JAVA(1)

📦 version 27.0.1.0.3.u1, in arch rolling

🦋 JAVA(1) — JDK Commands 🦋

Okay bestie, this is the java launcher page from the JDK Commands manual — and yes, she's long, but she's literally the “start a Java app” icon. 💅 We're gonna walk it together, no cap. 🫶

✨ NAME ✨

java — launch a Java application. 🌈

🌸 SYNOPSIS 🌸

To launch a class file:

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 ...]

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

To launch a source-file program:

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

options — Optional: Specifies command-line options separated by spaces. See Overview of Java Options for a description of available options. ✨
mainclass — Specifies the name of the class to be launched. Command-line entries following classname are the arguments for the main method.
-jar jarfile — Executes a program encapsulated in a JAR file. The jarfile argument is the name of a JAR file with a manifest that contains a line in the form Main-Class:classname that defines the class with the public static void main(String[] args) method that serves as your application's starting point. When you use -jar, the specified JAR file is the source of all user classes, and other class path settings are ignored. If you're using JAR files, then see jar. 🎀
-m or --module module[/mainclass] — Executes the main class in a module specified by mainclass if it is given, or, if it is not given, the value in the module. In other words, mainclass can be used when it is not specified by the module, or to override the value when it is specified. See Standard Options for Java. 🦋
source-file — Only used to launch a source-file program. Specifies the source file that contains the main class when using source-file mode. See Using Source-File Mode to Launch Source-Code Programs.
args ... — Optional: Arguments following mainclass, source-file, -jar jarfile, and -m or --module module/mainclass are 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 specified class, and calling that class's main() method. The method must be declared public and static, it must not return any value, and it must accept a String array as a parameter. The method declaration has the following form:

public static void main(String[] args)

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

Note: You can use the JDK_JAVA_OPTIONS launcher environment variable to prepend its content to the actual command line of the 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 class and resource files for the application. The startup class must be indicated by the Main-Class manifest header in its manifest file.

Arguments after the class file name or the JAR file name are passed to the main() method. 💫

🪟 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 to appear. The javaw launcher will, however, display a dialog box with error information if a launch fails. 😭

🧚 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. Entering source-file mode is determined 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 in the command line that would otherwise be the main class name.
The --source version option, if present.

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

Note: The valid values of N change for each release, with new values added and old values removed. You'll get an error message if you use a value of N that is no longer supported. The supported values of N are the current Java SE release (27) and a limited number of previous releases, detailed in the command-line help for 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 the source-file mode. The --source option is used for cases when the source file is a "script" to be executed and the name of the source file does not 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 are passed to the compiled class when it is 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.
Command-line argument files (@-files) may be used in the standard way. Long lists of arguments for either the VM or the program being invoked may be placed in files specified on the command-line by prefixing the filename with an @ character. 🌟

In source-file mode, compilation proceeds as follows:

Any command-line options that are relevant to the compilation environment are taken into account. These include: --class-path/-classpath/-cp, --module-path/-p, --add-exports, --add-modules, --limit-modules, --patch-module, --upgrade-module-path, --enable-preview.
The root of the source tree, <source-root> is computed from the package of the class being launched. For example, if HelloWorld.java declared its classes to be in the hello package, then the file HelloWorld.java is expected to reside in the directory somedir/hello/. In this case, somedir is computed to be the root of the source tree.
The root of the source tree serves as the source-path for compilation, so that other source files found in that tree and are needed by HelloWorld could be compiled.
Annotation processing is disabled, as if -proc:none is in effect.
If a version is specified, via the --source option, the value is used as the argument for an implicit --release option for the compilation. This sets both the source version accepted by compiler and the system API that may be used by the code in the source file.
If --enable-preview is specified, the --source N arguments can be omitted. If the Java runtime version is N, then --release N is implied when compiling source files.
If a module-info.java file exists in the <source-root> directory, its module declaration is used to define a named module that will contain all the classes compiled from .java files in the source tree. If module-info.java does not exist, all the classes compiled from source files will be compiled in the context of the unnamed module.
The source file that is launched should contain one or more top-level classes, the first of which is taken as the class to be executed.
For the source file that is launched, the compiler does not enforce the optional restriction defined at the end of JLS 7.6, 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. 👑
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 be 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. 💀

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 JDK_JAVA_OPTIONS is like… the launcher's little pre-game playlist 💅 it prepends its content to the options parsed from the command line. So basically it goes first, and your command line goes after it.

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 — same energy, same rules ✨

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 are preserved by simply removing the pair of quotes. Lowkey elegant. But bestie — 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. Any wildcard literal * in the JDK_JAVA_OPTIONS environment variable content isn't expanded and is passed as-is to the starting VM. No sneaky globbing here 🙈

So, 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. And 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

see how it just slid in there at the front? iconic. 👑

✨ 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). plus means on, minus means off, we love a binary 💅

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. math is mathing ✨ 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. 💀

🌸 STANDARD OPTIONS FOR JAVA 🌸

These are the most commonly used options supported by all implementations of the JVM. the everyday baddies 💖

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

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

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

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

-agentpath:pathname[=options] — Loads the native agent library specified by the absolute path name. This option is equivalent to -agentlib but uses the full path and file name of the library. same vibe, full address 🗺️
--class-path classpath, -classpath classpath, or -cp classpath — Specifies a list of directories, JAR files, and ZIP archives to search for class files. On Windows, semicolons (;) separate entities in this list; on other platforms it is a colon (:). Specifying classpath overrides any setting of the CLASSPATH environment variable. If the class path option isn't used and classpath isn't set, then the user class path consists of the current directory (.).

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

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

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

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

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

allow: This mode allows illegal native access in all modules, without any warnings.
warn: This mode is identical to allow except that a warning message is issued for the first illegal native access found in a module. This mode is the default for the current JDK but will change in a future release.
deny: This mode disables illegal native access. That is, any illegal native access causes an IllegalCallerException. This mode will become the default in a future release. To verify that your application is ready for a future version of the JDK, run it with --illegal-native-access=deny along with any necessary --enable-native-access options. future-proof your fave fr fr 🔮

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

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

---illegal-final-field-mutation=parameter — This option specifies a mode for how illegal final field mutation is handled:

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

allow: This mode allows illegal final field mutation in all modules, without any warnings.

warn — this one is literally allow but with a little heads-up: a warning message drops for the first illegal final field mutation performaed in a module 😭. This mode is the default for the current JDK but will change in a future release. iykyk ✨
debug — same as allow, except it prints a warning message and a stack trace for every illegal final field mutation. Maximum receipts, bestie 💅
deny — this mode disables final field mutation completely. That is, any illegal final field mutation access causes an IllegalAccessException. This mode will become the default in a future release. To verify that your application is ready for a future version of the JDK, run it with --illegal-final-field-mutation=deny along with any necessary --enable-final-field-mutation options. no cap, this one is strict 🫶
--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. simple vibe check 🌸
--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 (:). platform princess behavior 🎀
--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 (:). upgrade era 🦋
--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. root module main character energy 👑
--list-modules — lists the observable modules and then exits. show me the modules, then bye 💖
-d module_name or --describe-module module_name — describes a specified module and then exits. spill the module tea ☕
--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. test the vibe before committing ✨
--validate-modules — validates all modules and exits. This option is helpful for finding conflicts and other errors with modules on the module path. module wellness check 🌈
-Dproperty=value — sets a system property value. The -Dproperty variable is a string with no spaces that represents the name of the property. The value variable is a string that represents the value of the property. If value is a string with spaces, then enclose it in quotation marks (for example -Dfoo="foo bar"). property setting icon 💅
-disableassertions[:[packagename]...|:classname] or -da[:[packagename]...|:classname] — disables assertions. By default, assertions are disabled in all packages and classes. With no arguments, -disableassertions (-da) disables assertions in all packages and classes. With the packagename argument ending in ..., the switch disables assertions in the specified package and any subpackages. If the argument is simply ..., then the switch disables assertions in the unnamed package in the current working directory. With the classname argument, the switch disables assertions in the specified class.

The -disableassertions (-da) option applies to all class loaders and to system classes (which don't have a class loader). There's one exception to this rule: If the option is provided with no arguments, then it doesn't apply to system classes. This makes it easy to disable assertions in all classes except for system classes. The -disablesystemassertions option enables you to disable assertions in all system classes. To explicitly enable assertions in specific packages or classes, use the -enableassertions (-ea) option. Both options can be used at the same time. 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. system classes getting quiet time 🙈
-enableassertions[:[packagename]...|:classname] or -ea[:[packagename]...|:classname] — enables assertions. By default, assertions are disabled in all packages and classes. With no arguments, -enableassertions (-ea) enables assertions in all packages and classes. With the 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). There's one exception to this rule: If the option is provided with no arguments, then it doesn't apply to system classes. This 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. 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. system classes, you're on 👑
-help, -h, or -? — prints the help message to the error stream. helpful but slightly dramatic 🎀
--help — prints the help message to the output stream. same help, nicer lane ✨
-javaagent:jarpath[=options] — loads the specified Java programming language agent. See java.lang.instrument. agent era 🕵️
--show-version — prints the product version to the output stream and continues. version, but make it output stream 🌈
-showversion — prints the product version to the error stream and continues. version, but make it error stream 💀
--show-module-resolution — shows module resolution output during startup. watch the modules resolve in real time 🦋
-splash:imagepath — shows the splash screen with the image specified by imagepath. HiDPI scaled images are automatically supported and used if available. The unscaled image file name, such as image.ext, should always be passed as the argument to the -splash option. The most appropriate scaled image provided is 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. aesthetic startup unlocked 🌸

-verbose:class — displays information about each loaded class. class-by-class commentary 💅
-verbose:gc — displays information about each garbage collection (GC) event. GC tea, served hot ☕
-verbose:jni — displays information about the use of native methods and other Java Native Interface (JNI) activity. JNI activity, tracked 🧚
-verbose:module — displays information about the modules in use. module gossip 🫶
--version — prints product version to the output stream and exits. version and dip 👑
-version — prints product version to the error stream and exits. version and dip, but error stream 😭
-X — prints the help on extra options to the error stream. extra options, extra drama 💀
--help-extra — prints the help on extra options to the output stream. extra options, calmer lane 🌈
@argfile — specifies one or more argument files prefixed by @ used by the java command. It isn't uncommon for the java command line to be very long because of the .jar files needed in the classpath. The @argfile option overcomes command-line length limitations by enabling the launcher to expand the contents of argument files after shell expansion, but before argument processing. Contents in the argument files are expanded because otherwise, they would 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 options. 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. argfile ate, periodt 💖

🔥 EXTRA OPTIONS FOR JAVA 🔥

The following java options are general purpose options that are specific to the Java HotSpot Virtual Machine. HotSpot era, bestie ⭐

-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. foreground compilation, no background sneaky vibes 🔥
-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 is a colon (:). boot class path append situation 🎀
-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:
- The thread doing the call is not attached to the JVM. not attached? JVM said absolutely not 💀
The thread doing the call is using the JNIEnv belonging to another thread. wrong-thread energy 😭
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: so they're giving warning-only vibes 👀

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

-Xcomp — Testing mode to exercise JIT compilers. This option should not be used in production environments. 🚨
-Xdebug — Does nothing; deprecated for removal in a future release. literally does nothing, bestie 💀
-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. 🐢
-Xinternalversion — Displays more detailed JVM version information than the -version option, and then exits. iconic behavior 👑
-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. hot methods only, periodt 🔥
-Xmn size — Sets the initial and maximum size (in bytes) of the heap for the young generation (nursery) in the generational collectors. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too small, then a lot of minor garbage collections are performed. If the size is too large, then only full garbage collections are performed, which can take a long time to complete. It is recommended that you do not set the size for the young generation for the G1 collector, and keep the size for the young generation greater than 25% and less than 50% of the overall heap size for other collectors. The following examples show how to set the initial and maximum size of young generation to 256 MB using various units:

-Xmn256m
-Xmn262144k
-Xmn268435456

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

-Xms size — Sets the minimum and the initial size (in bytes) of the heap. This value must be a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The following examples show how to set the size of allocated memory to 6 MB using various units:

-Xms6291456
-Xms6144k
-Xms6m

If you do not set this option, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation. The initial size of the heap for the young generation can be set using the -Xmn option or the -XX:NewSize option. Note that the -XX:InitialHeapSize option can also be used to set the initial heap size. If it appears after -Xms on the command line, then the initial heap size gets set to the value specified with -XX:InitialHeapSize.

-Xmx size — Specifies the maximum size (in bytes) of the heap. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -Xms and -Xmx are often set to the same value. The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:

-Xmx83886080
-Xmx81920k
-Xmx80m

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

-Xnoclassgc — Disables garbage collection (GC) of classes. This can save some GC time, which shortens interruptions during the application run. When you specify -Xnoclassgc at startup, the class objects in the application are left untouched during GC and are always considered live. This can result in more memory being permanently occupied which, if not used carefully, throws an out-of-memory exception. 🫠
-Xrs — Reduces the use of operating system signals by the JVM. Shutdown hooks enable the orderly shutdown of a Java application by running user cleanup code (such as closing database connections) at shutdown, even if the JVM terminates abruptly. 🛑
- Non-Windows:
  - The JVM catches signals to implement shutdown hooks for unexpected termination. The JVM uses SIGHUP, SIGINT, and SIGTERM to initiate the running of shutdown hooks.
  - Applications embedding the JVM frequently need to trap signals such as SIGINT or SIGTERM, which can lead to interference with the JVM signal handlers. The -Xrs option is available to address this issue. When -Xrs is used, the signal masks for SIGINT, SIGTERM, SIGHUP, and SIGQUIT aren't changed by the JVM, and signal handlers for these signals aren't installed.
- Windows:
  - The JVM watches for console control events to implement shutdown hooks for unexpected termination. Specifically, the JVM registers a console control handler that begins shutdown-hook processing and returns TRUE for CTRL_C_EVENT, CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, and CTRL_SHUTDOWN_EVENT.
  - The JVM uses a similar mechanism to implement the feature of dumping thread stacks for debugging purposes. The JVM uses CTRL_BREAK_EVENT to perform thread dumps.
  - If the JVM is run as a service (for example, as a servlet engine for a web server), then it can receive CTRL_LOGOFF_EVENT but shouldn't initiate shutdown because the operating system doesn't actually terminate the process. To avoid possible interference such as this, the -Xrs option can be used. When the -Xrs option is used, the JVM doesn't install a console control handler, implying that it doesn't watch for or process CTRL_C_EVENT, CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, or CTRL_SHUTDOWN_EVENT.

There are two consequences of specifying -Xrs:

Non-Windows: SIGQUIT thread dumps aren't available.
Windows: Ctrl + Break thread dumps aren't available. User code is responsible for causing shutdown hooks to run, for example, by calling System.exit() when the JVM is to be terminated.

-Xshare:mode — Sets the class data sharing (CDS) mode. Possible mode arguments for this option include the following:
- auto — Use shared class data if possible (default).
- on — Require using shared class data, otherwise fail.

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

off — Do not attempt to use shared class data.

-XshowSettings — Shows all settings and then continues. ✨
-XshowSettings:category — Shows settings and continues. Possible category arguments for this option include the following:
- all — Shows all categories of settings in verbose detail.
- locale — Shows settings related to locale.
- properties — Shows settings related to system properties.
- security — Shows all settings related to security. sub-category arguments for security include the following:
  - security:all : shows all security settings
  - security:properties : shows security properties
  - security:providers : shows static security provider settings
  - security:tls : shows TLS related security settings
- vm — Shows the settings of the JVM.
- system — Linux only: Shows host system or container configuration and continues.
-Xss size — Sets the thread stack size (in bytes). Append the letter k or K to indicate KB, m or M to indicate MB, or g or G to indicate GB. The actual size may be rounded up to a multiple of the system page size as required by the operating system. 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. Here are the glow-up examples that set the thread stack size to 1024 KB in different units:

-Xss1m
-Xss1024k
-Xss1048576

This option is similar to -XX:ThreadStackSize. ✨

--add-reads module=target-module(,target-module)* — updates module so it reads target-module, regardless of the module declaration. target-module can be ALL-UNNAMED to read all unnamed modules. 🫶
--add-exports module/package=target-module(,target-module)* — updates module so it exports 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 so it opens package to target-module, regardless of module declaration. iykyk 💅
--limit-modules module[,module...] — specifies the limit of the universe of observable modules. that's it, that's the whole vibe 🌌
--patch-module module=file(;file)* — overrides or augments a module with classes and resources in JAR files or directories. 🎀
--source version — sets the version of the source in source-file mode.
--sun-misc-unsafe-memory-access= value — allows or denies usage of the unsupported API sun.misc.Unsafe. value is one of these four besties:
- allow — allow use of the memory-access methods with no warnings at run time. 😌
- warn — allow use of the memory-access methods, but issues a warning on the first occasion that any memory-access method is used. At most one warning is issued.
- debug — allow use of the memory-access methods, but issue a one-line warning and a stack trace when any memory-access method is used. 🔍
- deny — disallow use of the memory-access methods by throwing an UnsupportedOperationException on every usage. 💀

The default value when the option is not specified is warn.

🍎 EXTRA OPTIONS FOR MACOS 🍎

The following extra options are macOS specific. Apple girlies rise 🙈

-XstartOnFirstThread — runs the main() method on the first (AppKit) thread.
-Xdock:name=application_name — overrides the default application name displayed in dock. 🛟
-Xdock:icon=path_to_icon_file — overrides the default icon displayed in dock. cute icon era 🌸

🔓 ADVANCED OPTIONS FOR JAVA 🔓

These java options can be used to enable other advanced options. like keys to a secret room, no cap 🗝️

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

Bestie, listen: 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 real

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

🧠 ADVANCED RUNTIME OPTIONS FOR JAVA 🧠

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

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

The VM normally determines the number of available processors from the operating system. This flag can be useful for partitioning CPU resources when running multiple Java processes in docker containers. This flag is honored even if UseContainerSupport is not enabled. See -XX:-UseContainerSupport for a description of enabling and disabling container support. 🐳

-XX:AllocateHeapAt=path — takes a path to the file system and uses memory mapping to allocate the object heap on 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. ✨

-XX:-CompactStrings — disables the Compact Strings feature. By default, this option is enabled. When this option 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. 💅

-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_pidpid.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. -XX:FlightRecorderOptions:help prints the available options, default redaction filters, and example command lines. Multiple parameters can be specified by separating them with a comma.

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

globalbuffersize=size — specifies the total amount of primary memory used for data retention. The default value is based on the value specified for memorysize. Change the memorysize parameter to alter the size of global buffers.
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} — tells the JVM whether event classes should get retransformed via JVMTI. If you set it to false, then instrumentation gets added whenever event classes are loaded instead. Default vibe? It's on — enabled. ✨
redact-argument=argument-filter — swaps out command-line arguments that match a semicolon-separated list of glob patterns — like, say, *secret*;password*. Matching is case-insensitive, and the wildcards you get are * and ?. Need to redact more than one argument? Use a literal space (' ') as the separator. So for example, to match the two arguments --auth username:token, you'd use the filter --auth *:*. Filters with spaces in them have to be quoted as ONE command-line argument, like -XX:FlightRecorderOptions='redact-argument=--auth *:*'. Heads up bestie: arguments that contain spaces might not get matched the way you expect 😭. Wanna load patterns from a file (one per line)? Use @<filename>. Wanna add to the default patterns instead of replacing them? Prefix the whole list with +, for example +*foo*;@redact.txt. And none (lowercase, okay?) disables all the redaction filters for command-line arguments. Redacted arguments get replaced with [REDACTED]. The redact-argument option is best-effort and only applies to command-line arguments in the jdk.JVMInformation event, plus the java.command system property in the jdk.InitialSystemProperty event, plus matching command-line argument text in the values of jdk.InitialEnvironmentVariable events. Other events — like jdk.ProcessStart (child processes) — are not redacted. Run -XX:FlightRecorderOptions:help to see the default filters that redact-argument uses. 💅
redact-key=key-filter — replaces the value of environment variables and system properties whose key matches a semicolon-separated list of glob patterns, e.g. *password*;*token*. Matching is case-insensitive, wildcards are * and ?. Load patterns from a file (one per line) with @<filename>. Add to the defaults instead of replacing them by prefixing the whole list with +, like +*cred*;@keys.txt. Use none (lowercase) to nuke all redaction filters for key matching. Redacted values become [REDACTED]. This one's also best-effort and only touches the jdk.InitialSystemProperty, jdk.InitialEnvironmentVariable and jdk.JVMInformation (-Dkey=...) events. Other events, like jdk.InitialSecurityProperty, are not redacted 🙈. Peep the default filters with -XX:FlightRecorderOptions:help. 🫶
stackdepth=depth — stack depth for stack traces. Default is 64 method calls. Max is 2048. Anything above 64 could create significant overhead and tank your performance, so… choose wisely. 💀
threadbuffersize=size — the per-thread local buffer size in bytes. Default is 8 kilobytes, minimum is 4 kilobytes. Overriding this could reduce performance and is not recommended — the JVM devs said what they said. 🎀

-XX:LargePageSizeInBytes=size — sets the max large page size (in bytes) that the JVM uses. The size argument has to be a valid page size that your environment actually supports for this to do literally anything. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. Default is 0, which means the JVM uses the environment's default large page size as the max for large pages. See Large Pages. 🌸

Example of setting the large page size to 1 gigabyte (GB):

-XX:LargePageSizeInBytes=1g

-XX:MaxDirectMemorySize=size — the max total size (in bytes) of java.nio package direct-buffer allocations. Append k/K for kilobytes, m/M for megabytes, g/G for gigabytes. If you don't set it, the flag is ignored and the JVM picks the size for NIO direct-buffer allocations automatically. ✨

Here's NIO set to 1024 KB in three different units, iconic behavior:

-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 it's ignored on Windows. The only time you might need to disable it is on macOS, where using it imposes a maximum of 10240, which is lower than the actual system maximum. Rude but true. 😭
-XX:NativeMemoryTracking=mode — sets the mode for tracking JVM native memory usage. The possible mode arguments:
- off — don't track JVM native memory usage at all. This is the default behavior if you don't specify -XX:NativeMemoryTracking.
- summary — tracks memory usage only by JVM subsystems, like Java heap, class, code, and thread.
- detail — on top of tracking by JVM subsystems, also tracks memory usage by individual CallSite, individual virtual memory region, and its committed regions.
-XX:TrimNativeHeapInterval=millis — the interval, in ms, at which the JVM trims the native heap. Lower values reclaim memory more eagerly, but at the cost of higher overhead. A value of 0 (the default) disables native heap trimming. Trimming happens in a dedicated thread. 🧚

Heads up: this option is only supported on Linux with GNU C Library (glibc). Periodt.

-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 must be within the range of 8 and 256 (inclusive). This option is what 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 may not realize any benefits from using compressed pointers with large Java heap sizes. It's giving tradeoff. ⚖️

-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 must be enclosed in quotation marks.
- Non-Windows: here's how -XX:OnError can run the gcore command to create a core image, then start the gdb debugger to attach to the process when there's an irrecoverable error (the %p designates the current process identifier):

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

Windows: here's how -XX:OnError can run the userdump.exe utility to grab a crash dump on an irrecoverable error (the %p designates the current process identifier). This example assumes the path to userdump.exe is specified in the PATH environment variable:

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

-XX:OnOutOfMemoryError=string — sets a custom command, or a series of semicolon-separated commands, to run when an OutOfMemoryError exception is first thrown by the JVM. If the string contains spaces, it must be enclosed in quotation marks. For an example command string, see the description of the -XX:OnError option. ⚡

Real talk: this applies only to OutOfMemoryError exceptions caused by Java Heap exhaustion. It does not apply to OutOfMemoryError exceptions thrown directly from Java code, nor ones thrown by the JVM for other types of resource exhaustion (like native thread creation errors). Important distinction, bestie. 💀

-XX:+PrintCommandLineFlags — turns on printing of ergonomically selected JVM flags that appeared on the command line. Super useful for knowing the ergonomic values the JVM set, like heap space size and the selected garbage collector. Default is disabled, so flags aren't printed. 🦋
-XX:+PreserveFramePointer — picks between using the RBP register as a general purpose register (-XX:-PreserveFramePointer) and using the RBP register to hold the frame pointer of the currently executing method (-XX:+PreserveFramePointer). If the frame pointer is available, external profiling tools (like Linux perf, for example) can construct more accurate stack traces. 🔥
-XX:+PrintNMTStatistics — turns on printing of collected native memory tracking data at JVM exit, when native memory tracking is enabled (see -XX:NativeMemoryTracking). Default is disabled, so no native memory tracking data gets 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 you specify this, the JVM will load a CDS archive file only if it passes an integrity check based on CRC32 checksums. The whole point of this flag is catching unintentional damage to CDS archive files during transmission or storage. 🧐

But listen — to guarantee the security and proper operation of CDS, you the user must ensure the CDS archive files used by Java applications can't be modified without proper authorization. The flag ain't gonna do that part for you. 💅

-XX:SharedArchiveConfigFile=shared_config_file — specifies additional shared data added to the archive file. ✨
-XX:SharedClassListFile=file_name — tells the JVM which text file holds the names of the classes to stash in the class data sharing (CDS) archive. That file is literally one full class name per line, except slashes (/) replace the dots (.). Say you wanna include java.lang.Object and hello.Main — you'd make a text file with these two lines, periodt:

java/lang/Object
hello/Main

The classes you drop in this text file should include the ones your app uses all the time. They can be classes from the application, extension, or bootstrap class paths — your pick. See Application Class Data Sharing. 💅

-XX:+ShowCodeDetailsInExceptionMessages — turns on the glow-up version of NullPointerException messages. When your app throws a NullPointerException, this option lets the JVM actually read the program's bytecode instructions to figure out exactly which reference is null, then describes the source with a null-detail message. That null-detail message gets calculated and returned by NullPointerException.getMessage(), and it prints as the exception message right alongside the method, filename, and line number. 👑 By default, this option is enabled. Slay.
-XX:+ShowMessageBoxOnError — pops up a dialog box when the JVM hits an irrecoverable error 😭. This stops the JVM from exiting and keeps the process alive so you can attach a debugger to it and go investigate what went wrong. By default, this option is disabled.
-XX:StartFlightRecording:parameter=value — starts a JFR recording for the Java application. This option is the same thing as the JFR.start diagnostic command, which starts a recording during runtime. -XX:StartFlightRecording:help prints the available options and some example command lines. 🎀 You can set these parameter=value entries when you start a JFR recording:
- delay=time — the delay between the Java application launch time and the start of the recording. Append s for seconds, m for minutes, h for hours, or d for days (like 10m means 10 minutes). By default there's no delay, and this parameter is set to 0.
- disk={true|false} — whether to write data to disk while recording. By default, this parameter is enabled.
- dumponexit={true|false} — whether the running recording gets dumped when the JVM shuts down. If it's enabled and you didn't enter a filename, the recording is written to a file in the directory where the process was started. The file name is a system-generated one containing the process ID, recording ID, and current timestamp, something like hotspot-pid-47496-id-1-2018_01_25_19_10_41.jfr. By default, this parameter is disabled.
- duration=time — how long the recording lasts. Append s for seconds, m for minutes, h for hours, or d for days (like 5h means 5 hours). By default the duration isn't limited, and this parameter is set to 0.
- filename=path — the path and name of the file the recording gets written to when the recording is stopped, for example:
  - recording.jfr
  - /home/user/recordings/recording.jfr
  - c:\recordings\recording.jfr

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

name=identifier — takes both the name and the identifier of a recording.
maxage=time — 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 for seconds, m for minutes, h for hours, or d for days (like 30s means 30 seconds). By default the maximum age isn't limited, and this parameter is set to 0s.
maxsize=size — 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 for megabytes, or g or G for 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} — 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 genuinely useful for finding memory leaks, but collecting it is time-consuming 😩. Enable this option only when you start a recording for an application 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 gets included in the information collected.
report-on-exit=identifier — the name of the view to display when the Java Virtual Machine (JVM) shuts down. Wanna specify more than one view? Use the report-on-exit parameter repeatedly. This option is not available if the disk option is set to false. For a list of available views, see jfr help view. By default, no report is generated.
settings=path — the path and name of the event settings file (of type JFC). By default, the default.jfc file is used, which lives in JAVA_HOME/lib/jfr. This default settings file collects a predefined set of information with low overhead, so it barely touches performance and can be used with recordings that run continuously. ✨ A second settings file is also provided, profile.jfc, which gives you more data than the default configuration but can have more overhead and impact performance. Use that one for short periods of time when you need more info.

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

option=value — the option value to modify. To list available options, use the JAVA_HOME/bin/jfr tool.
event-setting=value — 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. If a parameter and a .jfc option conflict, the parameter takes precedence. The whitespace delimiter can be omitted for timespan values, i.e. 20ms. For more info about the settings syntax, see the Javadoc of the jdk.jfr package. And to only see warnings and errors from JFR during startup, set -Xlog:jfr+startup=warning. 💜

-XX:ThreadStackSize=size — sets the Java thread stack size (in kilobytes). If you use a scaling suffix like k, the kilobytes value gets scaled, so -XX:ThreadStackSize=1k sets the Java thread stack size to 1024*1024 bytes, aka 1 megabyte. The default value depends on the platform, bestie:
- Linux/x64: 1024 KB
- Linux/Aarch64: 2048 KB
- macOS/x64: 1024 KB
- macOS/Aarch64: 2048 KB
- Windows: the default value depends on virtual memory

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:-UseCompactObjectHeaders — disables compact object headers. By default this option is enabled and compact object headers are used. Using compact object headers reduces memory footprint in the Java heap by 4 bytes per object (on average) and often improves performance. 🔥 You'd use this option if you suspect performance regressions. In a future release, compact object headers is expected to become the only mode of operation — so enjoy the toggle while it lasts.
-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 — and by default that 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 you're running the app with Java heap sizes smaller than the compressed oops pointer range. ⭐ Heads up: 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. iykyk 💅
-XX:-UseContainerSupport — Linux only, bestie: 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 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)

🚀 ADVANCED JIT COMPILER OPTIONS FOR JAVA 🚀

These java options control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM.

-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:

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

You can also use the asterisk (*) as a wildcard for class and method names. For example, to exclude all indexOf() methods in all classes from being compiled, use the following:

-XX:CompileCommand=exclude,*.indexOf

The commas and periods are aliases for spaces, making it easier to pass compiler commands through a shell. You can pass arguments to -XX:CompileCommand using spaces as separators by enclosing the argument in quotation marks:

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

Note that after parsing the commands passed on the command line using the -XX:CompileCommand options, the JIT compiler then reads commands from the .hotspot_compiler file. You can add commands to this file or specify a different file using the -XX:CompileCommandFile option. To add several commands, either specify the -XX:CompileCommand option multiple times, or separate each argument with the new line separator (\n). The following commands are available:

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

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

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

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

CompilerOracle: exclude java/lang/String.indexOf

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

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

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

So that bitmask at the end? You literally just add up the compilation-level numbers you want, bestie, that's the whole math 💅

1 — the C1 JIT compiler, but with zero profiling.
10 — C1 again, this time with limited profiling.
100 — C1 with full profiling, she's doing the most. 1000. 1000 — C2, the big one: no profiling, full optimization.

If you don't pass a bitmask at all, then all levels are assumed. Heads up though: excluding specific compilation levels can mess with the normal state transitions between those levels, because the VM will not automatically work around the ones you excluded. Like, it's not gonna politely route around your mess — it'll just be weird. 😭

-XX:CompileCommandFile=filename — tells the JVM which file to read JIT compiler commands from. By default the .hotspot_compiler file is the one used to store commands performed by the JIT compiler ✨

Every single line in that command file is one command, plus a class name, plus a method name — that's the trio you're working with. For example, this line right here prints assembly code for the toString() method of the String class:

print java/lang/String toString

And if you're out here 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 onto 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).

ngl this one's got a buddy system requirement: -XX:CompilerDirectivesFile has to be used together with the -XX:UnlockDiagnosticVMOptions option, the one that unlocks diagnostic JVM options. No unlock, no vibes 🔒

-XX:+CompilerDirectivesPrint — prints the directives stack when the program starts, or when a new directive gets added. Iconic for debugging, honestly.

Same deal as above, fr fr: -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 to restrict compilation to, separated by commas. Only the methods you name get compiled, everyone else can sit down 🪑

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

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

-XX:CompileThresholdScaling=scale — your unified control knob for first compilation 🌸 It controls when methods are first compiled, for both the tiered and the nontiered modes of operation. The value is a floating point number between 0 and +Inf, and it scales the thresholds that correspond to the current mode of operation (tiered and nontiered alike).

Setting CompileThresholdScaling to something less than 1.0 gives you earlier compilation. Values greater than 1.0 delay compilation. And setting CompileThresholdScaling to 0? That's equivalent to disabling compilation entirely. 💀

-XX:+DoEscapeAnalysis — enables the use of escape analysis. This one's enabled by default, bestie, it's already on. If you wanna turn it off, specify -XX:-DoEscapeAnalysis.

-XX:InitialCodeCacheSize=size — sets the initial code cache size, in bytes. 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 the initial code cache size shouldn't be less than the system's minimal memory page size — like, don't be out here asking for less than the floor, okay? 🥺

Here's how you'd set the initial code cache size to 32 KB:

-XX:InitialCodeCacheSize=32k

-XX:+Inline — enables method inlining. This is enabled by default to increase performance (she's doing her job 👑). To disable method inlining, specify -XX:-Inline.

-XX:InlineSmallCode=size — sets the maximum code size, in bytes, for methods that are already compiled and might get 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 into a file named hotspot.log in the current working directory 📁 You can point it somewhere else with a different log file path and name using the -XX:LogFile option. By default this option is disabled, and compilation activity isn't logged.

Requirement check, bestie: -XX:+LogCompilation has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options 🔓

Also, if you want verbose diagnostic output with a message printed to the console every single time a method is compiled, that's the -XX:+PrintCompilation option's whole thing.

-XX:FreqInlineSize=size — sets the maximum bytecode size, in bytes, of a hot method that's allowed 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 that's allowed 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 that's allowed 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 that's allowed 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 — same energy, C1 edition: sets the maximum bytecode size, in bytes, of a trivial method that's allowed 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 that can be used during the compilation of a single method 🌿 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 that contains nonmethod code.

For context, a nonmethod code segment contains nonmethod code — think 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.

-XX:NonProfiledCodeHeapSize=size — sets the size in bytes of the code segment containing nonprofiled methods. This flag is used only if -XX:SegmentedCodeCache is enabled ✨

-XX:+OptimizeStringConcat — enables the optimization of String concatenation operations. Enabled by default, bestie 💖 To disable the optimization of String concatenation operations, specify -XX:-OptimizeStringConcat.

-XX:+PrintAssembly — enables printing of assembly code for bytecoded and native methods by using the external hsdis-<arch>.so or .dll library 🦋 For the 64-bit VM on Windows, it's hsdis-amd64.dll. This lets you actually see the generated code, which may help you diagnose performance issues 👀

By default this option is disabled and assembly code isn't printed.

Again with the buddy system: -XX:+PrintAssembly has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.

-XX:ProfiledCodeHeapSize=size — sets the size in bytes of the code segment containing profiled methods. This flag is used only if -XX:SegmentedCodeCache is enabled 🎀

-XX:+PrintCompilation — enables verbose diagnostic output from the JVM by printing a message to the console every single time a method is compiled 🗣️ This lets you see which methods actually get compiled. By default this option is disabled and diagnostic output isn't printed.

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

-XX:+PrintInlining — enables printing of inlining decisions, so you can see which methods are getting inlined 👑 By default this option is disabled and inlining information isn't printed.

You already know the drill: -XX:+PrintInlining has to be used together with the -XX:+UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.

-XX:ReservedCodeCacheSize=size — sets the maximum code cache size, in bytes, for JIT-compiled code 💅 Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes.

The default maximum code cache size is 240 MB. But if you disable tiered compilation with the option -XX:-TieredCompilation, then the default size is 48 MB.

⚠️ This option has a limit of 2 GB — go over that and an error is generated. Also, the maximum code cache size shouldn't be less than the initial code cache size; see the option -XX:InitialCodeCacheSize 🍓

-XX:+SegmentedCodeCache — turns on segmentation of the code cache; without it the code cache is just one big segment 😤. With this flag you get separate segments for non-method code, profiled method code, and non-profiled method code. Heads up: the segments are not resized at runtime, so no take-backs. The glow-up? Better control of the memory footprint, less code fragmentation, and better CPU iTLB (instruction translation lookaside buffer) + instruction cache behavior because ~locality~. It's enabled by default if tiered compilation is on (-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 yeet unused code once only the given percentage of the code cache is free. Default is 10%.
-XX:-TieredCompilation — disables tiered compilation. By default this option is enabled (yes, the enabled default is the thing, the flag here is the off-switch, don't @ me).
-XX:UseSSE=version — enables the SSE instruction set of a specified version. Defaults to the highest supported version available (x86 only).
-XX:UseAVX=version — enables 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 that has the necessary instructions. -XX:+UseAES is used in conjunction with UseAESIntrinsics. Also, flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions — like, mandatory, no cap.
-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, 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 for 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 SHA-224 and SHA-256 crypto hash functions. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseSHA512Intrinsics — enables intrinsics for 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 — generates CMove (scalar and vector) instructions regardless of profitability analysis. Like, vibes over ROI, we're doing it anyway 💀.
-XX:+UseCodeCacheFlushing — enables flushing of the code cache before shutting down the compiler. This option is enabled by default. To disable flushing of the code cache before shutting down the compiler, specify -XX:-UseCodeCacheFlushing.
-XX:+UseCondCardMark — enables checking if the card is already marked before updating the card table. Disabled by default. Use it only on machines with multiple sockets, where it increases the performance of Java applications that rely on concurrent operations.
-XX:+UseCountedLoopSafepoints — keeps safepoints in counted loops. Its default value depends on whether the selected garbage collector requires low latency safepoints. iykyk 🌀
-XX:LoopStripMiningIter=number_of_iterations — controls the number of iterations in the inner strip mined loop. Strip mining transforms counted loops into two-level nested loops; safepoints are kept in the outer loop while the inner loop can execute at full speed. This option controls the maximum number of iterations in the inner loop. Default is 1,000.
-XX:LoopStripMiningIterShortLoop=number_of_iterations — controls loop strip mining optimization. Loops with a number of iterations less than specified will not have safepoints in them. Default value is 1/10th of -XX:LoopStripMiningIter.
-XX:+UseFMA — enables hardware-based FMA intrinsics for hardware where FMA instructions are available (such as Intel and ARM64). FMA intrinsics are generated for the java.lang.Math.fma(a, b, c) methods that calculate the value of ( a * b + c ) expressions.
-XX:+UseSuperWord — enables the transformation of scalar operations into superword operations. Superword is a vectorization optimization ✨. This option is enabled by default. To disable the transformation of scalar operations into superword operations, specify -XX:-UseSuperWord.

✨ ADVANCED SERVICEABILITY OPTIONS FOR JAVA ✨

okay bestie, these java options are the whole “gather system information and do extensive debugging” vibe. Think detective mode, but make it JVM. 🔍💅

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

jstat -class file:///path/hsperfdata_pid

jstat -gc file:///path/hsperfdata_pid

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

🗑️ ADVANCED GARBAGE COLLECTION OPTIONS FOR JAVA 🗑️

these java options control how garbage collection (GC) is performed by the Java HotSpot VM. yes bestie, we’re talking about the cleanup crew of the JVM. iconic behavior only. ✨

-XX:+AlwaysPreTouch — requests the VM to touch every page on the Java heap after requesting it from the operating system and before handing memory out to the application. By default, this option is disabled and all pages are committed as the application uses the heap space.
-XX:ConcGCThreads=threads — sets the number of threads used for concurrent GC. Sets threads to approximately 1/4 of the number of parallel garbage collection threads. The default value depends on the number of CPUs available to the JVM. For example, to set the number of threads for concurrent GC to 2, specify the following option:

-XX:ConcGCThreads=2

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

-XX:G1HeapRegionSize=16m

-XX:G1HeapWastePercent=percent — sets the percentage of heap that you’re willing to waste. The Java HotSpot VM doesn’t initiate the mixed garbage collection cycle when the reclaimable percentage is less than the heap waste percentage. The default is 5 percent.
-XX:G1MaxNewSizePercent=percent — sets the percentage of the heap size to use as the maximum for the young generation size. The default value is 60 percent of your Java heap. This is an experimental flag. This setting replaces the -XX:DefaultMaxNewGenPercent setting.
-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. The default is 8 mixed garbage collections. The goal for mixed collections is to be within this target number.
-XX:G1MixedGCLiveThresholdPercent=percent — sets the occupancy threshold for an old region to be included in a mixed garbage collection cycle. The default occupancy is 85 percent. This is an experimental flag. This setting replaces the -XX:G1OldCSetRegionLiveThresholdPercent setting.
-XX:G1NewSizePercent=percent — sets the percentage of the heap to use as the minimum for the young generation size. The default value is 5 percent of your Java heap. This is an experimental flag. This setting replaces the -XX:DefaultMinNewGenPercent setting.
-XX:G1OldCSetRegionThresholdPercent=percent — sets an upper limit on the number of old regions to be collected during a mixed garbage collection cycle. The default is 10 percent of the Java heap.
-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 the percentage, ensure that you adjust the total Java heap by the same amount. By default, this option is set to 10%. The following example sets the reserved heap to 20%:

-XX:G1ReservePercent=20

-XX:+G1UseAdaptiveIHOP — controls adaptive calculation of the old generation occupancy to start background work preparing for an old generation collection. If enabled, G1 uses -XX:G1IHOP for the first few times as specified by the value of -XX:G1AdaptiveIHOPNumInitialSamples, and after that adaptively calculates a new optimum value for the initiating occupancy automatically. Otherwise, the old generation collection process always starts at the old generation occupancy determined by -XX:G1IHOP. The default is enabled.
-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 to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value is selected at run time based on the system configuration. The following examples show how to set the size of allocated memory to 6 MB using various units:
-XX:InitialHeapSize=size (continued from the previous bit) — okay so here's how the sizes look in every unit you could want, and yes they're all the same number bestie:

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

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

-XX:InitialRAMPercentage=percent — sets the initial amount of memory that the JVM will use for the Java heap before applying ergonomics heuristics, as a percentage of the available memory to the JVM process. Here's the vibe:

-XX:InitialRAMPercentage=5

🌸

-XX:InitialSurvivorRatio=ratio — sets the initial survivor space ratio used by the throughput garbage collector (which is the one you get with -XX:+UseParallelGC). Adaptive sizing is on by default with the throughput collector via -XX:+UseParallelGC, so the survivor space gets resized according to the application behavior, starting with the initial value. If adaptive sizing is disabled (via -XX:-UseAdaptiveSizePolicy), then you should use the -XX:SurvivorRatio option to set the size of the survivor space for the entire execution of the application.

The following formula can be used to calculate the initial size of survivor space (S) based on the size of the young generation (Y), and the initial survivor space ratio (R):

S=Y/(R+2)

The 2 in the equation denotes two survivor spaces. The larger the value specified as the initial survivor space ratio, the smaller the initial survivor space size. By default, the initial survivor space ratio is set to 8. If the default value for the young generation space size is used (2 MB), then the initial size of the survivor space is 0.2 MB. Math is giving slay today 🧮✨ and here's how to set the ratio to 4:

-XX:InitialSurvivorRatio=4

-XX:G1IHOP=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. Here's 75%:

-XX:G1IHOP=75

👑

-XX:MaxGCPauseMillis=time — sets a target for the maximum GC pause time (in milliseconds). Heads up: this is a soft goal, and the JVM will make its best effort to achieve it — it's not a promise, it's a vibe check 😭. Only G1 and Parallel support a maximum GC pause time target. For G1, the default maximum pause time target is 200 milliseconds. Setting it to 500 ms:

-XX:MaxGCPauseMillis=500

-XX:MaxHeapSize=size — sets the maximum size (in bytes) of the memory allocation pool. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value is selected at run time based on the system configuration. For server deployments, the options -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value — twinsies 🫶.

The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:

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

The -XX:MaxHeapSize option is equivalent to -Xmx. Same energy, different font. 💜

-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%.

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

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

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

-XX:MaxMetaspaceSize=size — sets the maximum amount of native memory that can be allocated for class metadata. By default, the size isn't limited 😳 — the amount of metadata for an application depends on the application itself, other running applications, and the amount of memory available on the system. Capping it at 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. That's the whole entry, she's short and iconic. ✨
-XX:MaxRAMPercentage=percent — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics, as a percentage of the available memory to the JVM process. The default value is 25 percent.

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. 💀

Here's how to set that percentage:

-XX:MaxRAMPercentage=75

-XX:MinRAMPercentage=percent — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics, as a percentage of the available memory to the JVM process, for small heaps. A small heap is a heap of approximately 125 MB. The default value is 50 percent. (Yes, MinRAMPercentage setting a maximum is confusing — that's just how it is, no cap. 🙈)

-XX:MinRAMPercentage=75

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

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

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

-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.

The following examples show how to 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. 🧚

-XX:NewRatio=ratio — sets the ratio between young and old generation sizes. By default, this option is set to 2. Here's how to set the young-to-old ratio to 1:

-XX:NewRatio=1

Main character ratio honestly. 👑

-XX:NewSize=size — sets the initial size (in bytes, bestie) of the heap for the young generation, aka the nursery 🍼. You can slap a k or K on the end for kilobytes, m or M for megabytes, or g or G for gigabytes.

Okay so context: the young generation region of the heap is where new objects go to be born, and GC happens in there way more often than in other regions. Which means if you set it too low, you get a tragic amount of minor GCs (💀 constantly). If you set it too high, then only full GCs happen, and those can take forever to finish. The recommendation is literally to keep the young gen size greater than 25% and less than 50% of the overall heap size. Here's how you'd set the initial young gen size to 256 MB in the various units:

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

Also -XX:NewSize is equivalent to -Xmn, iykyk.

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

So for example, to set the number of threads for G1 GC to 2, you'd specify:

-XX:ParallelGCThreads=2

-XX:+PrintAdaptiveSizePolicy — turns on printing of information about adaptive-generation sizing. By default it's disabled (so nothing gets printed until you ask).
-XX:SoftRefLRUPolicyMSPerMB=time — sets how long (in milliseconds) a softly reachable object stays active on the heap after the last time it was referenced. The default is one second of lifetime per free megabyte in the heap. The -XX:SoftRefLRUPolicyMSPerMB option accepts integer values representing milliseconds per one megabyte of the current heap size (that's for Java HotSpot Client VM) or the maximum possible heap size (for Java HotSpot Server VM).

The difference matters, so listen: 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 that second case, the value of the -Xmx option has a significant effect on how quickly soft references get garbage collected. Here's how you set the value to 2.5 seconds:

-XX:SoftRefLRUPolicyMSPerMB=2500

-XX:-ShrinkHeapInSteps — incrementally reduces the Java heap to the target size, which is specified by the option -XX:MaxHeapFreeRatio. This option is enabled by default. If you disable it, 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. Heads up though: you will likely encounter performance degradation when this option is disabled 😭.

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

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

Setting the target survivor space ratio to 30% looks like this:

-XX:TargetSurvivorRatio=30

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

Setting the initial TLAB size to 512 KB:

-XX:TLABSize=512k

-XX:+UseAdaptiveSizePolicy — enables 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 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. G1 is recommended for applications requiring large heaps (sizes of around 6 GB or larger) with limited GC latency requirements — like 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 a policy that limits the proportion of time the JVM spends on GC before an OutOfMemoryError exception gets 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 it, specify -XX:-UseGCOverheadLimit.
-XX:+UseNUMA — enables performance optimization of an application on a machine with nonuniform memory architecture (NUMA) by increasing the application's use of lower latency memory. The default value for this option depends on the garbage collector.
-XX:+UseParallelGC — enables the parallel scavenge garbage collector (also known as the throughput collector) to improve your app's performance by leveraging multiple processors 🔥. By default this option is disabled and the default collector is used.
-XX:+UseSerialGC — enables 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 it, 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. It's giving "sharing is caring" 🫶.

-XX:+UseTLAB — enables the use of thread-local allocation blocks (TLABs) in the young generation space. This option is enabled by default. To disable TLABs, specify -XX:-UseTLAB.
-XX:+UseZGC — enables the Z garbage collector (ZGC). It's a low latency garbage collector, giving 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. Like, 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 app 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's 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 lowers the memory footprint of the JVM and makes 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, so using a lower value will cause heap memory to be uncommitted earlier — at the risk of soon having to commit it again 🙈.
-XX:+UseShenandoahGC — turns on the Shenandoah garbage collector, bestie. It's a low pause time, concurrent collector, and here's the iconic part: its pause times are not proportional to the size of the heap. Like, your heap can be huge and she still won't make you wait. She also plays nice with compressed pointers — peep -XX:UseCompressedOops if you wanna know more about those. ✨
-XX:ShenandoahGCMode= mode — sets which GC mode Shenandoah uses. Default is satb (that's the vibe out of the box). Among other things, this decides which barriers are in use. Possible mode values, bestie:
- satb — snapshot-at-the-beginning concurrent GC (three pass mark-evac-update). It's a single generation GC. 🎀
- generational — also a snapshot-at-the-beginning and concurrent GC, but it's generational. Go read JEP 404 [https://openjdk.org/jeps/404] and JEP 521 [https://openjdk.org/jeps/521] for its advantages and risks. 👀
-XX:ShenandoahGCHeuristics= heuristics — sets the heuristics for Shenandoah GC. Default is adaptive. This fine-tunes the GC mode you picked — it chooses when to start the GC, how much to process on each cycle, and what other features to automatically enable. Heads up: when -XX:ShenandoahGCMode is generational, the only supported option is the default, adaptive. The possible heuristics are:
- adaptive — to maintain the given amount of free heap at all times, even during the GC cycle. 🫶
- static — trigger GC when free heap falls below a specified threshold.
- compact — run GC more frequently and with deeper targets to free up more memory.

🥀 DEPRECATED JAVA OPTIONS

Okay so these java options are deprecated and might get yeeted in a future JDK release. They're still accepted and still acted upon, but you get a warning when you use them. It's giving "we're keeping her for now but she's on thin ice." 💀

-Xloggc: filename — sets the file that verbose GC event info gets redirected to 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 Java Flight Recorder (JFR) while your app is running. Since JDK 8u40 this option has not been required to use JFR. So like... she's just here for the memories. 🙈
-XX:+ParallelRefProcEnabled — enables parallel reference processing. By default, collectors that employ multiple threads perform parallel reference processing if the number of parallel threads to use is larger than one. Lowkey important caveat: this option is available only when the throughput or G1 garbage collector is used (-XX:+UseParallelGC or -XX:+UseG1GC). Other collectors that employ multiple threads always perform reference processing in parallel anyway, so they don't need it. ⭐

💀 OBSOLETE JAVA OPTIONS

These java options are still accepted but ignored, and a warning is issued when they're used. Like, she'll hear you, she just won't do anything. 🫠

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

By default the flag was set to true only on Windows in 32-bit mode and false in all other cases. The "Client VM emulation" mode was not enabled if any of the following flags were used on the command line:

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

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

🗑️ REMOVED JAVA OPTIONS

No documented java options have been removed in JDK 27. Clean slate, no casualties, we love to see it. 🌈

For the lists and descriptions of options removed in previous releases, see the Removed Java Options section in:

The java Command, Release 26 [https://docs.oracle.com/en/java/javase/26/docs/specs/man/java.html]
The java Command, Release 25 [https://docs.oracle.com/en/java/javase/25/docs/specs/man/java.html]
The java Command, Release 24 [https://docs.oracle.com/en/java/javase/24/docs/specs/man/java.html]
The java Command, Release 23 [https://docs.oracle.com/en/java/javase/23/docs/specs/man/java.html]
The java Command, Release 22 [https://docs.oracle.com/en/java/javase/22/docs/specs/man/java.html]
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]

That's the whole archaeology dig, bestie — like a museum of options that used to be somebody. 🏛️✨

🎀 JAVA COMMAND-LINE ARGUMENT FILES 🎀

okay bestie, so you know how java commands can get unhinged long? like, a whole paragraph of flags and class names and you're just sitting there like 😭? yeah. enter argument files. you can shorten or simplify the java command 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 let's you to create java commands of any length on any operating system. literally any length, go wild 💅

on 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. cute, right? ✨

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. it's like a little off switch and it works from inside the house 🫶

🌸 the syntax rules (read these, fr fr) 🌸

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. that's the ceiling, babe, no cap 🔥
the launcher doesn't expand wildcards that are present within an argument file. that means an asterisk (*) is passed on as-is to the starting VM. for example *.java stays *.java and is not expanded to Foo.java Bar.java ..., as would happen with some command line shells. she said "i don't do that here" 💀
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. yes this trips everyone up. yes it will trip you up too 🙈
use the number sign # in the argument file to identify comments. all characters following the # are ignored until the end of line. gossip section, basically 💬
additional at sign @ prefixes to @ prefixed options act as an escape, (the first @ is removed and the rest of the arguments are presented to the launcher literally).
lines may be continued using the continuation character (\) at the end-of-line. the two lines are concatenated with the leading white spaces trimmed. to prevent trimming the leading white spaces, a continuation character (\) may be placed at the first column.
because backslash (\) is an escape character, a backslash character must be escaped with another backslash character.
partial quote is allowed and is closed by an end-of-file.
an open quote stops at end-of-line unless \ is the last character, which then joins the next line by removing all leading white space characters.
use of the at sign (@) to recursively interpret files isn't supported. no inception, sorry 🧚

🦋 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

see how the quote just kinda... hangs there until EOF? iconic behaviour honestly 🎀

🦋 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"

double the backslash, double the fun 😭 it's giving "escape character tax"

🦋 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

(that \ sitting in the first column is doing the lord's work, protecting your leading spaces 🙏)

⭐ 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

one file. one command. main character energy 👑

⭐ 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

say it with me: relative to the current working directory, not the argument file. i know. i know. 💜

🔥 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?

like, the code heap is being mysterious and you need answers 🔍 to provide this insight, a code heap state analytics feature has been implemented that enables on-the-fly analysis of the code heap. the analytics process is divided into two parts. the first part examines the entire code heap and aggregates all information that is believed to be useful or important. the second part consists of several independent steps that print the collected information with an emphasis on different aspects of the data. data collection and printing are done on an "on request" basis. so she's not doing all this unprompted, she's just ready when you ask 💅

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. the pdf is right there, bestie, it's got the whole lore 📄✨

🌈 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. one option to rule them all 🫶

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. glow-up complete ✨

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. it's the built-in tutorial, slay 📚
-Xlog:disable — turns off all logging and clears all configuration of the logging framework including the default configuration for warnings and errors. full reset, no crumbs 💀
-Xlog[:option] — applies multiple arguments in the order that they appear on the command line. multiple -Xlog arguments for the same output override each other in their given order.

the option is set 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.

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. ⚠️ so like, UTF-8 girlies only, everyone else be careful

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

so if you're ever wondering "wait, what is she even doing by default" — that's it. that's the whole vibe check 🌸

💅 Controlling Logging at Runtime

okay so here's the tea — you can also wrangle logging at run time through Diagnostic Commands (with the jcmd utility). Everything that can be specified on the command line can also be specified dynamically with the VM.log command. And since the diagnostic commands are automatically exposed as MBeans, you can use JMX to change logging configuration at run time too. main character flexibility, fr fr 🦋

✨ -Xlog Tags and Levels ✨

Every single log message has a level and a tag set riding along with it. The level corresponds to how detailed the message is, and the tag set corresponds to what the message contains or which JVM component it involves (like, gc, jit, or os — iykyk). Mapping GC flags to the Xlog configuration is described in Convert GC Logging Flags to Xlog, and mapping legacy runtime logging flags to the corresponding Xlog configuration is described in Convert Runtime Logging Flags to Xlog. we love a cross-reference 💖

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. And if you're feeling feral, specifying all instead of a tag combination matches all tag combinations. periodt 🔥

🎀 -Xlog Output 🎀

The -Xlog option supports the following types of outputs:

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

When using file=filename, specifying %p, %t and/or %hn in the file name expands to the JVM's PID, startup timestamp and host name, respectively. iconic little placeholders, no cap 🫶

You can also configure text files to handle file rotation based on file size and a number of files to rotate. For example, to rotate the log file every 10 MB and keep 5 files in rotation, specify the options filesize=10M, filecount=5. The target size of the files isn't guaranteed to be exact — it's just an approximate value, bestie, don't come for it 😭

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

🌸 -Xlog Output Mode 🌸

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

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

Write all logging asynchronously.

In asynchronous logging mode, log sites enqueue all logging messages to an intermediate buffer and a standalone thread is responsible for flushing them to the corresponding outputs. The intermediate buffer is bounded. On buffer exhaustion the enqueuing message is either discarded (async:drop), or logging threads are stalled until the flushing thread catches up (async:stall). If no specific mode is chosen, then async:drop is chosen by default. Log entry write operations are guaranteed to be non-blocking in the async:drop case. slay for not blocking 💅

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. your call, babe ✨

🌈 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. 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. 🎀

The 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 — here's the whole glow-up chart, bestie, every single row 👑

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

🔥 Convert Runtime Logging Flags to Xlog 🔥

These legacy flags are no longer recognized and will cause an error if used directly. Use their unified logging equivalent instead. like, do not pass go, do not collect $200, it's giving deprecated-and-proud 💀

Runtime Logging Flags to Xlog Configuration Mapping

okay so if you're migrating from the old legacy logging flags (rip to those, they were lowkey ancient) here's the translation table, bestie. Same info, just unified-logging-ified ✨

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

`-Xlog` Usage Examples

the following are -Xlog examples, and honestly? they ate. study these, bestie 🎀

-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. (yes, it's an OR situation, not an AND, don't come for me 💀)
-Xlog:gc+ref=debug — logs messages tagged with both gc and ref tags, using the debug level to stdout, with default decorations. Messages tagged only with one of the two tags won't be logged.
-Xlog:gc=debug:file=gc.txt:none — logs messages tagged with the gc tag using the debug level to a file called gc.txt with no decorations. The default configuration for all other messages at level warning is still in effect.
-Xlog:gc=trace:file=gctrace.txt:uptimemillis,pid:filecount=5,filesize=1024 — logs messages tagged with the gc tag using the trace level to a rotating file set with 5 files with size 1 MB with the base name gctrace.txt and uses decorations uptimemillis and pid. The default configuration for all other messages at level warning is still in effect. 🔥 rotating files, we love to see it
-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. the * is doing SO much heavy lifting here 🫡
-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. 🙈

bestie heads up: that -Xlog:disable starts the whole command line, so the default config is genuinely off the table. no warnings, no errors, nothing. iykyk.

Complex `-Xlog` Usage Examples

the following describes a few complex examples of using the -Xlog option. buckle up, these get spicy 🌶️

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

VALIDATE JAVA VIRTUAL MACHINE FLAG ARGUMENTS

so the vibes here are: the JVM is checking your homework 👀

You use values provided to all Java Virtual Machine (JVM) command-line flags for validation and, if the input value is invalid or out-of-range, then an appropriate error message is displayed.

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. 😭 so like, don't test the JVM. it will simply leave.

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. 🫶 look at those little empty brackets, they're literally telling on themselves

🐘 LARGE PAGES 💖

Okay so bestie, large pages (aka huge pages) are memory pages that are like. significantly bigger than the standard memory page size — and that standard size varies depending on your processor and operating system, just so you know. And what do large pages do? They optimize your processor's Translation-Lookaside Buffers. Slay. ✨

A Translation-Lookaside Buffer (TLB) is basically a page translation cache that holds the most-recently used virtual-to-physical address translations. And here's the tea: a TLB is a scarce system resource. A TLB miss is lowkey expensive because the processor then has to go read from the hierarchical page table, which can require multiple memory accesses. 😭 But the way using a larger memory page size fixes this?? A single TLB entry can represent a larger memory range. Less pressure on the TLB, and memory-intensive applications may have better performance. Iconic behavior honestly.

But — and this is the part everyone skips — 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 that slows down the entire system. 💀 Also, a system that's been up for a long time could produce excessive fragmentation, which could make it impossible to reserve enough large page memory. When that happens, either the OS or the JVM reverts to using regular pages. Not the vibe, but that's the vibe.

Linux and Windows support large pages. 🌈

🐧 Large Pages Support for Linux

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

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

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

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

So the above environment supports 2 MB and 1 GB large pages — but they need to be configured so the JVM can actually use them. 🫶 When you're using large pages and not enabling transparent huge pages (that's the -XX:+UseTransparentHugePages option), 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, log in as root and run:

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

Always check the value of nr_hugepages after the request, bestie — you gotta make sure the kernel was actually able to allocate the requested number of large pages. 💅

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).

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. 😭 Such a classic gotcha.

🪟 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:

Select Control Panel, Administrative Tools, and then Local Security Policy.
Select Local Policies and then User Rights Assignment.
Double-click Lock pages in memory, then add users and/or groups.
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. Yes really. 🙈

Application Class Data Sharing (AppCDS) stores classes used by your applications in an archive file. And since these classes are stored in a format that can be loaded very quickly (compared to classes stored in a JAR file), AppCDS can improve the start-up time of your applications. Obsessed. ✨ On top of that, AppCDS can reduce the runtime memory footprint by sharing parts of these classes across multiple processes. Queen behavior.

But heads up: 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 it's a good idea to archive only those classes that are actually used by your application — which usually is just a small portion of all available classes. For example, your application may use only a few APIs provided by a large library. 💅

🎀 Using CDS Archives

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

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

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

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

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

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

-XX:SharedArchiveFile=<static_archive>

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

-XX:SharedArchiveFile=<dynamic_archive>

🛠️ Manually Creating CDS Archives

CDS archives can be created manually using several methods:

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

One common operation in all these methods is a "trial run", where you run the application once to determine the classes that should be stored in the archive. Test flight first, archive second. 🦋

🧚 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.

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.

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

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

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

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

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

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. Speedy slay. 🔥

💜 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.

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.

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

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

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

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=hello.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 mentioned 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. 😭

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

No script edits, no drama. We love to see it. 🫶✨

Note: okay so to use jcmd <pid> VM.cds dynamic_dump, the JVM process identified by <pid> must be started with -XX:+RecordDynamicDumpInfo 💅. You can also pass that same flag 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` ✨

Okay bestie, -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. Iconic behavior fr fr. 👑

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. Slay. ✨

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. Sad little archive, no notes. 💀

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. lowkey important.
Only modular JAR files are supported in --module-path. Exploded modules are not supported. no cap.
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.) iykyk.
The CDS archive cannot be loaded if any JAR files in the class path or module path are modified after the archive is generated. so don't touch them after, bestie 😭.

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

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

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

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

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

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

Mismatched values for property jdk.module.addmods

runtime jdk.incubator.vector dump time jdk.jconsole

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

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

🌈 AHEAD-OF-TIME CACHE 🌈

okay bestie, so the JDK literally does optimizations before your app even runs — ahead-of-time (AOT). One example is Class Data Sharing (CDS), which we already talked about, where classes get parsed ahead of time. These AOT optimizations can make your Java apps start up faster and warm up faster. it's giving prepared queen. 💅

The Ahead-of-Time Cache (AOT cache) is a container introduced in JDK 24 for storing artifacts produced by AOT optimizations. Right now the AOT cache contains Java classes and heap objects. In future JDK releases, the AOT cache may contain additional artifacts, like execution profiles and compiled methods. 🔮

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

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

If any of the above changes, you must recreate the AOT cache. no cap — it's not gonna just vibe with a different setup. 😭

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

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

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

-XX:AOTCache=cachefile — Specifies the location of the AOT cache. The standard extension for cachefile is .aot. This option cannot be used together with -XX:AOTCacheOutput. This option is compatible with AOTMode settings of on, create, or auto (the default). The cachefile is read in AOT modes on and auto, and is ignored by mode off. The cachefile is written by AOT mode create. In that case, this option is equivalent to -XX:AOTCacheOutput=cachefile. 🫶
-XX:AOTCacheOutput=cachefile — Specifies the location of the AOT cache to write. The standard extension for cachefile is .aot. This option cannot be used together with -XX:AOTCache. This option is compatible with AOTMode settings of record, create, or auto (the default).
-XX:AOTConfiguration=configfile — Specifies the AOT Configuration file for the JVM to write to or read from. The standard extension for configfile is .aotconfig. This option is compatible with AOTMode settings of record, create, or auto (the default). The configfile is read by AOT mode create, and written by the other applicable modes. If the AOT mode is auto, then AOTCacheOutput must also be present.
-XX:AOTMode=mode — Specifies the AOT Mode for this run. mode must be one of the following: auto, off, record, create, on, or required. Note that on is an alias for required. All discussion below about required also applies to on. The use of on is discouraged. The support of on may be deprecated and removed in future releases. ⚠️

Then the modes themselves, bestie:

auto: This AOT mode is the default, and takes effect if no -XX:AOTMode option is present. It automatically sets the AOT mode to record, required, or off, as follows:
- If -XX:AOTCacheOutput=cachefile is specified, the AOT mode is changed to record (a training run, with a subsequent create operation).
- Otherwise, if an AOT cache can be loaded, the AOT mode is changed to required (a production run).
- Otherwise, the AOT mode is changed to off (a production run with no AOT cache).
off: No AOT cache is used. Other AOT command line options are ignored.
record: Execute the application in the training phase. At least one of -XX:AOTConfiguration=configfile and/or -XX:AOTCacheOutput=cachefile must be specified. If -XX:AOTConfiguration=configfile is specified, the JVM gathers statistical data and stores them into configfile. If -XX:AOTConfiguration=configfile is not specified, the JVM uses a temporary file name, which may be the string AOTCacheOutput+".config", or else a fresh implementation-dependent temporary file name. If -XX:AOTCacheOutput=cachefile is specified, a second JVM process is launched to perform the Assembly phase to write the optimization artifacts into cachefile.

Extra JVM options can be passed to the second JVM process using the environment variable JDK_AOT_VM_OPTIONS, with the same format as the environment variable JAVA_TOOL_OPTIONS, which is defined by JVMTI (https://docs.oracle.com/en/java/javase/24/docs/specs/jvmti.html#tooloptions). 💖

create: Perform the Assembly phase. -XX:AOTConfiguration=configfile must be specified. The JVM reads history and statistics from configfile and writes the optimization artifacts into cachefile. Note that the application itself is not executed in this phase. (iconic — it does the work without running your app at all ✨)
required: Execute the application in the Production phase. -XX:AOTCache=cachefile must be specified. The JVM tries to load cachefile as the AOT cache.

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

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

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

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

When the AOT cache fails to load:

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

In production environments, -XX:AOTMode=required should be used with care. Your application may fail to launch if an incompatible VM option is added without your knowledge. For example, a cloud provider might run your JVM with a JVMTI class-file load hook for monitoring purposes. 💀 like bestie pls don't let your app not launch because somebody else snuck an option in.

-XX:+AOTClassLinking — If this option is enabled, the JVM will perform more advanced optimizations (such as ahead-of-time resolution of invokedynamic instructions) when creating the AOT cache. As a result, the application will see further improvements in start-up and warm-up performance. However, an AOT cache created with this option cannot be used when certain command-line parameters are specified in the Production phase. Please see JEP 483 (https://openjdk.org/jeps/483) for a detailed discussion of -XX:+AOTClassLinking and its restrictions. When -XX:AOTMode is used in the command-line, AOTClassLinking is automatically enabled. To disable it, you must explicitly pass the -XX:-AOTClassLinking option. When -XX:AOTMode is not used in the command-line, AOTClassLinking is disabled by default to provide full compatibility with traditional CDS options such as -Xshare:dump.

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

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

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

🔥 PERFORMANCE TUNING EXAMPLES 🔥

You can use the Java advanced runtime options to optimize the performance of your applications. let's gooo 💅

✨ 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. ⚠️ (tiny heap, tiny footprint, but don't be delulu about the perf cost, bestie.)

👑 EXIT STATUS 👑

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

0: Successful completion 🎉
>0: An error occurred 😭

and that's the whole page, bestie!! you made it to the very end, i'm so proud of you 🫶✨ this vibe-checked rewrite is sourced from JDK 27.0.1.0.3, dated 2026, and it's the JAVA(1) page — go forth and slay your command line, fr fr 💖🎀🦋

📦 where this page comes from

arch rolling: jdk-openjdk 27.0.1.0.3.u1-1 · LicenseRef-Java
arch rolling: jre-openjdk 27.0.1.0.3.u1-1 · LicenseRef-Java
arch rolling: jre-openjdk-headless 27.0.1.0.3.u1-1 · LicenseRef-Java
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms

JAVA(1)

🦋 JAVA(1) — JDK Commands 🦋

✨ NAME ✨

🌸 SYNOPSIS 🌸

💜 DESCRIPTION 💜

🪟 javaw 🪟

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

🫶 USING THE JDK_JAVA_OPTIONS LAUNCHER ENVIRONMENT VARIABLE 🫶

✨ OVERVIEW OF JAVA OPTIONS ✨

🌸 STANDARD OPTIONS FOR JAVA 🌸

🔥 EXTRA OPTIONS FOR JAVA 🔥

🍎 EXTRA OPTIONS FOR MACOS 🍎

🔓 ADVANCED OPTIONS FOR JAVA 🔓

🧠 ADVANCED RUNTIME OPTIONS FOR JAVA 🧠

🚀 ADVANCED JIT COMPILER OPTIONS FOR JAVA 🚀

✨ ADVANCED SERVICEABILITY OPTIONS FOR JAVA ✨

🗑️ ADVANCED GARBAGE COLLECTION OPTIONS FOR JAVA 🗑️

🥀 DEPRECATED JAVA OPTIONS

💀 OBSOLETE JAVA OPTIONS

🗑️ REMOVED JAVA OPTIONS

🎀 JAVA COMMAND-LINE ARGUMENT FILES 🎀

🌸 the syntax rules (read these, fr fr) 🌸

🦋 Example of Open or Partial Quotes in an Argument File

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

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

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

⭐ Examples of Using Single Argument File

⭐ Examples of Using Argument Files with Paths

🔥 CODE HEAP STATE ANALYTICS 🔥

Overview

Syntax

🌈 ENABLE LOGGING WITH THE JVM UNIFIED LOGGING FRAMEWORK 🌈

Synopsis

Description

Default Configuration

💅 Controlling Logging at Runtime

✨ -Xlog Tags and Levels ✨

🎀 -Xlog Output 🎀

🌸 -Xlog Output Mode 🌸

🌈 Decorations 🌈

🦋 Logging Messages Decorations 🦋

💜 Convert GC Logging Flags to Xlog 💜

🔥 Convert Runtime Logging Flags to Xlog 🔥

Runtime Logging Flags to Xlog Configuration Mapping

-Xlog Usage Examples

Complex -Xlog Usage Examples

VALIDATE JAVA VIRTUAL MACHINE FLAG ARGUMENTS

🐘 LARGE PAGES 💖

🐧 Large Pages Support for Linux

🪟 Large Pages Support for Windows

📦 APPLICATION CLASS DATA SHARING 🌸

🎀 Using CDS Archives

🛠️ Manually Creating CDS Archives

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

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

🌟 Creating CDS Archive Files with jcmd

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

🌸 Restrictions on Class Path and Module Path 🌸

🦋 Module related options 🦋

🌈 AHEAD-OF-TIME CACHE 🌈

🔥 PERFORMANCE TUNING EXAMPLES 🔥

✨ Tuning for Higher Throughput ✨

🌸 Tuning for Lower Response Time 🌸

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

👑 EXIT STATUS 👑

`-Xlog` Usage Examples

Complex `-Xlog` Usage Examples

🧚 Creating a Static CDS Archive File with `-Xshare:dump`

💜 Creating a Dynamic CDS Archive File with `-XX:ArchiveClassesAtExit`

🌟 Creating CDS Archive Files with `jcmd`

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