-ea, dated 2026, and it lives at ALT-JAVA(1) in the manual — go forth and alt-java responsibly, mwah 🫶💖🎀