JAVA(1)

🌸 NAME 🌸

java — launch a Java application 💅 (yep bestie, this is the JAVA(1) page, straight out of JDK Commands — it's a long one, grab a snack 🍓)

✨ SYNOPSIS ✨

To launch a class file, the classic:

java [options] mainclass [args ...]

To launch the main class in a JAR file:

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

To launch the main class in a module:

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

or, same energy, different spelling:

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

To launch a source-file program:

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

• options — optional, and yes they're separated by spaces. Pop over to Overview of Java Options for the full rundown of what's available 👀
• mainclass — the name of the class you're launching. The command-line entries that come after classname are the arguments for the main method, no cap.
• -jar jarfile — runs a program that's all wrapped up inside a JAR file. The jarfile argument is the name of a JAR file whose manifest has a line like Main-Class:classname in it, defining the class with the public static void main(String[] args) method that's your app's whole starting point 🌟 When you use -jar, that JAR file is the source of all user classes and every other class path setting just gets ignored, rip. If you're doing JAR file things, then see jar.
• -m or --module module[/mainclass] — runs 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 one, or to override the value when it does. So flexible 💫 See Standard Options for Java.
• source-file — only used for launching a source-file program. It 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! Whatever follows mainclass, source-file, -jar jarfile, and -m or --module module/mainclass gets passed as arguments to the main class. 🫶

🌈 DESCRIPTION 🌈

The java command starts a Java application. It does this by starting the Java Virtual Machine (JVM), loading the class you specified, and calling that class's main() method. That method has to be declared public and static, it must not return any value, and it must accept a String array as a parameter. Literally the whole personality of the method declaration is this:

public static void main(String[] args)

In source-file mode, java can launch a class declared in a source file — see Using Source-File Mode to Launch Source-Code Programs for the full description of that mode ✨

📌 Note: you can use the JDK_JAVA_OPTIONS launcher environment variable to prepend its content to the actual command line of the java launcher. See Using the JDK_JAVA_OPTIONS Launcher Environment Variable.

By default, the first argument that isn't an option of the java command is the fully qualified name of the class to be called. 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, that's non-negotiable 🎀

Arguments after the class file name or the JAR file name get passed to the main() method. Simple as that.

🪟 javaw

Windows: the javaw command is identical to java, except that with javaw there's no associated console window. Use javaw when you don't want a command prompt window popping up like "hi 🙈". The javaw launcher will, however, display a dialog box with error information if a launch fails — so you're never just left guessing, icons only 💀

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

To launch a class declared in a source file, run the java launcher in source-file mode. Whether you're in source-file mode is decided by two items on the 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 it's 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, slay. The --source option can be used to specify the source version, or N, of the source code — that determines which API you can use. 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, old values get removed, it's giving seasonal rotation. You'll get an error message if you use a value of N that's no longer supported. The supported values of N are the current Java SE release (22) and a limited number of previous releases, detailed in the command-line help for 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 java command to use source-file mode. The --source option is the moment for 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 🎀

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:

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 — nope, not a thing, don't try it 🙈
• 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. Cute, honestly 🎀

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. Sorry to the annotation processors 💀
• 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 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, that a type in a named package should exist in a file whose name is composed from the type name followed by the .java extension.
• 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. 😭

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 — no main, no vibes.
• 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 👀
• 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-module 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. Name collisions? Not on our watch 🚫

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 bestie, listen — JDK_JAVA_OPTIONS is that girl who always shows up first and prepends her whole vibe to the options parsed from the command line. No cap. ✨

The content of the JDK_JAVA_OPTIONS environment variable is a list of arguments separated by white-space characters (as determined by isspace()). Those get prepended to the command line arguments passed to the java launcher. The encoding requirement for the environment variable is the same as the java command line on the system. And JDK_JAVA_OPTIONS environment variable content is treated in the same manner as that specified in the command line. Literally identical energy. 💅

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. But — and this is important — in case a matching quote is not found, the launcher will abort with an error message. 😭 @-files are supported as they are specified in the command line. However, as in @-files, use of a wildcard is not supported. 🚫

In order to mitigate potential misuse of JDK_JAVA_OPTIONS behavior, options that specify the main class (such as -jar) or cause the 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. 🫶

Example:

$ export JDK_JAVA_OPTIONS='-g @file1 -Dprop=value @file2 -Dws.prop="white spaces"'
$ 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 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. 🎀
• 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 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). ➕➖

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. 🙈
• Obsolete Java Options: Accepted but ignored --- a warning is issued when they're used. 💀
• Removed Java Options: Removed --- using them results in an error. ❌

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👑 STANDARD OPTIONS FOR JAVA 👑

These are the most commonly used options supported by all implementations of the JVM. The main character options, fr fr. 🌟

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. 🛣️
• --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"). 🌀

• --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. ✋
• --enable-preview — Allows classes to depend on preview features [https://docs.oracle.com/en/java/javase/12/language/index.html#JSLAN-GUID-5A82FE0E-0CA4-4F1F-B075-564874FE2823] of the release. 👀
• --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. Methods that provide native access are restricted, and by default their use causes warnings. This option allows code in the specified modules to use restricted methods without warnings. module can be ALL-UNNAMED to indicate code on the class path. When this option is present, any use of restricted methods by code outside the specified modules causes an IllegalCallerException. ⚠️
• --finalization=value — Controls whether the JVM performs finalization of objects. Valid values are "enabled" and "disabled". Finalization is enabled by default, so the value "enabled" does nothing. The value "disabled" disables finalization, so that no finalizers are invoked. 🗑️
• --module-path modulepath... or -p modulepath — Specifies where to find application modules with a list of path elements. The elements of a module path 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. 📦

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

• --upgrade-module-path modulepath... — Specifies where to find module replacements of upgradeable modules in the runtime image with a list of path elements. The elements of a module path 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. ⬆️

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

• --add-modules module[,module...] — Specifies the root modules to resolve in addition to the initial module. module can also be ALL-DEFAULT, ALL-SYSTEM, and ALL-MODULE-PATH. ➕
• --list-modules — Lists the observable modules and then exits. 📋
• -d module_name or --describe-module module_name — Describes a specified module and then exits. 🔍
• --dry-run — Creates the VM but doesn't execute the main method. This --dry-run option might be useful for validating the command-line options such as the module system configuration. 🧪
• --validate-modules — validates every module and then dips, no cap. It's the vibe-check option: perfect for sniffing out conflicts and other messy errors in the modules chilling on your module path. 🕵️‍♀️
• -Dproperty=value — sets a system property. The property bit is a string (no spaces, bestie, it's not that kind of party) that names the property, and value is a string holding its value. If your value has spaces in it, wrap it in quotation marks, like -Dfoo="foo bar". 💅
• -disableassertions[:[packagename]...|:classname] or -da[:[packagename]...|:classname] — turns assertions off. And by default? Assertions are already disabled in all packages and classes, so this is you being extra sure. With no arguments at all, -disableassertions (-da) disables assertions in all packages and classes. If the packagename argument ends in ..., the switch disables assertions in that package and any subpackages. If the argument is literally just ..., then it disables assertions in the unnamed package in the current working directory. And with the classname argument, it disables assertions in that specified class. 🚫

Okay so the -disableassertions (-da) option applies to all class loaders and to system classes (which don't even have a class loader, main character energy). There's exactly one exception to this rule: if you give the option with no arguments, then it does not apply to system classes. Which honestly is a feature — it makes it easy to disable assertions in all classes except the system ones. The -disablesystemassertions option is there so you can disable 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, they don't fight. 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, use the following command:

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

• -disablesystemassertions or -dsa — disables assertions in all system classes. That's it, that's the whole thing. 🎀
• -enableassertions[:[packagename]...|:classname] or -ea[:[packagename]...|:classname] — turns assertions on. (Reminder bestie: by default assertions are disabled in all packages and classes, so this is you opting in.) With no arguments, -enableassertions (-ea) enables assertions in all packages and classes. With a packagename argument ending in ..., the switch enables assertions in the specified package and any subpackages. If the argument is simply ..., then the switch enables assertions in the unnamed package in the current working directory. With the classname argument, the switch enables assertions in the specified class. ✅

The -enableassertions (-ea) option applies to all class loaders and to system classes (which don't have a class loader). One exception to this rule: if the option is provided with no arguments, then it doesn't apply to system classes. Which makes it easy to enable assertions in all classes except for system classes. The -enablesystemassertions option provides a separate switch to enable assertions in all system classes. To explicitly disable assertions in specific packages or classes, use the -disableassertions (-da) option. And if a single command contains multiple instances of these switches, then they're processed in order, before loading any classes. 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 the following command:

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

• -enablesystemassertions or -esa — enables assertions in all system classes. Iconic, tiny, does its job. ✨
• -help, -h, or -? — prints the help message to the error stream. Yes, the error stream, that's not a typo, that's just how it is. 😭
• --help — prints the help message to the output stream. The classy one. 🎀
• -javaagent:jarpath[=options] — loads the specified Java programming language agent. See java.lang.instrument for the lore. 🧚
• --show-version — prints the product version to the output stream and then continues on with whatever you asked for. 💖
• -showversion — prints the product version to the error stream and continues. Same energy, different stream. 💀
• --show-module-resolution — shows module resolution output during startup. Peek behind the curtain, bestie. 🦋
• -splash:imagepath — shows the splash screen with the image specified by imagepath. HiDPI scaled images are automatically supported and used if they're available. The unscaled image file name, such as image.ext, should always be passed as the argument to the -splash option — and then 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 application, use the following option:

-splash:images/splash.gif

See the SplashScreen API documentation for more information.

• -verbose:class — displays information about each loaded class. Every single one. 👑
• -verbose:gc — displays information about each garbage collection (GC) event. Watch the GC do its lil cleanup in real time. 🗑️
• -verbose:jni — displays information about the use of native methods and other Java Native Interface (JNI) activity. 🌈
• -verbose:module — displays information about the modules in use. 💜
• --version — prints product version to the output stream and exits. Bye! 👋
• -version — prints product version to the error stream and exits. 👋
• -X — prints the help on extra options to the error stream. A little sneak peek at the extras. 🙈
• --help-extra — prints the help on extra options to the output stream. 🎀
• @argfile — specifies one or more argument files prefixed by @ used by the java command. Real talk: the java command line gets long sometimes because of all the .jar files you need in the classpath, and the @argfile option overcomes those command-line length limitations by letting the launcher expand the contents of argument files after shell expansion, but before argument processing. The contents in the argument files get expanded, because otherwise they'd be specified on the command line until the --disable-@files option was encountered. 🤯

The argument files can also contain the main class name and all the options. So if an argument file contains all of the options required by the java command, then the command line could simply be:

java @argfile

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

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🔥 EXTRA OPTIONS FOR JAVA 🔥

The following java options are general purpose options that are specific to the Java HotSpot Virtual Machine. These are the "okay but why" options and honestly, we love them for it. 💅

• -Xbatch — disables background compilation. By default, the JVM compiles the method as a background task, running the method in interpreter mode until the background compilation is finished. The -Xbatch flag disables background compilation so that compilation of all methods proceeds as a foreground task until completed. This option is equivalent to -XX:-BackgroundCompilation. ⚡
• -Xbootclasspath/a:directories|zip|JAR-files — specifies 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 — performs additional checks for Java Native Interface (JNI) functions. 🕵️‍♀️

The following checks are considered indicative of significant problems with the native code, and the JVM terminates with an irrecoverable error in such cases — like, it's over, no saving it:

• 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 — annoying lil notes, not fatal:

• 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

⚠️ Expect a performance degradation when this option is used. You're paying for those checks in speed, bestie, no cap.

• -Xcomp — testing mode to exercise JIT compilers. This option should not be used in production environments. Fr fr, do not ship this. 🚨
• -Xdebug — does nothing; deprecated for removal in a future release. It's giving ghost of options past. 👻
• -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. Slow and steady, but mostly slow. 🐢
• -Xinternalversion — displays more detailed JVM version information than the -version option, and then exits. The lore dump version. 📜
• -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. 🔥
• -Xmn size — sets the initial and maximum size (in bytes) of the heap for the young generation (the nursery, aka where all the baby objects hang out 💕) in the generational collectors. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. This young-gen region is literally where new objects spawn, and GC runs here way more often than in other regions. So: too small → a million minor garbage collections, death by a thousand cuts 😭. Too large → only full garbage collections, which take forever to finish, no cap. Recommendation era: do not set the young-gen size for the G1 collector, and for other collectors keep it greater than 25% and less than 50% of the overall heap size. Here's how you'd set initial + max young gen to 256 MB in different units:

-Xmn256m
-Xmn262144k
-Xmn268435456

And instead of -Xmn (which does both initial and max), you can split it: -XX:NewSize for the initial size, -XX:MaxNewSize for the maximum. slay, options 💅

• -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 k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. Setting allocated memory to 6 MB, three ways:

-Xms6291456
-Xms6144k
-Xms6m

If you don't set this, the initial size is set as the sum of the sizes allocated for the old generation and the young generation. (The young-gen initial heap size can be set with the -Xmn option or the -XX:NewSize option.) Also note: -XX:InitialHeapSize can also set the initial heap size — and if it shows up after -Xms on the command line, the initial heap size gets set to whatever -XX:InitialHeapSize says. last one wins, bestie 🫶

• -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 k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. The default is chosen at runtime based on system configuration (so it's giving mystery ✨). For server deployments, -Xms and -Xmx are often set to the same value. Setting the max allowed size to 80 MB:

-Xmx83886080
-Xmx81920k
-Xmx80m

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

• -Xnoclassgc — disables garbage collection (GC) of classes. This can save some GC time, which shortens interruptions while your app is running — cute! When you specify -Xnoclassgc at startup, the class objects in the application are left untouched during GC and are always considered live. ⚠️ Downside: this can result in more memory being permanently occupied, which — if you're not careful — throws an out-of-memory exception. so like, use with intention 💀
• -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 (like 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. you're the main character now, act accordingly 👑

• *-Xshare:mode* — sets the class data sharing (CDS) mode. Here are the possible mode arguments, bestie:
  • auto — use shared class data if possible (default). the chill option ✨
  • 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. fr fr, don't 🙈
  • off — do not attempt to use shared class data.
• -XshowSettings — shows all settings and then continues. lowkey nosy but we respect it 💅
• *-XshowSettings:category* — shows settings and continues. Possible category arguments:
  • all — shows all categories of settings. This is the default value.
  • locale — shows settings related to locale.
  • properties — shows settings related to system properties.
  • 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 k or K for KB, m or M for MB, or g or G for GB. The actual size may be rounded up to a multiple of the system page size as required by the operating system (so it's giving "close enough" 🎀). The default value depends on the platform. Like:
  • Linux/x64: 1024 KB
  • Linux/Aarch64: 2048 KB
  • macOS/x64: 1024 KB
  • macOS/Aarch64: 2048 KB
  • Windows: the default value depends on virtual memory

Setting the thread stack size to 1024 KB, three ways:

-Xss1m
-Xss1024k
-Xss1048576

This option is similar to -XX:ThreadStackSize.

• --add-reads module=target-module(,target-module)* — updates *module* to read the *target-module*, regardless of the module declaration (we're overriding destiny here ✨). *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. opening doors, no key needed 🚪✨
• --limit-modules module[,module...] — specifies the limit of the universe of observable modules. the observable universe but make it modules 🌌
• --patch-module module=file(;file`)* — overrides or augments a module with classes and resources in JAR files or directories. patching it up like a cute little sewing kit 🧵
• --source version — sets the version of the source in source-file mode.

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🍎 EXTRA OPTIONS FOR MACOS 🍎

The following extra options are macOS specific. (yes diva, Apple-only 💅)

• -XstartOnFirstThread — runs the main() method on the first (AppKit) thread.
• *-Xdock:name=application_name* — overrides the default application name displayed in dock. rename your icon era 🎀
• *-Xdock:icon=path_to_icon_file* — overrides the default icon displayed in dock. custom icon slay 🦋

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🔓 ADVANCED OPTIONS FOR JAVA 🔓

These java options can be used to enable other advanced options. unlocking the secret menu, iykyk 👀

• -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 are enabled with the use of this option are not supported. If you encounter issues while using any of these options, it is very likely that you will be required to reproduce the problem without using any of these unsupported options before Oracle Support can assist with an investigation. It is also possible that any of these options may be removed or their behavior changed without any warning. so basically, you're on your own bestie 😭

• -XX:+UnlockExperimentalVMOptions — unlocks the options that provide experimental features in the JVM. By default, this option is disabled and experimental features aren't available. (experimental = we're all just vibing and hoping ✨🧪)

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🏃♀️ ADVANCED RUNTIME OPTIONS FOR JAVA 🏃♀️

These java options control the runtime behavior of the Java HotSpot VM. runtime behavior = how it moves, how it thinks 💅

• *-XX:ActiveProcessorCount=x* — overrides the number of CPUs that the VM will use to calculate the size of thread pools it will use for various operations such as Garbage Collection and ForkJoinPool. lying to the VM about how many CPUs you have, iconic honestly 🌈

okay so the VM normally determines the number of available processors from the operating system. This flag can be useful for partitioning CPU resources when you’re running multiple Java processes in docker containers. It’s honored even if UseContainerSupport is not enabled. See -XX:-UseContainerSupport for the whole enabling/disabling container-support situation. 💅

• -XX:AllocateHeapAt=path — Takes a path to the file system and uses memory mapping to allocate the object heap on the memory device. Using this option enables the HotSpot VM to allocate the Java object heap on an alternative memory device, such as 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 other memory structures (such as the code heap, metaspace, and thread stacks) continue to 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 provide a direct mapping of virtual memory to the physical memory on the device. The existing heap related flags (such as -Xmx and -Xms) and garbage-collection related flags continue to work as before. no cap.

• -XX:-CompactStrings — Disables the Compact Strings feature. By default, this option is enabled. When the Compact Strings feature is 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. This reduces, by 50%, the amount of space required for Strings containing only single-byte characters. For Java Strings containing at least one multibyte character: these are represented and stored as 2 bytes per character using UTF-16 encoding. Disabling the Compact Strings feature forces the use of UTF-16 encoding as the internal representation for all Java Strings. 😭💾

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

• When it’s known that an application overwhelmingly will be allocating multibyte character Strings
• In the unexpected event where a performance regression is observed in migrating from Java SE 8 to Java SE 9 and an analysis shows that Compact Strings introduces the regression

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

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

The following 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: The following example shows how to set the error log to /var/log/java/java_error.log:

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

• Windows: The following 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 will be overwritten. Otherwise, if the file can’t be created in the specified directory (due to insufficient space, permission problem, or another issue), then the file is 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. This option can be turned 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. 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.
• 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.
• 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. 🌸

The following example describes how to set the large page size to 1 gigabyte (GB):

-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:+NeverActAsServerClassMachine — turns on "Client VM emulation" mode ✨ which is a whole vibe: it only uses the C1 JIT compiler, a 32Mb CodeCache, and the Serial GC. The max memory the JVM can use (that's the thing controlled by the -XX:MaxRAM=n flag) gets set to 1GB by default. Oh, and the string "emulated-client" gets added to the JVM version string, so everyone knows. 💅

By default this flag is true only on Windows in 32-bit mode, and false everywhere else.

Important caveat bestie: the "Client VM emulation" mode will not be enabled if any of these flags show up on the command line:

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

• -XX:ObjectAlignmentInBytes=alignment — sets the memory alignment of Java objects, in bytes. Default is 8 bytes. The value you give should be a power of 2, and it must be within the range of 8 and 256 (inclusive). Why do you care? Because 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. So you might not actually realize any benefits from using compressed pointers with large Java heap sizes. 😭 It's giving tradeoffs.

• -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, it has to be enclosed in quotation marks.
  • Non-Windows: this example shows how -XX:OnError 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: this example shows how -XX:OnError 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 — same idea as above but for a different moment: sets a custom command, or a series of semicolon-separated commands, to run when an OutOfMemoryError exception is first thrown. If the string contains spaces, it must be enclosed in quotation marks. For an example of a command string, go peek at the description of the -XX:OnError option. 🫶
• -XX:+PrintCommandLineFlags — enables printing of ergonomically selected JVM flags that appeared on the command line. It's genuinely useful for knowing the ergonomic values set by the JVM, like the heap space size and the selected garbage collector. By default this option is disabled, and flags aren't printed. 🙈
• -XX:+PreserveFramePointer — picks between two lives: using the RBP register as a general purpose register (-XX:-PreserveFramePointer) or using the RBP register to hold the frame pointer of the currently executing method (-XX:+PreserveFramePointer). If the frame pointer is available, then 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 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. That 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 you can attach a debugger to it and 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. 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 isn't 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. ✨

• 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. 🦋

• settings=path — bestie, this one specifies the path and name of the event settings file (of type JFC). By default, the default.jfc file is used, and it lives 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 it 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 this 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 +.

You can specify values for multiple event settings and .jfc options by separating them with a comma. In case of a conflict between a parameter and a .jfc option, the parameter will take precedence. The whitespace delimiter can be omitted for timespan values, i.e. 20ms. For more information about the settings syntax, see Javadoc of the jdk.jfr package.

• -XX:ThreadStackSize=size — sets the Java thread stack size (in kilobytes). Use of a scaling suffix, such as k, results in the scaling of the kilobytes value so that -XX:ThreadStackSize=1k sets the Java thread stack size to 1024*1024 bytes or 1 megabyte. 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

The following examples show 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:-UseCompressedOops — disables the use of compressed pointers. By default, this option is enabled, and compressed pointers are used. This will automatically limit 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 allows the VM to determine the amount of memory and number of processors that are available to a Java process running in docker containers. It uses this information 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 to 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 — allows the user to specify VM options in a file, for example, 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 the following:

• 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)

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🔥 ADVANCED JIT COMPILER OPTIONS FOR JAVA 🔥

Okay bestie, these java options control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM. Compiler vibes: activated. 💅

• -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.

The following example shows how to set 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.

The following example shows 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. The following example shows how to set 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 the following:

-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 the following:

...and if you need to nail a specific method signature, you can pass the full descriptor too, like:

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

Bestie, the asterisk (*) is your wildcard bff here — you can use it for class names and method names. So say you want to boot every single indexOf() in every single class out of compilation town:

-XX:CompileCommand=exclude,*.indexOf

And here's the cute part 🎀 — commas and periods are literally just aliases for spaces, so passing compiler commands through a shell doesn't get all weird on you. Pass args to -XX:CompileCommand with spaces as separators by wrapping the whole thing in quotation marks:

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

Heads up though: after the JIT compiler parses all the commands you handed it on the command line via -XX:CompileCommand, it then reads commands from the .hotspot_compiler file. You can add commands to that file, or point it at a whole different file with the -XX:CompileCommandFile option. iykyk 💅

Wanna stack several commands? Easy — either repeat the -XX:CompileCommand option as many times as you want, or separate each argument with the new line separator (\n). Here's the full menu of commands, queen:

• break — sets a breakpoint when you're debugging the JVM, so it stops right at the beginning of compilation of the specified method.
• compileonly — excludes all the methods from compilation except for the specified one. As an alternative you can use the -XX:CompileOnly option, which lets you name several methods at once. Obsessed.
• dontinline — prevents inlining of the specified method. no thoughts, no inlining.
• exclude — excludes the specified method from compilation.
• help — prints a help message for the -XX:CompileCommand option. very demure, very mindful.
• inline — attempts to inline the specified method.
• log — excludes compilation logging (the kind you get with -XX:+LogCompilation) for all methods except the specified one. By default, logging happens for all compiled methods.
• option — passes a JIT compilation option to the specified method in place of the last argument (option). The compilation option gets set at the end, after the method name. For example, to enable the BlockLayoutByFrequency option for the append() method of the StringBuffer class:

-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. show me the receipts 🔥
• quiet — instructs the JVM not to print the compile commands. By default, the commands you specify with -XX:CompileCommand do get printed; so like, if you exclude the indexOf() method of the String class from compilation, 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. order matters, bestie.

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💜 -XX:CompileCommandFile=filename

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

Every line in the command file is a command, a class name, and a method name that the command applies to. 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 go peek at 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, the one that unlocks diagnostic JVM options. they're a package deal, fr fr.

⭐ -XX:+CompilerDirectivesPrint

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

Same deal: -XX:+CompilerDirectivesPrint 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) that compilation should be restricted to. Only the specified methods get compiled — everybody else, take a seat.

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

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

💅 -XX:CompileThresholdScaling=scale

Gives you unified control of first compilation ✨. This option controls when methods get first compiled for both the tiered and the nontiered modes of operation. CompileThresholdScaling has a floating point value between 0 and +Inf, and it scales the thresholds that correspond to the current mode of operation (both tiered and nontiered). Setting CompileThresholdScaling to a value less than 1.0 means earlier compilation, while values greater than 1.0 delay compilation. And setting CompileThresholdScaling to 0 is equivalent to disabling compilation entirely. one dial, all the vibes 🎛️

🎀 -XX:+DoEscapeAnalysis

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

💖 -XX:InitialCodeCacheSize=size

Sets the initial code cache size (in bytes). Append the letter k or K to mean kilobytes, m or M to mean megabytes, or g or G to mean gigabytes. The default value depends on the platform. Also, the initial code cache size shouldn't be less than the system's minimal memory page size. Like so — here's how you set it 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 k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. The default value depends on the platform and on whether tiered compilation is enabled. In this example it's 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 with the -XX:LogFile option.

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

Oh and — you can get verbose diagnostic output, with a message printed to the console every single 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 k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. The default value depends on the platform. Here it's 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 k or K for kilobytes, m or M for megabytes, or g or G for 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 k or K for kilobytes, m or M for megabytes, or g or G for 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 k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. By default the maximum bytecode size of a trivial method is set to 6 bytes:

-XX:MaxTrivialSize=6

🦋 -XX:C1MaxTrivialSize=size

Sets the maximum bytecode size (in bytes) of a trivial method to be inlined. This flag only applies to the C1 compiler. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. By default the maximum bytecode size of a trivial method is set to 6 bytes:

-XX:MaxTrivialSize=6

💅 -XX:MaxNodeLimit=nodes

Sets the maximum number of nodes to be used during single method compilation. By default the value depends on the features enabled. In this example the maximum number of nodes is set to 100,000:

-XX:MaxNodeLimit=100000

🎀 -XX:NonNMethodCodeHeapSize=size

Sets the size in bytes of the code segment containing nonmethod code.

A nonmethod code segment contains nonmethod code, like compiler buffers and the bytecode interpreter. This code type stays in the code cache forever 💀. And this flag is used only if -XX:SegmentedCodeCache is enabled. periodt.

• -XX:NonProfiledCodeHeapSize=size — sets the size, in bytes, of the code segment that holds nonprofiled methods. Heads up: this flag only does anything if -XX:SegmentedCodeCache is enabled, iykyk 💅
• -XX:+OptimizeStringConcat — turns on optimization of String concatenation operations. It's on by default ✨. Wanna turn it off? Then say -XX:-OptimizeStringConcat and the optimization is gone.
• -XX:+PrintAssembly — makes the JVM print assembly code for bytecoded and native methods, using the external hsdis-<arch>.so or .dll library. For 64-bit VM on Windows, that's hsdis-amd64.dll. This is the moment you get to see the generated code, which lowkey helps you diagnose performance issues 👑

By default this option is disabled and no assembly code gets printed. Also, -XX:+PrintAssembly has to be used together with -XX:UnlockDiagnosticVMOptions, the option that unlocks diagnostic JVM options. No unlocking, no assembly. Periodt.

• -XX:ProfiledCodeHeapSize=size — sets the size in bytes of the code segment holding profiled methods. Again: only used if -XX:SegmentedCodeCache is enabled 🎀
• -XX:+PrintCompilation — gives you verbose diagnostic output from the JVM, printing a message to the console every single time a method gets compiled. Perfect for seeing which methods actually get compiled 👀. By default it's off and no diagnostic output is printed.

You can also log compilation activity to a file with the -XX:+LogCompilation option, if the console vibe isn't it.

• -XX:+PrintInlining — prints the inlining decisions so you can see which methods are getting inlined. By default it's disabled and no inlining info is printed. And just like PrintAssembly, -XX:+PrintInlining has to be used together with -XX:+UnlockDiagnosticVMOptions to work 🫶
• -XX:ReservedCodeCacheSize=size — sets the maximum code cache size (in bytes) for JIT-compiled code. Add k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. The default max is 240 MB; if you disable tiered compilation with -XX:-TieredCompilation, the default becomes 48 MB. This option caps out at 2 GB — go past that and an error is generated 😭. Also, the maximum code cache size shouldn't be less than the initial code cache size; see -XX:InitialCodeCacheSize.
• -XX:RTMAbortRatio=abort_ratio — the RTM abort ratio, given as a percentage (%) of all executed RTM transactions. If the number of aborted transactions goes above this ratio, the compiled code gets deoptimized. This ratio is used when -XX:+UseRTMDeopt is enabled. Default is 50, meaning compiled code is deoptimized if 50% of all transactions are aborted.
• -XX:RTMRetryCount=number_of_retries — how many times the RTM locking code is retried when it's aborted or busy, before falling back to the normal locking mechanism. Default is 5. Requires -XX:UseRTMLocking to be enabled.
• -XX:+SegmentedCodeCache — enables segmentation of the code cache; without it, the code cache is just one big segment. With -XX:+SegmentedCodeCache, separate segments are used for non-method, profiled method, and non-profiled method code. Segments are not resized at runtime. The glow-up: better control of the memory footprint, reduced code fragmentation, and better CPU iTLB (instruction translation lookaside buffer) and instruction cache behavior thanks to improved locality ✨

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 when only the given percentage of the code cache is free. Default value is 10%.
• -XX:-TieredCompilation — disables tiered compilation. By default, this option is enabled (so this flag turns the default off, bestie).
• -XX:UseSSE=version — enables the use of the SSE instruction set of a specified version. Defaults to the highest supported version available (x86 only).
• -XX:UseAVX=version — enables the use of the AVX instruction set of a specified version. Defaults to the highest supported version available (x86 only).
• -XX:+UseAES — enables hardware-based AES intrinsics for hardware that supports it. On by default on hardware with the necessary instructions 🔥. -XX:+UseAES is used in conjunction with UseAESIntrinsics. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseAESIntrinsics — enables AES intrinsics. Specifying -XX:+UseAESIntrinsics is equivalent to also enabling -XX:+UseAES. To disable hardware-based AES intrinsics, specify -XX:-UseAES -XX:-UseAESIntrinsics. For example, to enable hardware AES, use the following flags:

-XX:+UseAES -XX:+UseAESIntrinsics

Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

• -XX:+UseAESCTRIntrinsics — analogous to -XX:+UseAESIntrinsics, this 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 the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseChaCha20Intrinsics — enables ChaCha20 intrinsics. On by default for supported platforms. To disable ChaCha20 intrinsics, specify -XX:-UseChaCha20Intrinsics. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UsePoly1305Intrinsics — enables Poly1305 intrinsics. On by default for supported platforms. To disable Poly1305 intrinsics, specify -XX:-UsePoly1305Intrinsics. Flags that control intrinsics now require the option -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 the option -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 the option -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 the option -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 the option -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 using the sun.security.provider.Sun provider for SHA operations. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

To disable all hardware-based SHA intrinsics, specify -XX:-UseSHA. To disable only a 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 the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseSHA256Intrinsics — enables intrinsics for the SHA-224 and SHA-256 crypto hash functions. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseSHA512Intrinsics — enables intrinsics for the SHA-384 and SHA-512 crypto hash functions. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseMathExactIntrinsics — enables intrinsification of various java.lang.Math.*Exact() functions. Enabled by default. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseMultiplyToLenIntrinsic — enables intrinsification of BigInteger.multiplyToLen(). Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseSquareToLenIntrinsic — enables intrinsification of BigInteger.squareToLen(). Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseMulAddIntrinsic — enables intrinsification of BigInteger.mulAdd(). Enabled by default on platforms that support it. 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 — spits out CMove instructions (both scalar and vector ones) with zero regard for whether the profitability analysis thinks it's a good idea. It just does it. Unconditionally. Iconic behavior honestly 💅
• -XX:+UseCodeCacheFlushing — turns on flushing of the code cache before the compiler shuts down. This one's enabled by default. Wanna turn it off? Say -XX:-UseCodeCacheFlushing and the code cache stays un-flushed before shutdown.
• -XX:+UseCondCardMark — makes the JVM check whether the card is already marked before it goes and updates the card table. Off by default, and ngl only use it on machines with multiple sockets — that's where it actually boosts Java apps that lean on concurrent operations. Otherwise it's just vibes ✨
• -XX:+UseCountedLoopSafepoints — keeps safepoints in counted loops. Its default depends on whether your chosen garbage collector needs low-latency safepoints. So like, it's giving "it depends" energy 🦋
• -XX:LoopStripMiningIter=number_of_iterations — controls how many iterations the inner strip-mined loop gets. Okay so strip mining (no, not the Minecraft kind 💀) transforms counted loops into two-level nested loops: safepoints live in the outer loop while the inner loop zooms at full speed. This option caps the max iterations in that inner loop. Default is 1,000.
• -XX:LoopStripMiningIterShortLoop=number_of_iterations — controls the loop strip mining optimization. Loops with fewer iterations than the number you specify won't get safepoints in them at all. Default value is 1/10th of -XX:LoopStripMiningIter. Math! We love her 🎀
• -XX:+UseFMA — enables hardware-based FMA intrinsics on hardware where FMA instructions actually exist (think Intel and ARM64). These FMA intrinsics get generated for the java.lang.Math.fma(a, b, c) methods, which calculate the value of ( a * b + c ) expressions. Big math energy 🔥
• -XX:+UseRTMDeopt — autotunes RTM locking based on the abort ratio, which you set with the -XX:RTMAbortRatio option. If the number of aborted transactions goes over that abort ratio, then the method holding the lock gets deoptimized and recompiled with all locks as normal locks. Disabled by default, and heads up: -XX:+UseRTMLocking must be enabled for this to work.
• -XX:+UseRTMLocking — generates Restricted Transactional Memory (RTM) locking code for all inflated locks, with the normal locking mechanism as the fallback handler. Disabled by default. Also, RTM-related options are only available on x86 CPUs that support Transactional Synchronization Extensions (TSX). Let's get into it:

RTM is part of Intel's TSX, which is an x86 instruction set extension that makes building multithreaded applications easier. RTM brings in the new instructions XBEGIN, XABORT, XEND, and XTEST. The XBEGIN and XEND instructions wrap a set of instructions to run as a transaction. If no conflict pops up while running the transaction, then the memory and register modifications get committed together at the XEND instruction. The XABORT instruction can explicitly abort a transaction, and XTEST checks whether a set of instructions is being run inside a transaction.

A lock on a transaction gets inflated when another thread tries to access that same transaction, which blocks the thread that didn't originally request access to it. RTM requires that you specify a fallback set of operations in case a transaction aborts or fails. An RTM lock is a lock that's been delegated to the TSX's system.

RTM improves performance for highly contended locks with low conflict in a critical region (critical region = code that must not be accessed by more than one thread at the same time). RTM also improves the performance of coarse-grain locking, which typically doesn't perform well in multithreaded applications. (Coarse-grain locking = the strategy of holding locks for long periods to minimize the overhead of taking and releasing locks, whereas fine-grained locking = trying to achieve maximum parallelism by locking only when necessary and unlocking as soon as possible, iykyk.) Also, for lightly contended locks used by different threads, RTM can reduce false cache line sharing — also known as cache line ping-pong 🏓. That happens when multiple threads from different processors access different resources, but those resources share the same cache line. Result: the processors keep invalidating each other's cache lines, which forces them to read from main memory instead of their cache. Rough.

• -XX:+UseSuperWord — enables transforming scalar operations into superword operations. Superword is a vectorization optimization, and this is enabled by default. Want to turn that transformation off? Specify -XX:-UseSuperWord and the scalars stay scalars.

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🔍 ADVANCED SERVICEABILITY OPTIONS FOR JAVA 🔍

These java options give you the ability to gather system information and do some serious, extensive debugging. Detective mode fr 🕵️‍♀️

• -XX:+DisableAttachMechanism — disables the mechanism that lets tools attach to the JVM. By default this option is disabled, which means the attach mechanism is enabled and you can use diagnostics and troubleshooting tools like jcmd, jstack, jmap, and jinfo. Yes, the double-negative is confusing, we're all doing our best 😭

Note: the tools such as jcmd, jinfo, jmap, and jstack shipped with the JDK aren't supported when you use tools from one JDK version to troubleshoot a different JDK version. Keep your versions matched, bestie 🫶

• -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 dumping the Java heap to a file in the current directory using the heap profiler (HPROF) when a java.lang.OutOfMemoryError exception gets thrown. You can explicitly set the heap dump file path and name with -XX:HeapDumpPath. By default this option is disabled and the heap isn't dumped when an OutOfMemoryError exception is thrown. Sad but true 💀
• -XX:HeapDumpPath=path — sets the path and file name for writing the heap dump that the heap profiler (HPROF) provides when -XX:+HeapDumpOnOutOfMemoryError is set. By default the file gets created in the current working directory and it's named java_pid<pid>.hprof, where <pid> is the identifier of the process that caused the error. Here's how to set the default file explicitly (%p represents the current process identifier):

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

• Non-Windows: here's how to set the heap dump file to /var/log/java/java_heapdump.hprof:

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

• Windows: here's 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 for where log data gets written. By default the file is created in the current working directory and it's named hotspot.log.
  • Non-Windows: here's how to set the log file to /var/log/java/hotspot.log:

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

• Windows: here's how to set the log file to C:/log/java/hotspot.log:

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

• -XX:+PrintClassHistogram — enables printing a class instance histogram after one of these events:
  • Non-Windows: Control+\ (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 these events:
  • Non-Windows: Control+\ (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. Range check slay ✅
• -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 you ran. Use the jstat command to display the performance data inside that file like this:

jstat -class file:///path/hsperfdata_pid

jstat -gc file:///path/hsperfdata_pid

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

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🗑️ ADVANCED GARBAGE COLLECTION OPTIONS FOR JAVA 🗑️

These java options control how garbage collection (GC) gets done by the Java HotSpot VM. The cleanup crew 👑

• -XX:+AggressiveHeap — enables Java heap optimization. It sets 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 is disabled and heap sizes are configured less aggressively. Turn it on and it goes feral 🔥
• -XX:+AlwaysPreTouch — requests that the VM 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 get committed as the application uses the heap space.
• -XX:ConcGCThreads=threads — sets how many threads get to do the concurrent GC thing 💅. It sets threads to roughly 1/4 of the number of parallel garbage collection threads. The default value? Depends on how many CPUs the JVM has available, so like, it varies bestie.

Example time — want concurrent GC running on 2 threads? Say it with me:

-XX:ConcGCThreads=2

• -XX:+DisableExplicitGC — turns on the option that turns off processing of calls to the System.gc() method 🙈. This option is disabled by default, which means calls to System.gc() are processed. And heads up: if processing of calls to System.gc() is disabled, the JVM still performs GC when it's necessary. So it's not like you're banning GC forever, okay?
• -XX:+ExplicitGCInvokesConcurrent — enables invoking concurrent GC by using the System.gc() request ✨. This one is disabled by default, and you can only enable it with the -XX:+UseG1GC option. Package deal, no substitutions.
• -XX:G1AdaptiveIHOPNumInitialSamples=number — when -XX:UseAdaptiveIHOP is enabled, this sets the number of completed marking cycles used to gather samples until G1 adaptively figures out the optimum value of -XX:InitiatingHeapOccupancyPercent 🧠. Before all that, G1 uses the value of -XX:InitiatingHeapOccupancyPercent directly for this purpose. Default value is 3.
• -XX:G1HeapRegionSize=size — sets the size of the regions the Java heap gets subdivided into when you're 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.

Wanna set the subdivisions to 16 MB? Like this:

-XX:G1HeapRegionSize=16m

• -XX:G1HeapWastePercent=percent — sets the percentage of heap 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. Default is 5 percent.
• -XX:G1MaxNewSizePercent=percent — sets the percentage of the heap size to use as the maximum for the young generation size. Default value is 60 percent of your Java heap.

Heads up bestie: this is an experimental flag. This setting replaces the -XX:DefaultMaxNewGenPercent setting.

• -XX:G1MixedGCCountTarget=number — sets the target number of mixed garbage collections after a marking cycle to collect old regions with at most G1MixedGCLiveThresholdPercent live data 🔥. Default is 8 mixed garbage collections. The goal for mixed collections is to stay within this target number.
• -XX:G1MixedGCLiveThresholdPercent=percent — sets the occupancy threshold for an old region to be included in a mixed garbage collection cycle. Default occupancy is 85 percent.

Experimental flag alert 🚨: this setting replaces the -XX:G1OldCSetRegionLiveThresholdPercent setting.

• -XX:G1NewSizePercent=percent — sets the percentage of the heap to use as the minimum for the young generation size. Default value is 5 percent of your Java heap 🌸.

Also experimental, bestie: this setting replaces the -XX:DefaultMinNewGenPercent setting.

• -XX:G1OldCSetRegionThresholdPercent=percent — sets an upper limit on the number of old regions to be collected during a mixed garbage collection cycle. Default is 10 percent of the Java heap.
• -XX:G1ReservePercent=percent — 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 this percentage, make sure you adjust the total Java heap by the same amount. By default, this option is set to 10%.

Setting the reserved heap to 20%? Iconic, here's how:

-XX:G1ReservePercent=20

• -XX:+G1UseAdaptiveIHOP — controls the adaptive calculation of old generation occupancy to start background work preparing for an old generation collection 🧚. If enabled, G1 uses -XX:InitiatingHeapOccupancyPercent for the first few times as specified by the value of -XX:G1AdaptiveIHOPNumInitialSamples, and after that it 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:InitiatingHeapOccupancyPercent.

The default is enabled.

• -XX:InitialHeapSize=size — 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 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.

Here's how you 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. And if -Xms appears after -XX:InitialHeapSize on the command line, then the initial heap size gets set to the value specified with -Xms. Order matters, periodt.

• -XX:InitialRAMPercentage=percent — sets the initial amount of memory the JVM will use for the Java heap, before applying ergonomics heuristics, as a percentage of the maximum amount determined as described in the -XX:MaxRAM option ⭐. The default value is 1.5625 percent.

Example — setting the percentage of the initial amount of memory used for the Java heap:

-XX:InitialRAMPercentage=5

• -XX:InitialSurvivorRatio=ratio — 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.

You can use this formula 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. Fun fact: the larger the value specified as the initial survivor space ratio, the smaller the initial survivor space size. Math is wild like that.

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.

Setting the initial survivor space ratio to 4:

-XX:InitialSurvivorRatio=4

• -XX:InitiatingHeapOccupancyPercent=percent — 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 👀.

Setting the initiating heap occupancy to 75%:

-XX:InitiatingHeapOccupancyPercent=75

• -XX:MaxGCPauseMillis=time — 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. The specified value doesn't adapt to your heap size. By default, for G1 the maximum pause time target is 200 milliseconds. The other generational collectors don't use a pause time goal by default.

Setting the maximum target pause time to 500 ms:

-XX:MaxGCPauseMillis=500

• -XX:MaxHeapSize=size — sets the maximum size (in byes — yes, that's how it's written in the original, copying it exactly 💅) of the memory allocation pool. This value must be a multiple of 1024 and greater than 2 MB. Append the letter 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. For server deployments, the options -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value.

Setting 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 🫶.

• -XX:MaxHeapFreeRatio=percent — 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%.

Want to minimize the Java heap size? Lower 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. So try different values for these parameters until they're as low as possible yet still retain acceptable performance. Basically: tune responsibly bestie, your mileage will absolutely vary 😭.

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

And if you're a customer (lol yes you, the one trying to keep the heap smol 🥺) you should also add the option -XX:-ShrinkHeapInSteps. See Performance Tuning Examples for a whole description of using this option to keep the Java heap small by reducing the dynamic footprint for embedded applications. slay.

• -XX:MaxMetaspaceSize=size — caps the maximum amount of native memory that can be allocated for class metadata. By default the size isn't limited at all — no ceiling, no cap, live your truth 🦋. The amount of metadata an application needs depends on the application itself, other running applications, and how much memory is available on the system.

Here's how you set the max class metadata size to 256 MB:

-XX:MaxMetaspaceSize=256m

• -XX:MaxNewSize=size — sets the maximum size (in bytes) of the heap for the young generation (nursery) 🍼. The default value is set ergonomically — i.e. the JVM picks it for you based on vibes and your machine.
• -XX:MaxRAM=size — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics. The default value is the maximum amount of available memory to the JVM process or 128 GB, whichever is lower.

The maximum amount of available memory to the JVM process is the minimum of the machine's physical memory and any constraints set by the environment (e.g. container) 📦.

Heads up: 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, iykyk.

Example — set the maximum amount of available memory for sizing the Java heap to 2 GB:

-XX:MaxRAM=2G

• -XX:MaxRAMPercentage=percent — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics, expressed as a percentage of the maximum amount determined as described in the -XX:MaxRAM option. Default is 25 percent ✨.

Same warning as above, bestie: specifying this 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.

Example — set the percentage of the maximum amount of memory used for the Java heap:

-XX:MaxRAMPercentage=75

• -XX:MinRAMPercentage=percent — okay so this one also says "maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics", but it's the percentage of the maximum amount determined as described in the -XX:MaxRAM option for small heaps. A small heap is a heap of approximately 125 MB 🐣. The default value is 50 percent.

Example — set the percentage of the maximum amount of memory used for the Java heap for small heaps:

-XX:MinRAMPercentage=75

• -XX:MaxTenuringThreshold=threshold — sets the maximum tenuring threshold used in adaptive GC sizing. The largest value is 15. The default value is 15 for the parallel (throughput) collector. The way that objects "age"... it's giving immortality arc 💀

Example — set the maximum tenuring threshold to 10:

-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 garbage collection 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%.

Want to minimize Java heap size? Lower 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 🫶.

• -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 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.

Examples — set the minimum size of allocated memory to 6 MB using various units:

-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. cute 💅

• -XX:NewRatio=ratio — sets the ratio between young and old generation sizes. By default, this option is set to 2. Example — set 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 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 low, then a large number of minor GCs are performed. If the size is too high, then only full GCs are performed, which can take a long time to complete 😭. It is recommended that you keep the size for the young generation greater than 25% and less than 50% of the overall heap size.

Examples — set the initial size of the young generation to 256 MB using various units:

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

The -XX:NewSize option is equivalent to -Xmn. twin behavior fr fr 👯

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

For example, to set the number of threads for G1 GC to 2, specify the following option:

-XX:ParallelGCThreads=2

• -XX:+ParallelRefProcEnabled — enables parallel reference processing ✨. By default, this option is disabled.
• -XX:+PrintAdaptiveSizePolicy — enables printing of information about adaptive-generation sizing. By default, this option is disabled. turn it on if you're nosy, no judgment 💖
• -XX:+ScavengeBeforeFullGC — enables GC of the young generation before each full GC. This option is enabled by default. It is recommended that you don't disable it, because scavenging the young generation before a full GC can reduce the number of objects reachable from the old generation space into the young generation space. To disable GC of the young generation before each full GC, specify -XX:-ScavengeBeforeFullGC. (yes the +/- flip is the whole trick, main character energy 🔥)
• -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. The default value 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 that 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 are garbage collected.

Example — set 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 ⚠️ — not a vibe, just facts.

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. This is sometimes referred to as 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 set to 8. The following example shows how to set 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 set to 50%. The following example shows how to set 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 the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. If this option is set to 0, then the JVM selects the initial size automatically. The following example shows how to set 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.
• -XX:+UseGCOverheadLimit — enables the use of a policy that limits the proportion of time spent by the JVM on GC before an OutOfMemoryError exception is thrown. This option is enabled, 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. By default, this option is disabled and no optimization for NUMA is made. The option is available only when the parallel garbage collector is used (-XX:+UseParallelGC).
• -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.
• -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.
• -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.

💀 DEPRECATED JAVA OPTIONS 💀

These 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.

• -Xfuture — enables strict class-file format checks that enforce close conformance to the class-file format specification. Developers should use this flag when developing new code. Stricter checks may become the default in future releases.
• -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 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:InitialRAMFraction=ratio — sets the initial amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a ratio of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 64. Use the option -XX:InitialRAMPercentage instead.
• -XX:MaxRAMFraction=ratio — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a fraction of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 4. 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 information about compressed oops. Use the option -XX:MaxRAMPercentage instead.
• -XX:MinRAMFraction=ratio — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a fraction of the maximum amount determined as described in the -XX:MaxRAM option for small heaps. A small heap is a heap of approximately 125 MB. The default value is 2. Use the option -XX:MinRAMPercentage instead.

🫥 OBSOLETE JAVA OPTIONS 🫥

These java options are still accepted but ignored, and a warning is issued when they're used.

• --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:+UseHugeTLBFS — Linux only: this option is the equivalent of specifying -XX:+UseLargePages. This option is disabled by default. This option pre-allocates all large pages up-front, when memory is reserved; consequently the JVM can't dynamically grow or shrink large pages memory areas; see -XX:UseTransparentHugePages if you want this behavior.
• -XX:+UseSHM — Linux only: enables the JVM to use shared memory to set up large pages.

💀 REMOVED JAVA OPTIONS 💀

okay so the tea here is, like, nothing. in JDK 22, no documented java options have been removed. that's it. that's the section. we love a drama-free era ✨

but bestie, if you're feeling nostalgic (or debugging something from the stone age), the lists and descriptions of options removed in previous releases are chilling in the Removed Java Options section of:

• The java Command, Release 21 — https://docs.oracle.com/en/java/javase/21/docs/specs/man/java.html
• The java Command, Release 20 — https://docs.oracle.com/en/java/javase/20/docs/specs/man/java.html
• The java Command, Release 19 — https://docs.oracle.com/en/java/javase/19/docs/specs/man/java.html
• The java Command, Release 18 — https://docs.oracle.com/en/java/javase/18/docs/specs/man/java.html
• The java Command, Release 17 — https://docs.oracle.com/en/java/javase/17/docs/specs/man/java.html
• The java Command, Release 16 — https://docs.oracle.com/en/java/javase/16/docs/specs/man/java.html
• The java Command, Release 15 — https://docs.oracle.com/en/java/javase/15/docs/specs/man/java.html
• The java Command, Release 14 — https://docs.oracle.com/en/java/javase/14/docs/specs/man/java.html
• The java Command, Release 13 — 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
• 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
• Java Platform, Standard Edition Tools Reference, Release 10 — 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
• 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
• 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

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🎀 JAVA COMMAND-LINE ARGUMENT FILES 🎀

so you know how your java command can get, like, unreasonably long? yeah. you can shorten or simplify it by using @ argument files to specify one or more text files that contain arguments, such as options and class names, which are passed to the java command. this lets you create java commands of any length on any operating system 🫶

in the command line, use the at sign (@) prefix to identify an argument file that contains java options and class names. when the java command encounters 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. iconic behaviour, honestly.

the java launcher expands the argument file contents until it encounters the --disable-@files option. you can use the --disable-@files option anywhere on the command line, including in an argument file, to stop @ argument files expansion. slay.

the following items describe the syntax of java 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.
• 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.
• 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.
• wildcards (*) aren't allowed in these lists (such as specifying *.java).
• 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 java arguments:

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:

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, bestie. 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. very main character energy 💅

⭐ 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 — for a detailed description of the code heap state analytics feature, the supported functions, and the granularity options.

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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. periodt.

🎀 Synopsis 🎀

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

-Xlog:directive

• 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.
• 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 🦋

the Java Virtual Machine (JVM) unified logging framework provides a common logging system for all components of the JVM. GC logging for the JVM has been changed to use the new logging framework. the mapping of old GC flags to the corresponding new Xlog configuration is described in Convert GC Logging Flags to Xlog. in addition, runtime logging has also been changed to use the JVM unified logging framework. the mapping of legacy runtime logging flags to the corresponding new Xlog configuration is described in Convert Runtime Logging Flags to Xlog.

the following provides quick reference to the -Xlog command and syntax for options:

• -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.
• -Xlog:disable — turns off all logging and clears all configuration of the logging framework including the default configuration for warnings and errors.
• -Xlog[:option] — ok so this one applies multiple arguments in the order that they appear on the command line 👑 and like, multiple -Xlog arguments for the same output override each other in their given order. periodt.

The option is set as:

[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. no vibes-based guessing, bestie.

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. The default configuration is equivalent to entering the following on the command line:

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

🎀 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. As the diagnostic commands are automatically exposed as MBeans, you can use JMX to change logging configuration at run time 💅 iconic behaviour honestly.

🦋 -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. dozens!! 💀

💜 -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 and/or %t in the file name expands to the JVM's PID and startup timestamp, 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. 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 🙈 so like, be aware.

💖 -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. But you can instead use asynchronous logging mode by specifying:

• -Xlog:async — 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 and on buffer exhaustion the enqueuing message is discarded 💀 Log entry write operations are guaranteed non-blocking. slay for latency, awkward for your missing log lines.

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.

✨ 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 table (it's a list now, love that for us). 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 🧚

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 🎀

okay so these legacy flags are not recognized anymore, like at all — if you use them directly you straight up get an error 😭. So instead you use their unified logging equivalent. Here's the full glow-up mapping, bestie:

• 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.

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🌈 -Xlog Usage Examples 🌈

these are the -Xlog examples, and honestly they eat every time 🍓:

• -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,pids: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 💅

okay these are the spicy ones — the following describes a few complex examples of using the -Xlog option:

• -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 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 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.

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👑 VALIDATE JAVA VIRTUAL MACHINE FLAG ARGUMENTS 👑

so the values you hand to all Java Virtual Machine (JVM) command-line flags get used for validation, and if the input value is invalid or out-of-range, then an appropriate error message is displayed. no cap, the JVM will absolutely tell you 💀.

Whether they're set ergonomically, in a command line, by an input tool, or 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.

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). The JVM is terminated if a value violates either the range or constraint check and an appropriate error message is printed on the error stream.

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. slay 🦋.

🌸 LARGE PAGES 🌸

okay so bestie, buckle up, because this section is a whole thing 💅

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, so don't @ me if yours is different 🫶). And what do large pages do? They optimize processor Translation-Lookaside Buffers. Iconic behavior, honestly.

A Translation-Lookaside Buffer (TLB) is a page translation cache that holds the most-recently used virtual-to-physical address translations. It's a scarce system resource — like the last charger in the house, okay? A TLB miss can be costly because the processor must then read from the hierarchical page table, which may require multiple memory accesses 😭. But 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. ✨

⚠️ BUT — and this is the important part — 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. No cap, that's just how it is.

Linux and Windows support large pages. Periodt. 👑

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💖 Large Pages Support for Linux 💖

Linux supports large pages since version 2.6. Want a vibe check on whether 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 — congrats bestie, we're so back 🎉. 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. Like, literally always. Check it. Don't just assume. 🙈

Note: The values contained in /proc and /sys reset after you reboot your system, so you may want to set them in an initialization script (for example, rc.local or sysctl.conf). Otherwise you'll be redoing this at 2am wondering why nothing works 😭

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 little thieves, honestly 🎀

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💜 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 them. 💀 That's not a bug, that's just Windows being Windows.

🦋 APPLICATION CLASS DATA SHARING 🦋

Okay so AppCDS — Application Class Data Sharing — is literally the glow-up your app's startup time has been waiting for. It stores the classes your applications actually use in an archive file, and because those classes are kept in a format that loads so much faster than classes chillin' inside a JAR file, AppCDS can improve your app's start-up time. Not only that — it can also shrink the runtime memory footprint by sharing bits of those classes across multiple processes. Sharing is caring, bestie. 🫶

Here's the tea 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. So yeah — it's a good idea to archive only those classes your application actually uses. And usually that's just a small portion of all the available classes. Like, your app might only touch a few APIs out of some giant library. Don't archive the whole library, babe. 🎀

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Using CDS Archives

By default, in most JDK distributions — unless you go out of your way and specify -Xshare:off — 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). That archive contains about 1300 core library classes that most applications use. Free real estate, no cap. ✨

Wanna use CDS for the exact set of classes your application uses? That's what -XX:SharedArchiveFile is for, and it 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 only significance is that the "static" archive is loaded first and the "dynamic" archive is loaded second. That's it. That's the whole vibe. 💅)

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

-XX:SharedArchiveFile=<static_archive>

And for convenience — this is such a nice touch — the <dynamic_archive> records the location of the <static_archive>. Which means you can omit the <static_archive> entirely and just say:

-XX:SharedArchiveFile=<dynamic_archive>

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Manually Creating CDS Archives

You can create CDS archives 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. Basically a test drive before you commit. 🚗💨

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:

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 directories, bestie, I'm serious.

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

java -Xshare:dump -XX:SharedArchiveFile=hello.jsa -XX:Shared
ClassListFile=hello.classlist -cp hello.jar

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

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:

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

Heads up! 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. ⚡

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. Efficiency icon. 👑

The following steps create a dynamic CDS archive file that contains the classes that are used by the test.Hello application, excluding those that are already 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:

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

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

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.,

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

To run the application using this dynamic CDS archive:

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

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

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

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

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. Ugh, we've all been there. 😭

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

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. It's giving set-it-and-forget-it. 🌸

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. Old news, we don't read her. 💀

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.

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.
• 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. 🚨

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🔥 PERFORMANCE TUNING EXAMPLES 🔥

You can use the Java advanced runtime options to optimize the performance of your applications. Let's make her fast. 💨

Tuning for Higher Throughput

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

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:

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. Small but mighty — just don't take it too far, okay? 🧚

✨ EXIT STATUS ✨

okay bestie, so when the launcher gets called with arguments that just do not vibe (wrong ones, honestly), or when something seriously breaks, or when the JVM throws an exception — these are the exit values you'll typically see. like, this is the launcher's default behavior, not a promise 💅

🫶 important caveat though: a Java application can literally choose to return any value it wants by calling System.exit(exitValue). so if your exit code looks unhinged, that might just be the app being its own main character. 💀

The values are:

• 0 — Successful completion 🌸 everything ate, no notes, the build is green, go touch grass
• >0 — An error occurred 😭 something went sideways, time to read the logs bestie

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and that's the whole page!! you made it to the very end, which honestly is iconic behavior 👑💖

this was JAVA(1) — source/version JDK 22, dated 2024 🎀

go write some slay code, bestie — ngl I'm obsessed with you for reading all nineteen parts 🦋✨💜