JAVA(1)

📦

version 27.0.0.0.35 · 48 versions

💅 JAVA(1) — JDK Commands 💅

🌸 NAME 🌸

java — launch a Java application. That's the manual name, bestie: java, hanging out with the JDK Commands crew.

✨ 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 💅

Okay so 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 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 bestie, JDK_JAVA_OPTIONS is literally the pre-game playlist for your java launcher 💅 It prepends its content to the options parsed from the command line. The content of the JDK_JAVA_OPTIONS environment variable is a list of arguments separated by white-space characters (as determined by isspace()). These are 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. JDK_JAVA_OPTIONS environment variable content is treated in the same manner as that specified in the command line.

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. 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. In order to mitigate potential misuse of JDK_JAVA_OPTIONS behavior, options that specify the main class (such as -jar) or cause the java launcher to exit without executing the main class (such as -h) are disallowed in the environment variable. If any of these options appear in the environment variable, the launcher will abort with an error message. When JDK_JAVA_OPTIONS is set, the launcher prints a message to stderr as a reminder.

Example:

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

is equivalent to the command line:

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

🌈 OVERVIEW OF JAVA OPTIONS 🌈

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

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

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

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

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

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

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

Deprecated Java Options: Accepted and acted upon --- a warning is issued when they're used.
Obsolete Java Options: Accepted but ignored --- a warning is issued when they're used.
Removed Java Options: Removed --- using them results in an error.

💅 STANDARD OPTIONS FOR JAVA 💅

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

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

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

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

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

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

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

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

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

--disable-@files — Can be used anywhere on the command line, including in an argument file, to prevent further @filename expansion. This option stops expanding @-argfiles after the option.
--enable-preview — Allows classes to depend on preview features [https://docs.oracle.com/en/java/javase/12/language/index.html#JS-LAN-GUID-5A82FE0E-0CA4-4F1F-B075-564874FE2823] of the release.
--enable-native-access module[,module...] — Native access involves access to code or data outside the Java runtime. This is generally unsafe and, if done incorrectly, might crash the JVM or result in memory corruption. 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.

--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 — okay so this one sets the mode for how the JVM handles illegal final field mutation. The options are, and I'm quoting the actual modes here:

Heads up bestie: this option will be removed in a future release. Don't get too attached 💀

allow — this mode lets illegal final field mutation happen in all modules, no warnings, no nothing. Zero drama, zero receipts.
warn — same as allow except it throws a warning message for the first illegal final field mutation performed in a module. This mode is the default for the current JDK but that's changing in a future release, bestie, so don't rely on it.
debug — same as allow except a warning message and a stack trace get printed for every illegal final field mutation. It's giving "spill all the tea" energy 🔥
deny — this mode straight-up disables final field mutation. 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. Future-proofing, we love to see it ✨

[--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" literally does nothing (iconic no-op behavior, honestly). The value "disabled" disables finalization, so that no finalizers are invoked.
--module-path modulepath... or -p modulepath — tells java where to find your application modules, via a list of path elements. Each element 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 🫶

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

--upgrade-module-path modulepath... — specifies where to find module replacements of upgradeable modules in the runtime image, again 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.

Same deal: on Windows, semicolons (;) separate path elements; on other platforms it's a colon (:).

--add-modules module[,module...] — specifies the root modules to resolve in addition to the initial module. module can also be ALL-DEFAULT, ALL-SYSTEM, and ALL-MODULE-PATH. Like adding extra people to the group chat 💅
--list-modules — lists the observable modules and then exits. Window shopping, but for modules ✨
-d module_name or --describe-module module_name — describes a specified module and then exits.
--dry-run — creates the VM but doesn't execute the main method. This --dry-run option might be useful for validating the command-line options, like the module system configuration. It's giving rehearsal energy 🎭
--validate-modules — validates all modules and exits. This option is helpful for finding conflicts and other errors with modules on the module path. Basically a vibe check for your modules 🔍
-Dproperty=value — sets a system property value. The property 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").
-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.
-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.
-help, -h, or -? — prints the help message to the error stream. Yes, the error stream, don't ask 😭
--help — prints the help message to the output stream. Much more chill of it.
-javaagent:jarpath[=options] — loads the specified Java programming language agent. See java.lang.instrument.
--show-version — prints the product version to the output stream and continues.
-showversion — prints the product version to the error stream and continues. (Same info, different stream, that's the whole bit 💀)
--show-module-resolution — shows module resolution output during startup.
-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. So bougie, so effortless ✨

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

-splash:images/splash.gif

See the SplashScreen API documentation for more information.

-verbose:class — displays information about each loaded class. Every single one. She's thorough 💅
-verbose:gc — displays information about each garbage collection (GC) event.
-verbose:jni — displays information about the use of native methods and other Java Native Interface (JNI) activity.
-verbose:module — displays information about the modules in use.
--version — prints product version to the output stream and exits. Bye bestie 👋
-version — prints product version to the error stream and exits. Again with the error stream, it's a whole thing.
-X — prints the help on extra options to the error stream.
--help-extra — prints the help on extra options to the output stream.
@argfile — specifies one or more argument files prefixed by @ used by the java command. 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.

✨ EXTRA OPTIONS FOR JAVA ✨

The following java options are general purpose options that are specific to the Java HotSpot Virtual Machine. Yes, we're going deeper. Buckle up 🎀

-Xbatch — turns off background compilation, bestie. Normally the JVM compiles a method as a background task and runs it in interpreter mode until that background compile finishes. With -Xbatch, background compilation is off, so compiling all the methods happens as a foreground task until it's done. This flag is equivalent to -XX:-BackgroundCompilation.
-Xbootclasspath/a:directories|zip|JAR-files — hands the JVM a list of directories, JAR files, and ZIP archives to append to the end of the default bootstrap class path. 🎀 On Windows, semicolons (;) separate the entities in this list; on every other platform it's a colon (:).
-Xcheck:jni — does extra checks for Java Native Interface (JNI) functions. It's giving very much security-guard energy.

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

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

These next checks only result in warnings being printed:

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

Expect a performance degradation when this option is used. 😭 like, you're paying for those checks, no cap.

-Xcomp — a testing mode to exercise JIT compilers. This option should not be used in production environments. Periodt. 🙅‍♀️
-Xdebug — does absolutely nothing; deprecated for removal in a future release. 💀 literally a ghost.
-Xdiag — shows additional diagnostic messages. More info, more better. ✨
-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. So, like, every last line goes through the interpreter. Slow queen behavior but valid. 🐢
-Xinternalversion — displays more detailed JVM version information than the -version option, and then exits. An info dump and a dip-out in one move.
-Xlog:option — configure or enable logging with the Java Virtual Machine (JVM) unified logging framework. See Enable Logging with the JVM Unified Logging Framework. 🪵
-Xmixed — executes all bytecode by the interpreter except for hot methods, which are compiled to native code. On by default. Use -Xint to switch off. This is the main character default setting, fr fr. 👑
-Xmn size — sets the initial and maximum size (in bytes) of the heap for the young generation (nursery) in the generational collectors. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. 🌱 The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too small, then a lot of minor garbage collections are performed. If the size is too large, then only full garbage collections are performed, which can take a long time to complete. It 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. Options upon options, iykyk. 🎀

-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. Last one wins, bestie. 🫶

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

-Xmx83886080
-Xmx81920k
-Xmx80m

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

-Xnoclassgc — disables garbage collection (GC) of classes. This can save some GC time, which shortens interruptions during the application run. When you specify -Xnoclassgc at startup, the class objects in the application are left untouched during GC and are always be considered live. ⚠️ This can result in more memory being permanently occupied which, if not used carefully, throws an out-of-memory exception. So like, use it with your whole chest but also maybe don't.
-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. You're the main character here, you gotta do it yourself. 💅

-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. 😭 No cap, it will just bail on you.

off — do not attempt to use shared class data.

-XshowSettings — shows all settings and then continues. A full vibe check of every setting, and then it keeps going. ✨
-XshowSettings:category — shows settings and continues. Possible category arguments for this option include the following:
all — shows literally every category of settings, in verbose detail ✨
locale — shows the settings that have to do with locale 🌍
properties — shows the settings that have to do with system properties
security — shows, you guessed it, all the security-related settings 🔐

the security sub-category arguments are these ones:

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, bestie: shows host system or container configuration, and then continues on its way

-Xss size — sets the thread stack size, and it's in bytes ngl. Append the letter k or K to say KB, m or M for MB, or g or G for GB. Heads up: the actual size may get rounded up to a multiple of the system page size, because that's what the operating system requires 💅 The default value depends on the platform, like so:
- Linux/x64: 1024 KB
- Linux/Aarch64: 2048 KB
- macOS/x64: 1024 KB
- macOS/Aarch64: 2048 KB
- Windows: The default value depends on virtual memory

These examples all set the thread stack size to 1024 KB, just in different units 🎀

-Xss1m
-Xss1024k
-Xss1048576

This option is similar to -XX:ThreadStackSize.

--add-reads module=target-module(,target-module)* — updates module to read the target-module, regardless of the module declaration. target-module can be ALL-UNNAMED to read all unnamed modules 👑
--add-exports module/package=target-module(,target-module)* — updates module to export package to target-module, regardless of module declaration. target-module can be ALL-UNNAMED to export to all unnamed modules ✨
--add-opens module/package=target-module(,target-module)* — updates module to open package to target-module, regardless of module declaration.
--limit-modules module[,module...] — specifies the limit of the universe of observable modules 🌌
--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:
- 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 🍎

These extra options are macOS specific, bestie. 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 🖼️

🔥 ADVANCED OPTIONS FOR JAVA 🔥

Okay so these java options are the ones that unlock other advanced options. Keys to the kingdom energy 👑

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

Real talk: 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's also possible that any of these options may be removed or their behavior changed without any warning. Not me warning you twice 😭

-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 settings 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. Slay 💅

-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 — sets the total amount of primary memory used for data retention ✨. The default value is based on whatever you set for memorysize — so if you want to change the size of your global buffers, go tweak memorysize instead. iykyk 💅
maxchunksize=size — the maximum size (in bytes) of the data chunks inside a recording 🎀. Add m or M to say it in megabytes (MB), or g or G for gigabytes (GB). By default the max chunk size is 12 MB, and the minimum allowed is 1 MB — that's the floor, bestie, don't go below it.
memorysize=size — decides how much buffer memory gets used, and sets the globalbuffersize and numglobalbuffers parameters based on the size you give it 🌸. Append m or M for megabytes (MB), or g or G for gigabytes (GB). Default is 10 MB.
numglobalbuffers — how many global buffers you're rolling with 🦋. The default is based on the memory size specified, so change memorysize to alter the number of global buffers, not this one directly.
old-object-queue-size=number-of-objects — the maximum number of old objects to track. Default is 256 objects. no cap... well, actually, cap 🧢
preserve-repository={true|false} — says whether files stored in the disk repository stick around after the JVM has exited 💖. If false, the files get deleted. By default this parameter is disabled.
repository=path — picks the repository (a directory) for temporary disk storage 🎀. By default the system's temporary directory is used.
retransform={true|false} — whether event classes should be retransformed using JVMTI ✨. If false, instrumentation gets added when event classes are loaded. By default this parameter is enabled.
redact-argument=argument-filter — this one's the privacy icon 👑. It replaces command-line arguments that match a semicolon-separated list of glob patterns, like *secret*;password*. Matching is case-insensitive, and the wildcards you get are * and ?.
- To redact multiple arguments, use a literal space (' ') as the separator. For example, to match the two arguments --auth username:token, use the filter --auth *:*.
- Filters containing spaces must be quoted as a single command-line argument, like -XX:FlightRecorderOptions='redact-argument=--auth *:*'.
- Arguments containing spaces might not be matched as expected 🙈 — heads up.
- To load patterns from a file (one per line), use @<filename>. To add to the default patterns instead of replacing them, prefix the whole list with +, like +*foo*;@redact.txt.
- Use none (lowercase!) to disable all redaction filters for command-line arguments.
- Redacted arguments get replaced with [REDACTED].
- The redact-argument option is best-effort, and it applies only to command-line arguments in the jdk.JVMInformation event, and to the java.command system property in the jdk.InitialSystemProperty event, and to matching command-line argument text in the values of jdk.InitialEnvironmentVariable events. Other events, like jdk.ProcessStart (child processes), are not redacted — so don't assume it's covering everything, fr fr.
- Use -XX:FlightRecorderOptions:help to see the default filters that redact-argument uses 💜
redact-key=key-filter — the sibling icon 🫶. Replaces the value of environment variables and system properties whose key matches a semicolon-separated list of glob patterns, like *password*;*token*. Case-insensitive matching, wildcards * and ?.
- To load patterns from a file (one per line), use @<filename>. To add to the default patterns instead of replacing them, prefix the whole list with +, like +*cred*;@keys.txt.
- Use none (lowercase) to disable all redaction filters for key matching.
- Redacted values get replaced with [REDACTED].
- redact-key is also best-effort, and it applies only to the jdk.InitialSystemProperty, jdk.InitialEnvironmentVariable and jdk.JVMInformation (-Dkey=...) events. Other events, like jdk.InitialSecurityProperty, are not redacted 😭.
- Use -XX:FlightRecorderOptions:help to see the default filters that redact-key uses.
stackdepth=depth — how deep the stack goes for stack traces 🪜. Default is 64 method calls, and the max is 2048. Values greater than 64 could create significant overhead and reduce performance — so like, be careful, that's a real warning ⚠️
threadbuffersize=size — the per-thread local buffer size (in bytes) 🌈. Default local buffer size is 8 kilobytes, with a minimum value of 4 kilobytes. Overriding this parameter could reduce performance and is not recommended — so maybe don't 💀

[-XX:LargePageSizeInBytes=]size — sets the maximum large page size (in bytes) that the JVM uses 💖. The size argument has to be a valid page size supported by the environment for it to have any effect. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. By default it's set to 0, which means the JVM uses the default large page size for the environment as the maximum size for large pages. See Large Pages ✨

Here's how you'd set the large page size to 1 gigabyte (GB):

-XX:LargePageSizeInBytes=1g

[-XX:MaxDirectMemorySize=]size — sets the maximum total size (in bytes) of the java.nio package's direct-buffer allocations 🦋. Append k or K for kilobytes, m or M for megabytes, or g or 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 — it's got it 💅

These examples show setting the NIO size to 1024 KB in different units:

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

[-XX:-MaxFDLimit] — disables the attempt to set the soft limit for the number of open file descriptors to the hard limit 🎀. By default this option is enabled on all platforms, but 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. lowkey a downgrade 🙈
[-XX:NativeMemoryTracking=]mode — specifies the mode for tracking JVM native memory usage 👑. The possible mode arguments are:
- off — don't track JVM native memory usage at all. This is the default behavior if you don't specify the -XX:NativeMemoryTracking option. it's giving "nothing to see here" 💀
- summary — tracks memory usage only by JVM subsystems, such as Java heap, class, code, and thread.
- detail — on top of tracking memory usage by JVM subsystems, this also tracks memory usage by individual CallSite, individual virtual memory region, and its committed regions. the full lore 🔥
[-XX:TrimNativeHeapInterval=]millis — the interval, in ms, at which the JVM trims the native heap 🌸. Lower values reclaim memory more eagerly, at the cost of higher overhead. A value of 0 (the default) disables native heap trimming. Native heap trimming is performed in a dedicated thread 🧵

This option is only supported on Linux with GNU C Library (glibc) — everywhere else it's just not that girl, sorry 😭

[-XX:ObjectAlignmentInBytes=]alignment — sets the memory alignment of Java objects (in bytes) ✨. By default the value is 8 bytes. Whatever value you give should be a power of 2, and must be within the range of 8 and 256 (inclusive). This option makes it possible to use compressed pointers with large Java heap sizes 🎀

The heap size limit in bytes is calculated as:

4GB * ObjectAlignmentInBytes

Note: as the alignment value increases, the unused space between objects also increases too 📈. As a result, you may not realize any benefits from using compressed pointers with large Java heap sizes — so plot twist, it might not be worth it 💀

[-XX:OnError=]string — sets a custom command, or a series of semicolon-separated commands, to run when an irrecoverable error occurs 🔥. If the string contains spaces, it has to be enclosed in quotation marks.
- Non-Windows: this example shows how -XX:OnError can run the gcore command to create a core image, and start the gdb debugger to attach to the process in case of an irrecoverable error (the %p designates the current process identifier) 👑

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

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

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

-XX:OnOutOfMemoryError=string — okay so this one sets a custom command, or a whole series of commands separated by semicolons, that gets run the very first time the JVM throws an OutOfMemoryError exception. Sounds dramatic bc it is 💀. If your string has spaces in it, you gotta wrap it in quotation marks or it's not gonna work. Want an example of what a command string looks like? Peep the description of the -XX:OnError option ✨. Heads up tho: this applies only to OutOfMemoryError exceptions caused by Java heap exhaustion. It does not apply to OutOfMemoryError exceptions thrown directly from Java code, and it doesn't apply to ones the JVM throws for other types of resource exhaustion either (like native thread creation errors). iykyk 🙈
-XX:+PrintCommandLineFlags — turns on printing of the ergonomically selected JVM flags that showed up on the command line. Honestly super useful when you wanna know which ergonomic values the JVM picked for itself, like the heap space size and which garbage collector it chose 👑. Default vibe? This option is disabled and flags aren't printed. periodt.
-XX:+PreserveFramePointer — this is the selector between two moods: using the RBP register as a general purpose register (-XX:-PreserveFramePointer), or using the RBP register to hold the frame pointer of the currently executing method (-XX:+PreserveFramePointer) 🎀. And why do we care? If the frame pointer is available, external profiling tools (Linux perf, for example) can construct way more accurate stack traces. slay for accurate stacks 🔥
-XX:+PrintNMTStatistics — enables printing of collected native memory tracking data at JVM exit, but only when native memory tracking is enabled (see -XX:NativeMemoryTracking). By default this option is disabled and no native memory tracking data is printed. no cap 🫶
-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 option, the JVM will load a CDS archive file only if it passes an integrity check based on CRC32 checksums ⭐. The whole purpose of this flag is to check for unintentional damage to CDS archive files during transmission or storage. And listen — to guarantee the security and proper operation of CDS, the user must ensure that the CDS archive files used by Java applications can't be modified without proper authorization. That's on you, bestie 🫵
-XX:SharedArchiveConfigFile=shared_config_file — specifies additional shared data added to the archive file ✨
-XX:SharedClassListFile=file_name — specifies the text file that contains the names of the classes to store in the class data sharing (CDS) archive 🌸. This file contains the full name of one class per line — except slashes (/) replace dots (.), which is giving very specific. So for example, to specify the classes java.lang.Object and hello.Main, you create a text file that contains the following two lines:

java/lang/Object
hello/Main

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

See Application Class Data Sharing.

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

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

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

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

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

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

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

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

Okay so bestie, you can specify values for multiple event settings and .jfc options by separating them with a comma. In case of a conflict between a parameter and a .jfc option, the parameter will take precedence (no cap, the parameter wins). The whitespace delimiter can be omitted for timespan values, i.e. 20ms. For more information about the settings syntax, see Javadoc of the jdk.jfr package. ✨

To only see warnings and errors from JFR during startup set -Xlog:jfr+startup=warning. That's the vibe check you need. 💅

`-XX:ThreadStackSize=size` 🧵

Sets the Java thread stack size (in kilobytes). Use of a scaling suffix, such as k, results in the scaling of the kilobytes value so that -XX:ThreadStackSize=1k sets the Java thread stack size to 1024*1024 bytes or 1 megabyte. The default value depends on the platform. For example:

Linux/x64: 1024 KB
Linux/Aarch64: 2048 KB
macOS/x64: 1024 KB
macOS/Aarch64: 2048 KB
Windows: The default value depends on virtual memory

Bestie, the default lowkey changes by platform — check the list above! 👀

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. Periodt. 💫

`-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. That's literally so iconic. 🔥

This option can be used if performance regressions are suspected. In a future release compact object headers is expected to become the only mode of operation. Iykyk. 👑

`-XX:-UseCompressedOops` 🔗

Disables the use of compressed pointers. By default, this option is enabled, and compressed pointers are used. This will automatically limit the maximum ergonomically determined Java heap size to the maximum amount of memory that can be covered by compressed pointers. By default this range is 32 GB. 💀

With compressed oops enabled, object references are represented as 32-bit offsets instead of 64-bit pointers, which typically increases performance when running the application with Java heap sizes smaller than the compressed oops pointer range. This option works only for 64-bit JVMs. Ngl, that's a vibe. 🦋

It's possible to use compressed pointers with Java heap sizes greater than 32 GB. See the -XX:ObjectAlignmentInBytes option. Slay. 💅

`-XX:-UseContainerSupport` 🐳

Linux only: The VM now provides automatic container detection support, which allows the VM to determine the amount of memory and number of processors that are available to a Java process running in docker containers. It uses this information to allocate system resources. The default for this flag is true, and container support is enabled by default. It can be disabled with -XX:-UseContainerSupport. Fr fr, it's auto. ✨

Unified Logging is available to help to diagnose issues related to this support. 🕵️‍♀️

Use -Xlog:os+container=trace for maximum logging of container information. See Enable Logging with the JVM Unified Logging Framework for a description of using Unified Logging. That's the tea. ☕

`-XX:+UseLargePages` 📄

Enables the use of large page memory. By default, this option is disabled and large page memory isn't used. Lowkey off by default. 🙈

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. So like, use with caution bestie. ⚠️

`-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. Nope, not by default. 🙅‍♀️

`-XX:VMOptionsFile=filename` 📁

Allows user to specify VM options in a file, for example, java -XX:VMOptionsFile=/var/my_vm_options HelloWorld. So convenient, bestie. 🎀

`-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. Highkey important. 🔒

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. It's giving main character energy for your code. 💅

`-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

Obsessed with this default. 😌

`-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. Lowkey thread-local. 🧵

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. Defaults are giving mystery. 🕵️‍♀️

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

Platform-dependent? It's giving customizable. ✨

`-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. No cap, it changes based on the object type. 🤯

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

Sixteen bytes, iconic. 🎀

`-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. 🧊

Pick your fighter. 💅

`-XX:+BackgroundCompilation` 🎧

Enables background compilation. This option is enabled by default. To disable background compilation, specify -XX:-BackgroundCompilation (this is equivalent to specifying -Xbatch). So like, it's on by default — don't @ me. 😌

`-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

Auto-selected? It's giving smart. 🤖

`-XX:+UseDynamicNumberOfCompilerThreads` 🔄

Dynamically create compiler thread up to the limit specified by -XX:CICompilerCount. This option is enabled by default. Dynamic threads? Slay. 💫

`-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"

Quoting is key, bestie. Don't forget the semicolon situation. 😭

okay so 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

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

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

If the bitmask is not specified, all levels are assumed.

Note: Excluding specific compilation levels may disrupt normal state transitions between the levels, as the VM will not automatically work around the excluded ones. 😭

-XX:CompileCommandFile=_filename_ — Sets the file from which JIT compiler commands are read. By default, the .hotspot_compiler file is used to store commands performed by the JIT compiler.

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

print java/lang/String toString

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

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

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

-XX:+CompilerDirectivesPrint — Prints the directives stack when the program starts or when a new directive is added.

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

-XX:CompileOnly=_methods_ — Sets the list of methods (separated by commas) to which compilation should be restricted. Only the specified methods are compiled.

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

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

-XX:CompileThresholdScaling=_scale_ — Provides unified control of first compilation. This option controls when methods are first compiled for both the tiered and the nontiered modes of operation. The CompileThresholdScaling option has a floating point value between 0 and +Inf and scales the thresholds corresponding to the current mode of operation (both tiered and nontiered). Setting CompileThresholdScaling to a value less than 1.0 results in earlier compilation while values greater than 1.0 delay compilation. Setting CompileThresholdScaling to 0 is equivalent to disabling compilation.
-XX:+DoEscapeAnalysis — Enables the use of escape analysis. This option is enabled by default. To disable the use of escape analysis, specify -XX:-DoEscapeAnalysis.
-XX:InitialCodeCacheSize=_size_ — Sets the initial code cache size (in bytes). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value depends on the platform. The initial code cache size shouldn't be less than the system's minimal memory page size. The following example shows how to set the initial code cache size to 32 KB:

-XX:InitialCodeCacheSize=32k

-XX:+Inline — Enables method inlining. This option is enabled by default to increase performance. To disable method inlining, specify -XX:-Inline.
-XX:InlineSmallCode=_size_ — Sets the maximum code size (in bytes) for already compiled methods that may be inlined. This flag only applies to the C2 compiler. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value depends on the platform and on whether tiered compilation is enabled. In the following example it is set to 1000 bytes:

-XX:InlineSmallCode=1000

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

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

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

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

-XX:FreqInlineSize=325

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

-XX:MaxInlineSize=35

-XX:C1MaxInlineSize=_size_ — Sets the maximum bytecode size (in bytes) of a cold method to be inlined. This flag only applies to the C1 compiler. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. By default, the maximum bytecode size is set to 35 bytes:

-XX:MaxInlineSize=35

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

-XX:MaxTrivialSize=6

-XX:C1MaxTrivialSize= size — sets the max bytecode size (in bytes) of a trivial method that's allowed to get inlined. Important: this flag only applies to the C1 compiler, ok? Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. By default the max bytecode size of a trivial method is set to 6 bytes:

-XX:MaxTrivialSize=6

-XX:MaxNodeLimit= nodes — sets the maximum number of nodes that get used during compilation of a single method. By default the value depends on which features are enabled. In this example the max number of nodes is set to 100,000:

-XX:MaxNodeLimit=100000

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

So like, a nonmethod code segment contains nonmethod code — compiler buffers, the bytecode interpreter, that kind of thing. This code type stays in the code cache forever, no eviction, periodt. This flag is used only if -XX:SegmentedCodeCache is enabled.

-XX:NonProfiledCodeHeapSize= size — sets the size in bytes of the code segment that holds nonprofiled methods. Again, this flag is used only if -XX:SegmentedCodeCache is enabled.
-XX:+OptimizeStringConcat — turns on the optimization of String concatenation operations. This option is enabled by default bestie. To disable the optimization of String concatenation operations, specify -XX:-OptimizeStringConcat.
-XX:+PrintAssembly — turns on printing of assembly code for bytecoded and native methods, using the external hsdis-<arch>.so or .dll library. For 64-bit VM on Windows, it's hsdis-amd64.dll. This lets you see the generated code, which may help you diagnose performance issues. Kind of iconic honestly.

By default this option is disabled and assembly code isn't printed. And heads up: the -XX:+PrintAssembly option has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options. No unlocking, no assembly. 💅

-XX:ProfiledCodeHeapSize= size — sets the size in bytes of the code segment that holds profiled methods. This flag is used only if -XX:SegmentedCodeCache is enabled.
-XX:+PrintCompilation — turns on 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.

-XX:+PrintInlining — turns on printing of inlining decisions. This lets you see which methods are getting inlined.

By default this option is disabled and inlining information isn't printed. The -XX:+PrintInlining option 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 max 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 — otherwise, an error is generated. Also, the max 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 chunky segment. With -XX:+SegmentedCodeCache, separate segments get used for non-method, profiled method, and non-profiled method code. The segments are not resized at runtime. The advantages are: better control of the memory footprint, reduced code fragmentation, and better CPU iTLB (instruction translation lookaside buffer) and instruction cache behavior due to improved locality. Slay.

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

-XX:StartAggressiveSweepingAt= percent — forces stack scanning of active methods to aggressively remove unused code when only the given percentage of the code cache is free. The default value is 10%.
-XX:-TieredCompilation — disables the use of tiered compilation. By default this option is enabled.
-XX:UseSSE= version — enables the use of the SSE instruction set of a specified version. Is set by default to the highest supported version available (x86 only).
-XX:UseAVX= version — enables the use of the AVX instruction set of a specified version. Is set by default to the highest supported version available (x86 only).
-XX:+UseAES — enables hardware-based AES intrinsics for hardware that supports it. This option is on by default on hardware that has the necessary instructions. The -XX:+UseAES is used in conjunction with UseAESIntrinsics. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseAESIntrinsics — enables AES intrinsics. Specifying -XX:+UseAESIntrinsics is equivalent to also enabling -XX:+UseAES. To disable hardware-based AES intrinsics, specify -XX:-UseAES -XX:-UseAESIntrinsics. For example, to enable hardware AES, use the following flags:

-XX:+UseAES -XX:+UseAESIntrinsics

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

-XX:+UseAESCTRIntrinsics — analogous to -XX:+UseAESIntrinsics, 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. This option is 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. This option is on by default for supported platforms. To disable Poly1305 intrinsics, specify -XX:-UsePoly1305Intrinsics. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseBASE64Intrinsics — controls the use of accelerated BASE64 encoding routines for java.util.Base64. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseAdler32Intrinsics — controls the use of the Adler32 checksum algorithm intrinsic for java.util.zip.Adler32. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseCRC32Intrinsics — controls the use of CRC32 intrinsics for java.util.zip.CRC32. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseCRC32CIntrinsics — controls the use of CRC32C intrinsics for java.util.zip.CRC32C. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseSHA — enables hardware-based intrinsics for SHA crypto hash functions on some hardware. The UseSHA option is used in conjunction with the UseSHA1Intrinsics, UseSHA256Intrinsics, and UseSHA512Intrinsics options.

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

This feature is applicable only when using the sun.security.provider.Sun provider for SHA operations. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

To disable all hardware-based SHA intrinsics, specify -XX:-UseSHA. To disable only a particular SHA intrinsic, use the appropriate corresponding option. For example: -XX:-UseSHA256Intrinsics.

-XX:+UseSHA1Intrinsics — enables intrinsics for the SHA-1 crypto hash function. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseSHA256Intrinsics — enables intrinsics for the SHA-224 and SHA-256 crypto hash functions. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseSHA512Intrinsics — enables intrinsics for the SHA-384 and SHA-512 crypto hash functions. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMathExactIntrinsics — enables intrinsification of various java.lang.Math.*Exact() functions. Enabled by default. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMultiplyToLenIntrinsic — enables intrinsification of BigInteger.multiplyToLen(). Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseSquareToLenIntrinsic — turns on intrinsification of BigInteger.squareToLen(). It's enabled by default on platforms that support it. Heads up bestie: flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMulAddIntrinsic — turns on intrinsification of BigInteger.mulAdd(). Enabled by default on platforms that support it. And again, flags controlling intrinsics now need -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMontgomeryMultiplyIntrinsic — turns on intrinsification of BigInteger.montgomeryMultiply(). Enabled by default on platforms that support it. Same deal — flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMontgomerySquareIntrinsic — turns on intrinsification of BigInteger.montgomerySquare(). Enabled by default on platforms that support it. And yep, flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
-XX:+UseCMoveUnconditionally — generates CMove (scalar and vector) instructions regardless of profitability analysis. Like, it just does it, no vibe check on whether it's "worth it" 😌
-XX:+UseCodeCacheFlushing — enables flushing of the code cache before shutting down the compiler. This one is enabled by default. Wanna stop the flushing? Specify -XX:-UseCodeCacheFlushing and she's off 💅
-XX:+UseCondCardMark — enables checking if the card is already marked before updating the card table. This option is disabled by default. Only use it on machines with multiple sockets, where it increases the performance of Java applications that rely on concurrent operations. Multi-socket girlies only, no gatekeeping 🫶
-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. The default value is 1,000. Slay, nested loops ✨
-XX:LoopStripMiningIterShortLoop= number_of_iterations — controls loop strip mining optimization. Loops with the number of iterations less than specified will not have safepoints in them. The 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. Math but make it hardware 💖
-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. Scalar era over, superword era now 🌈

🩺 ADVANCED SERVICEABILITY OPTIONS FOR JAVA 🩺

These java options provide the ability to gather system information and perform extensive debugging. Literally the diagnostic queen toolkit 👑

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

Note: The tools such as jcmd, jinfo, jmap, and jstack shipped with the JDK aren't supported when using the tools from one JDK version to troubleshoot a different JDK version. Version match or it's a no from me 🙈

-XX:+DTraceAllocProbes — Linux and macOS: enable dtrace tool probes for object allocation.
-XX:+DTraceMethodProbes — Linux and macOS: enable dtrace tool probes for method-entry and method-exit.
-XX:+DTraceMonitorProbes — Linux and macOS: enable dtrace tool probes for monitor events.
-XX:+HeapDumpOnOutOfMemoryError — enables the dumping of the Java heap to a file in the current directory by using the heap profiler (HPROF) when a java.lang.OutOfMemoryError exception is thrown by the JVM. You can explicitly set the heap dump file path and name using the -XX:HeapDumpPath option. By default, this option is disabled and the heap isn't dumped when an OutOfMemoryError exception is thrown. This applies only to OutOfMemoryError exceptions caused by Java Heap exhaustion; it does not apply to OutOfMemoryError exceptions thrown directly from Java code, nor by the JVM for other types of resource exhaustion (such as native thread creation errors). Receipts on what it does and doesn't cover, bestie 🧾
-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. Cleanup crew, but make it configurable ✨

-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. Touch grass? No bestie, touch heap 💀
-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. So like, she'll still clean up — just not because you asked 😌
-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 the Java heap gets subdivided into when you're using the garbage-first (G1) collector. okay so 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. lowkey important ✨
-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. heads up, bestie 🫶

-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. the way that target number matters 💜
-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. experimental era 🦋

-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. noted and noted 🎀

-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. cap behavior, but make it GC ⭐
-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. adaptive slay 🌈

-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=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, bestie 💖

-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.

The following example shows how to set the percentage of the initial amount of memory used for the Java heap:

-XX:InitialRAMPercentage=5

-XX:InitialSurvivorRatio=ratio — Sets the initial survivor space ratio used by the throughput garbage collector (which is enabled by the -XX:+UseParallelGC option). Adaptive sizing is enabled by default with the throughput garbage collector by using the -XX:+UseParallelGC option, and the survivor space is resized according to the application behavior, starting with the initial value. If adaptive sizing is disabled (using the -XX:-UseAdaptiveSizePolicy option), then the -XX:SurvivorRatio option should be used to set the size of the survivor space for the entire execution of the application.

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

S=Y/(R+2)

The 2 in the equation denotes two survivor spaces. The larger the value specified as the initial survivor space ratio, the smaller the initial survivor space size.

By default, the initial survivor space ratio is set to 8. If the default value for the young generation space size is used (2 MB), then the initial size of the survivor space is 0.2 MB.

The following example shows how to set the initial survivor space ratio to 4:

-XX:InitialSurvivorRatio=4

-XX:G1IHOP=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.

The following example shows how to set the initiating heap occupancy to 75%:

-XX:G1IHOP=75

-XX:MaxGCPauseMillis=time — Sets a target for the maximum GC pause time (in milliseconds). This is a soft goal, and the JVM will make its best effort to achieve it. Only G1 and Parallel support a maximum GC pause time target. For G1, the default maximum pause time target is 200 milliseconds. soft goal, no cap 🌸

The following example shows how to set the maximum target pause time 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.

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. iconic alias behavior 👑

-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. tuning era fr fr 🧚

-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. metadata main character energy 💀

The following example shows how to set the maximum class metadata size to 256 MB:

-XX:MaxMetaspaceSize=256m

-XX:MaxNewSize=size — Sets the maximum size (in bytes) of the heap for the young generation (nursery). The default value is set ergonomically. nursery says bye at the max size 🍓
-XX:MaxRAMPercentage=_percent_ — sets the max memory the JVM is allowed to use for the Java heap before ergonomics heuristics kick in, expressed as a percentage of the memory available to the JVM process. Default is 25 percent 💖

Heads up: if you set this, it disables automatic use of compressed oops whenever the combined result of this option and any other options that influence the maximum memory ends up bigger than the range of memory addressable by compressed oops. Peep -XX:UseCompressedOops for the whole compressed-oops lore 🫶

Setting the percentage of the maximum memory used for the Java heap goes like this:

-XX:MaxRAMPercentage=75

-XX:MinRAMPercentage=_percent_ — sets the max memory the JVM may use for the Java heap before ergonomics heuristics apply, as a percentage of available memory to the JVM process — but for small heaps specifically. A small heap is a heap of roughly 125 MB. Default value is 50 percent ✨

Example of setting that percentage for small heaps:

-XX:MinRAMPercentage=75

-XX:MaxTenuringThreshold=_threshold_ — the maximum tenuring threshold for adaptive GC sizing. Biggest value allowed is 15, and the default is 15 for the parallel (throughput) collector 🎀

Setting the maximum tenuring threshold to 10:

-XX:MaxTenuringThreshold=10

-XX:MetaspaceSize=_size_ — sets the size of the allocated class metadata space that triggers a garbage collection the first time it's exceeded. That GC threshold then gets increased or decreased depending on how much metadata is actually used. Default size depends on your platform 🌸
-XX:MinHeapFreeRatio=_percent_ — the minimum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space drops below this value, the heap gets expanded. Default is 40% 💅

Wanna minimize Java heap size? Lower the values of the parameters MaxHeapFreeRatio (default 70%) and MinHeapFreeRatio (default 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 heap size without too much performance regression — but fr fr, results may vary greatly depending on your application 😭 Try different values for these parameters until they're as low as possible while still keeping acceptable performance:

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

If you're trying to keep the heap small, also add the option -XX:-ShrinkHeapInSteps. See Performance Tuning Examples for how to use this option to keep the Java heap small by reducing the dynamic footprint for embedded applications 🦋

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

Setting the minimum size of allocated memory to 6 MB in 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. Default is 2. Setting the young-to-old ratio to 1:

-XX:NewRatio=1

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

The young generation region of the heap is where new objects live. GC happens in this region way more often than in the other regions. If the young generation size is too low, you get a whole bunch of minor GCs 😩 If it's too high, you only get full GCs, which can take a long time to finish 💀 The recommendation is to keep the young generation size greater than 25% and less than 50% of the overall heap size.

Setting the initial young generation size to 256 MB in various units:

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

FYI -XX:NewSize is equivalent to -Xmn ✨

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

To set the number of threads for G1 GC to 2:

-XX:ParallelGCThreads=2

-XX:+PrintAdaptiveSizePolicy — turns on printing of information about adaptive-generation sizing. By default this option is disabled 🔥
-XX:SoftRefLRUPolicyMSPerMB=_time_ — sets how long (in milliseconds) a softly reachable object stays active on the heap after the last time it was referenced. Default is one second of lifetime per free megabyte in the heap 💜

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

Setting the value to 2.5 seconds:

-XX:SoftRefLRUPolicyMSPerMB=2500

-XX:-ShrinkHeapInSteps — incrementally reduces the Java heap down to the target size specified by the option -XX:MaxHeapFreeRatio. This option is enabled by default. If you disable it, the Java heap immediately shrinks to the target size instead of needing multiple garbage collection cycles. Disable it if you want to minimize the Java heap size — but you'll likely hit performance degradation when it's disabled ⚠️

See Performance Tuning Examples for how to use the MaxHeapFreeRatio option to keep the Java heap small by reducing the dynamic footprint for embedded applications 🌈

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

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

-XX:SurvivorRatio=_ratio_ — sets the ratio between eden space size and survivor space size. Default is 8. Setting the eden/survivor space ratio to 4:

-XX:SurvivorRatio=4

-XX:TargetSurvivorRatio=_percent_ — sets the desired percentage of survivor space (0 to 100) used after young garbage collection. Default is 50% 🍓

Setting the target survivor space ratio to 30%:

-XX:TargetSurvivorRatio=30

-XX:TLABSize=_size_ — sets the initial size (in bytes) of a thread-local allocation buffer (TLAB). Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. If this option is set to 0, 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 at multiprocessor machines with a large amount of RAM. This option meets GC pause time goals with high probability while maintaining good throughput 👑 The G1 collector is recommended for applications requiring large heaps (sizes of around 6 GB or larger) with limited GC latency requirements (a stable and predictable pause time below 0.5 seconds). By default this option is enabled and G1 is used as the default garbage collector 💅
-XX:+UseGCOverheadLimit — turns on a policy that caps how much of the JVM's time can go to GC before it throws an OutOfMemoryError 💀. This one is enabled by default, and the parallel GC will yeet 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, you can use this to stop apps from running for ages with little to no progress. To turn it off, specify -XX:-UseGCOverheadLimit.
-XX:+UseNUMA — enables performance optimization of an application on a machine with nonuniform memory architecture (NUMA) by increasing how much lower-latency memory the app uses ✨. The default value for this option depends on the garbage collector, iykyk.
-XX:+UseParallelGC — enables the parallel scavenge garbage collector (aka the throughput collector) so your app goes faster by leveraging multiple processors 🔥. By default, this option is disabled and the default collector is used.
-XX:+UseSerialGC — enables the serial garbage collector. Genuinely the best pick for small and simple applications that don't need any special functionality from GC 🎀. 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. So what does it even do? It reduces the memory footprint of String objects on the Java heap by taking advantage of the fact that lots of String objects are literally identical 🫶. Instead of each String object pointing to its own character array, identical String objects can point to and share the same character array. Slay, shared memory.
-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 collector, giving max pause times of a few milliseconds, at some throughput cost. Pause times are independent of what heap size you're using, and it supports heap sizes from 8MB to 16TB. Not a typo, bestie.
-XX:ZAllocationSpikeTolerance=factor — sets the allocation spike tolerance for ZGC. By default it's 2.0. This factor describes the level of allocation spikes to expect — for example, 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 it's set to 0 (disabled).
-XX:ZFragmentationLimit=percent — sets the maximum acceptable heap fragmentation (in percent) for ZGC. By default it's 25. Using a lower value makes the heap get compacted more aggressively — you reclaim more memory, but you pay for it in 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. Super useful if the app is mostly idle or allocates very few objects but you still want to keep the heap size down and let reference processing 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 JVM's memory footprint and hands that memory back so other processes can use it 🎀.
-XX:ZUncommitDelay=seconds — sets how long (in seconds) heap memory must have been unused before it gets uncommitted. By default this is 300 (5 minutes). Committing and uncommitting memory are relatively expensive operations, so heads up: a lower value means heap memory gets uncommitted earlier, at the risk of soon having to commit it again 🙈.
-XX:+UseShenandoahGC — enables the Shenandoah garbage collector. It's a low pause time, concurrent GC, and its pause times are not proportional to the size of the heap ✨. Shenandoah can work with compressed pointers — see -XX:UseCompressedOops for more on compressed pointers.
-XX:ShenandoahGCMode=mode — sets the GC mode for Shenandoah GC. By default this is satb. Among other things, this defines which barriers are in use. Possible mode values:
- 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. Check out 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. By default this is adaptive. This fine-tunes the GC mode you selected by choosing when to start the GC, how much to process on each cycle, and what other features to automatically enable. When -XX:ShenandoahGCMode is generational, the only supported option is the default, adaptive — no notes.

Possible heuristics:

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 removed in a future JDK release 😭. They're still accepted and acted upon, but you get a warning when you use them. Consider this your vibe check.

-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 the use of Java Flight Recorder (JFR) while your app runs. Since JDK 8u40, this option has not been required to use JFR 🦋.
-XX:+ParallelRefProcEnabled — enables parallel reference processing. By default, collectors employing multiple threads perform parallel reference processing if the number of parallel threads to use is larger than one. 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.

🌈 OBSOLETE JAVA OPTIONS 🌈

These java options are still accepted but ignored, and a warning is issued when you use them. It's giving ghosting, honestly.

--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 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 were configured less aggressively.

💀 REMOVED JAVA OPTIONS 💀

Good news bestie: no documented java options have been removed in JDK 27. Like, zero. Nothing. We're so back. 🎀

But if you're feeling nostalgic (or you're debugging something ancient and cursed), the full lists and descriptions of options that got yeeted in previous releases live in the Removed Java Options section of these:

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]

📎 JAVA COMMAND-LINE ARGUMENT FILES 📎

Okay so picture this: your java command is so long it's basically a paragraph. Unreadable. Unhinged. Enter argument files — you use the @ argument files to point at one or more text files stuffed with arguments (options, class names, whatever) that get passed to the java command. The whole point: you can build java commands of any length on any operating system. No more 4,000-character one-liners in your terminal history. Slay. ✨

On the command line you prefix with the at sign (@) to flag a file that holds java options and class names. The moment the java command bumps into a file starting with @, it expands that file's contents into an argument list — exactly like you'd typed them on the command line yourself. Magic, but make it filesystem.

The java launcher keeps expanding argument file contents until it encounters the --disable-@files option. And you can drop --disable-@files anywhere on the command line — including inside an argument file — to stop the @ expansion. Emergency brake, but for @ files. 🛑

Here's the syntax tea for java argument files, all of it, no skips:

The argument file must contain only ASCII characters, or characters in the system default encoding that's ASCII friendly — like UTF-8.
The argument file size must not exceed MAXINT (2,147,483,647) bytes.
The launcher doesn't expand wildcards that live inside an argument file. So an asterisk (*) gets passed on as-is to the starting VM. Like *.java stays *.java and does not get expanded into Foo.java Bar.java ..., even though some command line shells would absolutely do that to you.
Use white space or new line characters to separate the arguments in the file.
White space includes a white space character, \t, \n, \r, and \f.
- For example, you can have a path with a space, like c:\Program Files, and you can write it either as "c:\\Program Files" or — to dodge an escape — as c:\Program" "Files.
Any option containing spaces, like a path component, must be wrapped in quotation (") characters in its entirety. Whole thing, bestie. 💅
A string inside quotation marks may contain the characters \n, \r, \t, and \f. They get converted to their respective ASCII codes.
If a file name has embedded spaces, put the whole file name in double quotation marks.
File names inside an argument file are relative to the current directory, not to the location of the argument file. Important!! Don't get got. 🙈
Use the number sign # in the argument file to write comments. Everything after the # is ignored until the end of the line.
Extra at sign @ prefixes on @-prefixed options act as an escape (the first @ is removed and the rest of the arguments are handed to the launcher literally).
Lines may be continued using the continuation character (\) at the end-of-line. The two lines get concatenated with the leading white spaces trimmed. To prevent trimming those leading white spaces, you can put a continuation character (\) 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 @file inside @file, bestie. We don't do inception here. 💀

🌸 Example of Open or Partial Quotes in an Argument File 🌸

In the argument file,

-cp "lib/
cool/
app/
jars

this is interpreted as:

-cp lib/cool/app/jars

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

To output the following:

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

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

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

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

In the argument file,

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

This is interpreted as:

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

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

In the argument file,

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

This is interpreted as:

-cp /lib/cool app/jars

💖 Examples of Using Single Argument File 💖

You can use a single argument file — like myargumentfile in the example below — to hold all the required java arguments:

java @myargumentfile

🌈 Examples of Using Argument Files with Paths 🌈

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

java @path1/options @path2/classes

🧠 CODE HEAP STATE ANALYTICS 🧠

Overview

Sometimes you just need to peer into the current state of the JVM code heap, you know? Like, to answer questions such as:

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

To give you that insight, a code heap state analytics feature exists — it enables on-the-fly analysis of the code heap. The analytics process splits into two parts:

The first part examines the entire code heap and aggregates all the information believed to be useful or important.
The second part is several independent steps that print the collected info with an emphasis on different aspects of the data.

Data collection and printing both happen on an "on request" basis. No lurking. No background nonsense. 👑

Syntax

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

jcmd pid Compiler.CodeHeap_Analytics [function] [granularity]

If you're just curious how the code heap looks after running a sample workload, use the command line option:

-Xlog:codecache=Trace

And 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. Go read the PDF, it's giving very detailed. 🫶

🔥 ENABLE LOGGING WITH THE JVM UNIFIED LOGGING FRAMEWORK 🔥

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

Synopsis

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

-Xlog:directive

what — Specifies a combination of tags and levels in the form tag1[+tag2...][*][=level][,...]. Unless the wildcard (*) is specified, only log messages tagged with exactly the tags you 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 💅

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

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

-Xlog — Enables JVM logging on an info level.
-Xlog:help — Prints -Xlog usage syntax and available tags, levels, and decorators along with example command lines with explanations.
-Xlog:disable — Turns off all logging and clears all configuration of the logging framework including the default configuration for warnings and errors.
-Xlog[:option] — 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.

🌸 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

Bestie, that means if you just yeet -Xlog in there with no extra sauce, you’re getting warning/error energy only. 💅

🦋 Controlling Logging at Runtime 🦋

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

✨ -Xlog Tags and Levels ✨

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

Available log levels:

off
trace
debug
info
warning
error

Available log tags:

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

🎀 -Xlog Output 🎀

The -Xlog option supports the following types of outputs:

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

When using file=filename, specifying %p, %t and/or %hn in the file name expands to the JVM's PID, startup timestamp and host name, respectively. You can also configure text files to handle file rotation based on file size and a number of files to rotate. For example, to rotate the log file every 10 MB and keep 5 files in rotation, specify the options filesize=10M, filecount=5. The target size of the files isn't guaranteed to be exact, it's just an approximate value. Files are rotated by default with up to 5 rotated files of target size 20 MB, unless configured otherwise. Specifying filecount=0 means that the log file shouldn't be rotated. There's a possibility of the pre-existing log file getting overwritten.

Ngl, that last bit is the “oops, my old log got clapped” warning. Keep an eye on it, bestie. 😭

💜 -Xlog Output Mode 💜

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

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

Write all logging asynchronously.

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

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

🌈 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.

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

🎀 Logging Messages Decorations 🎀

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

💅 Convert GC Logging Flags to Xlog

okay so like, all your old GC logging flags? they've moved on and they're living their best life in -Xlog land now. here's the whole glow-up chart, no cap 🦋

✨ Legacy GC Logging Flags to Xlog Configuration Mapping

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

🫶 Convert Runtime Logging Flags to Xlog

okay bestie, heads up: these legacy flags are no longer recognized and will straight-up cause an error if you use them directly 😭 Use their unified logging equivalent instead, fr fr.

💖 Runtime Logging Flags to Xlog Configuration Mapping

TraceExceptions → -Xlog:exceptions=info — Not Applicable
TraceClassLoading → -Xlog:class+load=level — Use level=info for regular information, or level=debug for additional information. In Unified Logging syntax, -verbose:class equals -Xlog:class+load=info,class+unload=info.
TraceClassLoadingPreorder → -Xlog:class+preorder=debug — Not Applicable
TraceClassUnloading → -Xlog:class+unload=level — Use level=info for regular information, or level=trace for additional information. In Unified Logging syntax, -verbose:class equals -Xlog:class+load=info,class+unload=info.
VerboseVerification → -Xlog:verification=info — Not Applicable
TraceClassPaths → -Xlog:class+path=info — Not Applicable
TraceClassResolution → -Xlog:class+resolve=debug — Not Applicable
TraceClassInitialization → -Xlog:class+init=info — Not Applicable
TraceLoaderConstraints → -Xlog:class+loader+constraints=info — Not Applicable
TraceLoaderData → -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. queue them up, they're all lowkey iconic 💅

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

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

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

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

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

🌈 Complex -Xlog Usage Examples

The following describes a few complex examples of using the -Xlog option. buckle up bestie, these are the main character ones 👑

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

💖 VALIDATE JAVA VIRTUAL MACHINE FLAG ARGUMENTS 💖

okay so bestie, this section is literally the vibe check for JVM flags. 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. No cap.

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. iykyk.

Range and constraints are validated either when all flags have their values set during JVM initialization or a flag's value is changed during runtime (for example using the jcmd tool). 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. periodt — bad flag, bye. 😭

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

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

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

For example,

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

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

size_t NewSize         [   ...                  ] {product}

This helps to identify the flags that need to be implemented. The automatic testing framework can skip those flags that don't have values and aren't implemented. slay, automation. 💅

🦋 LARGE PAGES 🦋

You use large pages, also known as huge pages, as memory pages that are significantly larger than the standard memory page size (which varies depending on the processor and operating system). Large pages optimize processor Translation-Lookaside Buffers. it's giving big-page energy. ✨

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

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

Linux and Windows support large pages.

🌸 Large Pages Support for Linux 🌸

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

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

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

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

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

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

It is always recommended to check the value of nr_hugepages after the request to make sure the kernel was able to allocate the requested number of large pages.

Note: The values contained in /proc and /sys reset after you reboot your system, so may want to set them in an initialization script (for example, rc.local or sysctl.conf). do not forget, bestie. 🫶

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. the way that plot twist ate. 😭

🌸 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, even the admin. iconic. 👑

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

Classes in the CDS archive are stored in an optimized format that's about 2 to 5 times larger than classes stored in JAR files or the JDK runtime image. Therefore, it's a good idea to archive only those classes that are actually used by your application. These usually are just a small portion of all available classes. For example, your application may use only a few APIs provided by a large library. archive smart, not hard. ✨

✨ 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.

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. history lesson but make it JVM. 🦋

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

-XX:SharedArchiveFile=<static_archive>

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

-XX:SharedArchiveFile=<dynamic_archive>

✨ 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. vibe check 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. deterministic output ate, and so does -Xmixed when you don't need it. 🔥

💅 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’re created with one fewer step than the comparable static archive. bestie, dynamic archives are lowkey efficient 💅

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 we 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

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

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

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

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

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

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

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

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

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

🦋 Restrictions on Class Path and Module Path 🦋

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

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

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

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

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

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

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

Mismatched values for property jdk.module.addmods

runtime jdk.incubator.vector dump time jdk.jconsole

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

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

✨ AHEAD-OF-TIME CACHE ✨

Okay bestie, gather round: the JDK supports ahead-of-time (AOT) optimizations, and those can happen before an application is executed. One example is Class Data Sharing (CDS), as described above, which parses classes ahead of time. AOT optimizations can improve the start-up and warm-up performance of Java applications. 💅

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

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.

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

Training: We execute the application with a representative workload 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.

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.
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.

-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 replaced by the JVM's startup timestamp. (After replacement there must be no further occurrences of %p or %t, to prevent problems with sub-processes.) For example:

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 get into the tuning tea. 🫶

🔥 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.

🚪 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

💖 That's the whole page, bestie — this is the JAVA(1) manual page from JDK 27-ea, dated 2026. Periodt, bye! 🎀

📦 where this page comes from

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💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-06: 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 💜

-XX:ThreadStackSize=size 🧵

-XX:-UseCompactObjectHeaders 📦

-XX:-UseCompressedOops 🔗

-XX:-UseContainerSupport 🐳

-XX:+UseLargePages 📄

-XX:+UseTransparentHugePages 🌫️

-XX:+AllowUserSignalHandlers 📡

-XX:VMOptionsFile=filename 📁

-XX:UseBranchProtection=mode 🛡️

🔥 ADVANCED JIT COMPILER OPTIONS FOR JAVA 🔥

-XX:AllocateInstancePrefetchLines=lines 🚀

-XX:AllocatePrefetchDistance=size 📏

-XX:AllocatePrefetchInstr=instruction 🎛️

-XX:AllocatePrefetchLines=lines 🧮

-XX:AllocatePrefetchStepSize=size 📐

-XX:AllocatePrefetchStyle=style 🎨

-XX:+BackgroundCompilation 🎧

-XX:CICompilerCount=threads 🧵

-XX:+UseDynamicNumberOfCompilerThreads 🔄

-XX:CompileCommand=command,method[,option] 🎯

🩺 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 📎

🌸 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

✨ Legacy GC Logging Flags to Xlog Configuration Mapping

🫶 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 🚀

`-XX:ThreadStackSize=size` 🧵

`-XX:-UseCompactObjectHeaders` 📦

`-XX:-UseCompressedOops` 🔗

`-XX:-UseContainerSupport` 🐳

`-XX:+UseLargePages` 📄

`-XX:+UseTransparentHugePages` 🌫️

`-XX:+AllowUserSignalHandlers` 📡

`-XX:VMOptionsFile=filename` 📁

`-XX:UseBranchProtection=mode` 🛡️

`-XX:AllocateInstancePrefetchLines=lines` 🚀

`-XX:AllocatePrefetchDistance=size` 📏

`-XX:AllocatePrefetchInstr=instruction` 🎛️

`-XX:AllocatePrefetchLines=lines` 🧮

`-XX:AllocatePrefetchStepSize=size` 📐

`-XX:AllocatePrefetchStyle=style` 🎨

`-XX:+BackgroundCompilation` 🎧

`-XX:CICompilerCount=threads` 🧵

`-XX:+UseDynamicNumberOfCompilerThreads` 🔄

`-XX:CompileCommand=command,method[,option]` 🎯

🎀 Creating CDS Archive Files with `jcmd` 🎀

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