JAVA(1)

📦 version 25.0.5.0.3.u1, in arch rolling

🎀 JAVA(1) 🎀

okay bestie, you've landed on the java page from the JDK Commands manual, and honestly? it's a whole vibe. grab a boba 🧋, we're talking about launching Java applications the way the JVM intended. ✨

🌸 NAME 🌸

java — launch a Java application. that's it, that's the whole thing. and yet there are like six different ways to do it 😭 keep reading.

🦋 SYNOPSIS 🦋

so. to launch a class file, you go:

java [options] mainclass [args ...]

to launch the main class inside a JAR file, you go:

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

to launch the main class inside a module, you go:

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

or, alternate spelling, same energy:

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

and to launch a source-file program, you go:

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

now the deets on each lil piece 👇

options — optional, and yes it means what you think: command-line options, separated by spaces. want the full menu? see Overview of Java Options for a description of the available options. 📋
mainclass — the name of the class to be launched, obviously. everything you type on the command line after classname? those are the arguments for the main method. periodt. 💅
-jar jarfile — executes a program that's all wrapped up inside a JAR file. jarfile is the name of a JAR file whose manifest has a line shaped like Main-Class:classname, and that line defines the class with the public static void main(String[] args) method that serves as your application's starting point. and heads up: when you use -jar, the JAR you named is the source of all user classes, and every other class path setting gets ignored. rude but true. if you're doing JAR file stuff, go see jar. 🫙
-m or --module module[/mainclass] — executes the main class in a module. which class? the one specified by mainclass if you gave one, or, if you didn't, the value in the module. in other words: mainclass is there for when the module doesn't specify it, or to override the value when it does. 💫 see Standard Options for Java.
source-file — only used to launch a source-file program. it specifies the source file that contains the main class when you're in 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 get passed as arguments to the main class. 🎁

💖 DESCRIPTION 💖

the java command starts a Java application. how? by starting the Java Virtual Machine (JVM), loading the class you specified, and calling that class's main() method. main character energy, honestly. 👑

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 declaration looks like this, exactly:

public static void main(String[] args)

in source-file mode, the java command can launch a class that's declared in a source file. see Using Source-File Mode to Launch Source-Code Programs for a description of how that mode works. 🧚

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. and if -jar is specified, then its argument is the name of the JAR file containing the class and resource files for the application — and the startup class must be indicated by the Main-Class manifest header in its manifest file. no header, no launch, that's the rule. 💀

arguments after the class file name or the JAR file name? those get passed to the main() method. easy.

🪟 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. however — and this is important — the javaw launcher will display a dialog box with error information if a launch fails. so you're not flying blind, you just don't get a whole terminal. 🙈

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

to launch a class that's declared in a source file, you run the java launcher in source-file mode. entering source-file mode is decided by two items on the java command line:

the first item on the command line that is not an option or part of an option. in other words, the item 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 — and 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. no sneaking ahead to future APIs, bestie. 🚫

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 (25) and a limited number of previous releases, detailed in the command-line help for javac, under the --source and --release options. ⭐

if the file does not have the .java extension, the --source option must be used to tell the java command to use source-file mode. the --source option is 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. scripts deserve love too. 🍓

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 you placed after the name of the source file in the original command line get passed to the compiled class when it's executed. 🎀

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

java HelloWorld.java

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

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

where <source-root> is computed

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

the launcher scans the options specified before the source file for any that are relevant in order to compile the source file.

this includes: --class-path, --module-path, --add-exports, --add-modules, --limit-modules, --patch-module, --upgrade-module-path, and any variant forms of those options. it also includes the new --enable-preview option, described in JEP 12. 🔍

no provision is made to pass any additional options to the compiler, such as -processor or -Werror. sorry, no extras. 🙅
command-line argument files (@-files) may be used in the standard way. long lists of arguments for either the VM or the program being invoked may be placed in files specified on the command line by prefixing the filename with an @ character. 📁

in source-file mode, compilation proceeds as follows:

any command-line options that are relevant to the compilation environment are taken into account. these include: --class-path/-classpath/-cp, --module-path/-p, --add-exports, --add-modules, --limit-modules, --patch-module, --upgrade-module-path, --enable-preview. 🧾
the root of the source tree, <source-root>, is computed from the package of the class being launched. for example, if HelloWorld.java declared its classes to be in the hello package, then the file HelloWorld.java is expected to reside in the directory somedir/hello/. in this case, somedir is computed to be the root of the source tree. 🌳
the root of the source tree serves as the source-path for compilation, so that other source files found in that tree and needed by HelloWorld could be compiled. teamwork makes the dream work. 🫂
annotation processing is disabled, as if -proc:none is in effect. ✂️
if a version is specified, via the --source option, the value is used as the argument for an implicit --release option for the compilation. this sets both the source version accepted by the compiler and the system API that may be used by the code in the source file. 🎯
if --enable-preview is specified, the --source N arguments can be omitted. if the Java runtime version is N, then --release N is implied when compiling source files. sneaky but iconic. ✨
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. first one wins, no favorites. 🥇
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. duh. 💅
it is an error if there is a class on the application class path whose name is the same as that of the class to be executed. name collision = instant no. ⚠️

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, this env var is lowkey iconic. JDK_JAVA_OPTIONS prepends its content to the options parsed from the command line (yes, it goes first, main character energy). The content is a list of arguments separated by white-space characters (as determined by isspace()). Those get prepended to the command line arguments you pass to the java launcher. The encoding requirement for the env var is exactly the same as the java command line on your system. And the content is treated in the same manner as whatever you specified on the command line. fr fr.

Single (') or double (") quotes can be used to enclose arguments that contain whitespace characters. Everything between the open quote and the first matching close quote is preserved — all it does is remove the pair of quotes. But 🫶 if a matching quote is not found, the launcher will abort with an error message. @-files are supported exactly as they are specified in the command line. Any wildcard literal * in the JDK_JAVA_OPTIONS environment variable content isn't expanded and gets passed as-is to the starting VM.

Now the part where it gets strict: 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. 💀 And whenever JDK_JAVA_OPTIONS is set, the launcher prints a message to stderr as a reminder.

Example, bestie:

$ 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, you can drop a comma-separated list of options specific to that library. If the option -agentlib:foo is specified, then the JVM attempts to load the library named foo using the platform specific naming conventions and locations:
- Linux and other POSIX-like platforms: the JVM attempts to load the library named libfoo.so in the location specified by the LD_LIBRARY_PATH system variable.
- macOS: the JVM attempts to load the library named libfoo.dylib in the location specified by the DYLD_LIBRARY_PATH system variable.
- Windows: the JVM attempts to load the library named foo.dll in the location specified by the PATH system variable.

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

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

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

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

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

--finalization=value — controls whether the JVM performs finalization of objects. Valid values are "enabled" and "disabled". Finalization is enabled by default, so the value "enabled" does nothing. The value "disabled" disables finalization, so that no finalizers are invoked. 🎀
--module-path modulepath... or -p modulepath — specifies where to find application modules with a list of path elements. The elements of a module path can be a file path to a module or a directory containing modules. Each module is either a modular JAR or an exploded-module directory. On Windows, semicolons (;) separate path elements in this list; on other platforms it is a colon (:). 🌸
--upgrade-module-path modulepath... — okay so this one tells the runtime where to find module replacements of upgradeable modules in the runtime image, using a list of path elements. A module path element can be a file path to a module or a directory containing modules. Each module is either a modular JAR or an exploded-module directory. On Windows, semicolons (;) separate path elements in this list; on other platforms it is a colon (:). slay 💅
--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. periodt ✨
--list-modules — lists the observable modules and then exits. short, sweet, iconic 🎀
-d module_name or --describe-module module_name — describes a specified module and then exits. iykyk 🦋
--dry-run — creates the VM but doesn't execute the main method. This --dry-run option might be useful for validating the command-line options such as the module system configuration. very "test the vibe before the party" 💛
--validate-modules — validates all modules and exit. This option is helpful for finding conflicts and other errors with modules on the module path. lowkey a safety check bestie 🍑
-Dproperty=value — sets a system property value. The -Dproperty part is the property variable: a string with no spaces that represents the name of the property. The value part 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"). yes, the exact -Dproperty token is right there, no cap 💖
-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

the way that command just stacks the vibes 💜

-disablesystemassertions or -dsa — disables assertions in all system classes. system-class business only 🧊
-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

assertions are literally getting their moment 🌟

-enablesystemassertions or -esa — enables assertions in all system classes. system classes, you're up 🫶
-help, -h, or -? — prints the help message to the error stream. error stream, not output stream, bestie 😭
--help — prints the help message to the output stream. this one is the polite output-stream version 💖
-javaagent:jarpath[=options] — loads the specified Java programming language agent. See java.lang.instrument. agent behavior, iconic 🕶️
--show-version — prints the product version to the output stream and continues. output stream + keeps going ⭐
-showversion — prints the product version to the error stream and continues. error stream + keeps going 🍓
--show-module-resolution — shows module resolution output during startup. module gossip at 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.

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. main character entrance energy 🎬

-verbose:class — displays information about each loaded class. class-by-class tea 🫖
-verbose:gc — displays information about each garbage collection (GC) event. GC events, narrated ♻️
-verbose:jni — displays information about the use of native methods and other Java Native Interface (JNI) activity. JNI activity, spicy 🌶️
-verbose:module — displays information about the modules in use. module roll call 📣
--version — prints product version to the output stream and exits. output stream + bye 👋
-version — prints product version to the error stream and exits. error stream + bye 💫
-X — prints the help on extra options to the error stream. extra-options help, error-stream edition 🔥
--help-extra — prints the help on extra options to the output stream. extra-options help, output-stream edition ✨
@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. long command lines could never 💅

🔥 EXTRA OPTIONS FOR JAVA 🔥

The following java options are general purpose options that are specific to the Java HotSpot Virtual Machine. bestie, we're in HotSpot territory now 🌈

-Xbatch — disables background compilation. By default, the JVM compiles the method as a background task, running the method in interpreter mode until the background compilation is finished. The -Xbatch flag disables background compilation so that compilation of all methods proceeds as a foreground task until completed. This option is equivalent to -XX:-BackgroundCompilation. no more background multitasking, it's foreground only now 💅
-Xbootclasspath/a:directories|zip|JAR-files — specifies a list of directories, JAR files, and ZIP archives to append to the end of the default bootstrap class path. On Windows, semicolons (;) separate entities in this list; on other platforms it is a colon (:). append era 🎀
-Xcheck:jni — performs additional checks for Java Native Interface (JNI) functions.

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

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

The following checks only result in warnings being printed:

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

Expect a performance degradation when this option is used. fr fr, this one is not free 💀

-Xcomp — testing mode to exercise JIT compilers. This option should not be used in production environments. do NOT use this in production, bestie 🚨
-Xdebug — does nothing; deprecated for removal in a future release. literally a no-op, not me wasting your time 🙈
-Xdiag — shows additional diagnostic messages. extra diagnostics, extra clues 🔍
-Xint — runs your app in interpreted-only mode 🐢. Compilation to native code is fully off, and every single bytecode gets executed by the interpreter. The whole speed glow-up from the just-in-time (JIT) compiler? Not here. It's giving "walking instead of teleporting," but hey, sometimes that's the vibe you need.
-Xinternalversion — displays JVM version info that's more detailed than -version gives you, then dips ✌️.
*-Xlog:option* — configure or enable logging with the Java Virtual Machine (JVM) unified logging framework ✨. See Enable Logging with the JVM Unified Logging Framework for the whole lore, iykyk.
-Xmixed — executes all bytecode via the interpreter except for the hot methods, which get compiled to native code 🔥. On by default, bestie. Use -Xint to switch it off.
-Xmn size — sets the initial and maximum size (in bytes) of the heap for the young generation (aka the nursery 🌱) in the generational collectors. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. This young-gen region of the heap is where new objects live — GC happens here way more often than in other regions.

If the young generation is too small, you get a lot of minor garbage collections 💀. If it's too large, then only full garbage collections happen, and those can take forever to complete 😭. Rec: don't set the young-gen size at all for the G1 collector, and for other collectors keep it greater than 25% and less than 50% of the overall heap size. The following examples show how to set the initial and maximum size of young generation to 256 MB using various units:

-Xmn256m
-Xmn262144k
-Xmn268435456

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

-Xms size — sets the minimum and the initial size (in bytes) of the heap. This value must be a multiple of 1024 and greater than 1 MB, no exceptions. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. The following examples show how to set the size of allocated memory to 6 MB using various units:

-Xms6291456
-Xms6144k
-Xms6m

If you don't set this option, the initial size gets 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 with -Xmn or -XX:NewSize. Also note: -XX:InitialHeapSize can also set the initial heap size — and if it appears after -Xms on the command line, then the initial heap size gets set to the value specified with -XX:InitialHeapSize. Order matters, bestie ✨.

-Xmx size — specifies the maximum size (in bytes) of the heap. This value must be a multiple of 1024 and greater than 2 MB. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. The default value is chosen at runtime based on your system configuration, very "I'll decide when I get there." 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 app are left untouched during GC and are always considered live.

⚠️ This can result in more memory being permanently occupied, which — if you're not careful — throws an out-of-memory exception. She's iconic but she does not clean up after herself 😭.

-Xrs — reduces the use of operating system signals by the JVM 🎀. Shutdown hooks enable the orderly shutdown of a Java application by running user cleanup code (like closing database connections) at shutdown, even if the JVM terminates abruptly.
- Non-Windows:
  - The JVM catches signals to implement shutdown hooks for unexpected termination. It 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. It 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 like this, the -Xrs option can be used. When -Xrs is used, the JVM doesn't install a console control handler, implying it doesn't watch for or process CTRL_C_EVENT, CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, or CTRL_SHUTDOWN_EVENT.

There are two consequences of specifying -Xrs:

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

*-Xshare:mode* — sets the class data sharing (CDS) mode 🦋. Possible mode arguments for this option include:
- auto — use shared class data if possible (this is the 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 can't be used (for example, when certain VM parameters are changed, or when a different JDK is used). This option should not be used in production environments. Fr fr, do not ship this 💀.

off — do not attempt to use shared class data.

-XshowSettings — shows all settings, then just… continues 🚶‍♀️.
*-XshowSettings:category* — shows settings and continues. Possible category arguments for this option include:
- all — shows all categories of settings in verbose detail 👑.
- locale — shows settings related to locale.
- properties — shows settings related to system properties.
- security — shows all settings related to security. Sub-category arguments for security include:
  - security:all : shows all security settings
  - security:properties : shows security properties
  - security:providers : shows static security provider settings
  - security:tls : shows TLS related security settings
- vm — shows the settings of the JVM.
- system — Linux only: shows host system or container configuration and continues 🌸.
-Xss size — sets the thread stack size (in bytes). Append k or K for KB, m or M for MB, or g or G for GB. The actual size may get rounded up to a multiple of the system page size, as required by the operating system. The default value depends on the platform. For example:
- Linux/x64: 1024 KB
- Linux/Aarch64: 2048 KB
- macOS/x64: 1024 KB
- macOS/Aarch64: 2048 KB
- Windows: the default value depends on virtual memory. The following examples set the thread stack size to 1024 KB in different units:

-Xss1m
-Xss1024k
-Xss1048576

This option is similar to -XX:ThreadStackSize 💜.

--add-reads module=target-module(,target-module)* — updates module 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 the 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 the 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* — allow or deny usage of the unsupported API sun.misc.Unsafe. value is one of:
- 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 — okay so this value lets you use the memory-access methods, but it gives you a one-line warning and a stack trace whenever any memory-access method is used.
deny — this one fully says no: it disallows use of the memory-access methods by throwing an UnsupportedOperationException on every usage.

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

✨ EXTRA OPTIONS FOR MACOS ✨

These extra options are macOS specific, bestie. 🍓

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

💅 ADVANCED OPTIONS FOR JAVA 💅

These java options can be used to enable other advanced options. Very main-character energy, honestly. 👑

-XX:+UnlockDiagnosticVMOptions — unlocks the options intended for diagnosing the JVM. By default, this option is disabled and diagnostic options aren't available. Command line options that are enabled with the use of this option are not supported. If you encounter issues while using any of these options, it is very likely that you will be required to reproduce the problem without using any of these unsupported options before Oracle Support can assist with an investigation. It is also possible that any of these options may be removed or their behavior changed without any warning. 🚨
-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. Runtime era, let's go. 🔥

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

-XX:+ExtensiveErrorReports — enables the reporting of more extensive error information in the ErrorFile. This option can be turned on in environments where maximal information is desired — even if the resulting logs may be quite large and/or contain information that might be considered sensitive. The information can vary from release to release, and across different platforms. By default this option is disabled.
-XX:FlightRecorderOptions=parameter=value (or) -XX:FlightRecorderOptions:parameter=value — sets the parameters that control the behavior of JFR. Multiple parameters can be specified by separating them with a comma. The following list contains the available JFR parameter=value entries:
- globalbuffersize=size — specifies the total amount of primary memory used for data retention. The default value is based on the value specified for memorysize. Change the memorysize parameter to alter the size of global buffers.
- maxchunksize=size — specifies the maximum size (in bytes) of the data chunks in a recording. Append m or M to specify the size in megabytes (MB), or g or G to specify the size in gigabytes (GB). By default, the maximum size of data chunks is set to 12 MB. The minimum allowed is 1 MB.
- memorysize=size — determines how much buffer memory should be used, and sets the globalbuffersize and numglobalbuffers parameters based on the size specified. Append m or M to specify the size in megabytes (MB), or g or G to specify the size in gigabytes (GB). By default, the memory size is set to 10 MB.
- numglobalbuffers — specifies the number of global buffers used. The default value is based on the memory size specified. Change the memorysize parameter to alter the number of global buffers.
- old-object-queue-size=number-of-objects — maximum number of old objects to track. By default, the number of objects is set to 256.
- preserve-repository={true|false} — specifies whether files stored in the disk repository should be kept after the JVM has exited. If false, files are deleted. By default, this parameter is disabled.
- repository=path — specifies the repository (a directory) for temporary disk storage. By default, the system's temporary directory is used.
- retransform={true|false} — specifies whether event classes should be retransformed using JVMTI. If false, instrumentation is added when event classes are loaded. By default, this parameter is enabled.
- stackdepth=depth — stack depth for stack traces. By default, the depth is set to 64 method calls. The maximum is 2048. Values greater than 64 could create significant overhead and reduce performance.
- threadbuffersize=size — specifies the per-thread local buffer size (in bytes). By default, the local buffer size is set to 8 kilobytes, with a minimum value of 4 kilobytes. Overriding this parameter could reduce performance and is not recommended.
-XX:LargePageSizeInBytes=size — sets the maximum large page size (in bytes) used by the JVM. The size argument must be a valid page size supported by the environment to have any effect. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM will use the default large page size for the environment as the maximum size for large pages. See Large Pages.

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

-XX:LargePageSizeInBytes=1g

-XX:MaxDirectMemorySize=size — caps the total size (in bytes) of the java.nio package's direct-buffer allocations. Slap a k or K on the end for kilobytes, m or M for megabytes, g or G for gigabytes. If you don't set it, the flag is literally ignored and the JVM picks a size for NIO direct-buffer allocations all on its own. Bestie, here's the same 1024 KB written three ways so you can see the units doing their thing:

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

-XX:-MaxFDLimit — turns off 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. So like, the "helpful" thing is lowkey holding you back there 🌸
-XX:NativeMemoryTracking=mode — picks the mode for tracking JVM native memory usage. The possible mode arguments are:
- off — instructs the JVM not to track native memory usage. This is the default behavior if you don't specify the -XX:NativeMemoryTracking option at all.
- summary — tracks memory usage only by JVM subsystems, like Java heap, class, code, and thread.
- detail — on top of tracking memory usage by JVM subsystems, it also tracks memory usage by individual CallSite, individual virtual memory region and its committed regions.
-XX:TrimNativeHeapInterval=millis — the interval, in ms, at which the JVM will trim 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. Heads up: this option is only supported on Linux with GNU C Library (glibc) 🐧
-XX:+NeverActAsServerClassMachine — enables the "Client VM emulation" mode, which only uses the C1 JIT compiler, a 32Mb CodeCache and the Serial GC. The maximum amount of memory the JVM may use (controlled by the -XX:MaxRAM=n flag) is set to 1GB by default. The string "emulated-client" gets added to the JVM version string. By default the flag is set to true only on Windows in 32-bit mode and false in all other cases. The "Client VM emulation" mode will not be enabled if any of these flags are used on the command line:

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

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

4GB * ObjectAlignmentInBytes

Note: as the alignment value increases, the unused space between objects also increases. As a result, you may not realize any benefits from using compressed pointers with large Java heap sizes. It's giving "trade-off", no cap 💀

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

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

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

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

-XX:OnOutOfMemoryError=string — sets a custom command, or a series of semicolon-separated commands, to run when an OutOfMemoryError exception is first thrown by the JVM. If the string contains spaces, then it must be enclosed in quotation marks. For an example of a command string, see the description of the -XX:OnError option. Important detail, bestie: 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). Read that twice ✨
-XX:+PrintCommandLineFlags — enables printing of ergonomically selected JVM flags that appeared on the command line. It can be useful to know the ergonomic values set by the JVM, such as the heap space size and the selected garbage collector. By default this option is disabled and flags aren't printed.
-XX:+PreserveFramePointer — selects between using the RBP register as a general purpose register (-XX:-PreserveFramePointer) and using the RBP register to hold the frame pointer of the currently executing method (-XX:+PreserveFramePointer). If the frame pointer is available, then external profiling tools (for example, Linux perf) can construct more accurate stack traces. Profilers literally say thank you 🫶
-XX:+PrintNMTStatistics — enables printing of collected native memory tracking data at JVM exit when native memory tracking is enabled (see -XX:NativeMemoryTracking). By default this option is disabled and native memory tracking data isn't printed.
-XX:SharedArchiveFile=path — specifies the path and name of the class data sharing (CDS) archive file. See Application Class Data Sharing.
-XX:+VerifySharedSpaces — if this option is specified, the JVM will load a CDS archive file only if it passes an integrity check based on CRC32 checksums. The purpose of this flag is to check for unintentional damage to CDS archive files in transmission or storage. And this part matters: to guarantee the security and proper operation of CDS, the user must ensure that the CDS archive files used by Java applications cannot be modified without proper authorization. That's on you, not the flag, periodt 🔥
-XX:SharedArchiveConfigFile=shared_config_file — specifies additional shared data added to the archive file.
-XX:SharedClassListFile=file_name — specifies the text file that contains the names of the classes to store in the class data sharing (CDS) archive. This file contains the full name of one class per line, except slashes (/) replace dots (.). For example, to specify the classes java.lang.Object and hello.Main, create a text file that contains the following two lines:

java/lang/Object
hello/Main

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

-XX:+ShowCodeDetailsInExceptionMessages — enables printing of improved NullPointerException messages. When an application throws a NullPointerException, the option enables the JVM to analyze the program's bytecode instructions to determine precisely which reference is null, and describes the source with a null-detail message. The null-detail message is calculated and returned by NullPointerException.getMessage(), and will be printed as the exception message along with the method, filename, and line number. By default this option is enabled — iconic behavior honestly 💅
-XX:+ShowMessageBoxOnError — enables the display of a dialog box when the JVM experiences an irrecoverable error. This prevents the JVM from exiting and keeps the process active so that you can attach a debugger to it to investigate the cause of the error. By default this option is disabled.
-XX:StartFlightRecording:parameter=value — starts a JFR recording for the Java application. This option is equivalent to the JFR.start diagnostic command that starts a recording during runtime. -XX:StartFlightRecording:help prints available options and example command lines. You can set the following parameter=value entries when starting a JFR recording:
- delay=time — specifies the delay between the Java application launch time and the start of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 10m means 10 minutes). By default there's no delay, and this parameter is set to 0.
- disk={true|false} — specifies whether to write data to disk while recording. By default this parameter is enabled.
- dumponexit={true|false} — specifies if the running recording is dumped when the JVM shuts down. If enabled and a filename 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 — tells the recording where to live once it's stopped, path and filename both. Real talk examples of what that looks like:
- recording.jfr
- /home/user/recordings/recording.jfr
- c:\recordings\recording.jfr

If you slip %p and/or %t in there, bestie, they expand to the JVM's PID and the current timestamp respectively ✨. You can also just hand it a directory, in which case the filename gets generated from the PID and the current date inside that directory. Cute, right?

name=identifier — takes both the name and the identifier of a recording. Both. At once. 💅
maxage=time — the maximum age of disk data to keep for the recording. Only valid when the disk parameter is set to true, no exceptions 🫶. Append s for seconds, m for minutes, h for hours, or d for days (so 30s literally means 30 seconds). By default the max age isn't limited, and this parameter is set to 0s.
maxsize=size — max size in bytes of disk data to keep. Again: valid only when disk is true. The value must not be less than the value for the maxchunksize parameter set with -XX:FlightRecorderOptions — read that twice, it's a real constraint. Append m or M for megabytes, or g or G for gigabytes. Default? Not limited, set to 0.
path-to-gc-roots={true|false} — do we collect the path to garbage collection (GC) roots at the end of a recording? By default, nope, disabled. Here's the tea: the path to GC roots is genuinely useful for finding memory leaks, but collecting it is time-consuming 😭. So enable this only when you start a recording for an application you suspect has a memory leak. And if the settings parameter is set to profile, the stack trace from where the potential leaking object was allocated gets included in the info collected. Chef's kiss 🔥.
report-on-exit=identifier — the name of the view to display when the Java Virtual Machine (JVM) shuts down. Wanna specify more than one view? Just use report-on-exit repeatedly, stack 'em. This option is not available if the disk option is set to false. For a list of available views, run jfr help view. By default, no report is generated.
settings=path — path and name of the event settings file (of type JFC). By default the default.jfc file is used, which lives in JAVA_HOME/lib/jfr. That default settings file collects a predefined set of information with low overhead, so it barely touches performance and can be used with recordings that run continuously — iconic behaviour honestly 👑.

A second settings file is also provided, profile.jfc, which gives you more data than the default configuration but can have more overhead and impact performance. Use that one for short periods of time when you need more information. Short. Periods. 🙈

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 '+'. 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 — parameter wins, always 💅. The whitespace delimiter can be omitted for timespan values, i.e. 20ms. For more information about the settings syntax, see the Javadoc of the jdk.jfr package. And to only see warnings and errors from JFR during startup set -Xlog:jfr+startup=warning.

`-XX:ThreadStackSize=size`

Sets the Java thread stack size, in kilobytes. Now listen, the scaling suffix is a whole vibe: using one like k scales the kilobytes value, so -XX:ThreadStackSize=1k sets the Java thread stack size to 1024*1024 bytes, aka 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

And here's how you'd set the thread stack size to 1 megabyte in different units:

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

This option is similar to -Xss. Siblings, basically 🫶.

`-XX:+UseCompactObjectHeaders`

Enables compact object headers. By default, this option is disabled. Enabling it reduces memory footprint in the Java heap by 4 bytes per object (on average) and often improves performance. Slay.

The feature remains disabled by default while it continues to be evaluated. In a future release it is expected to be enabled by default, and eventually will be the only mode of operation. So like… it's coming, bestie. Watch this space 👀.

`-XX:-UseCompressedOops`

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

With compressed oops enabled, object references are represented as 32-bit offsets instead of 64-bit pointers, which typically increases performance when running the application with Java heap sizes smaller than the compressed oops pointer range. This option works only for 64-bit JVMs, no cap. And yes, it's possible to use compressed pointers with Java heap sizes greater than 32 GB — see the -XX:ObjectAlignmentInBytes option for that little trick.

`-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 available to a Java process running in docker containers. It uses this info to allocate system resources. The default for this flag is true, and container support is enabled by default. It can be disabled with -XX:-UseContainerSupport.

Unified Logging is available to help diagnose issues related to this support. Use -Xlog:os+container=trace for maximum logging of container information. See Enable Logging with the JVM Unified Logging Framework for a description of using Unified Logging.

`-XX:+UseLargePages`

Enables the use of large page memory. By default this option is disabled and large page memory isn't used. See Large Pages. That's the whole thing, short and sweet 🎀.

`-XX:+UseTransparentHugePages`

Linux only again 🌈. Enables the use of large pages that can dynamically grow or shrink. This option is disabled by default.

Heads up: 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. Not your production main character moment, iykyk 💀.

`-XX:+AllowUserSignalHandlers`

Non-Windows: enables installation of signal handlers by the application. By default this option is disabled and the application isn't allowed to install signal handlers.

`-XX:VMOptionsFile=filename`

Lets you specify VM options in a file instead of vomiting them all onto the command line. For example:

java -XX:VMOptionsFile=/var/my_vm_options HelloWorld

`-XX:UseBranchProtection=mode`

Linux AArch64 only: specifies the branch protection mode. All options other than none require the VM to have been built with branch protection enabled. In addition, for full protection, any native libraries provided by applications should be compiled with the same level of protection. Team effort, fr fr 🤝.

Possible mode arguments for this option include:

none — do not use branch protection. This is the default value.
standard — enables all branch protection modes available on the current platform.
pac-ret — enables protection against ROP based attacks. (AArch64 8.3+ only)

🔥 ADVANCED JIT COMPILER OPTIONS FOR JAVA 🔥

These java options control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM. This is where the compiler gets spicy 🌶️.

`-XX:AllocateInstancePrefetchLines=lines`

Sets the number of lines to prefetch ahead of the instance allocation pointer. By default the number of lines to prefetch is set to 1:

-XX:AllocateInstancePrefetchLines=1

`-XX:AllocatePrefetchDistance=size`

Sets the size, in bytes, of the prefetch distance for object allocation. Memory about to be written with the value of new objects is prefetched up to this distance starting from the address of the last allocated object. Each Java thread has its own allocation point — everybody gets their own 💅.

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. Here's how you 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 — so don't @ me about what 2 does on your machine 😭. By default, the prefetch instruction is set to 0:

-XX:AllocatePrefetchInstr=0

...which is literally just the tail end of that example, no notes needed 💅 (it's showing you what setting the prefetch instruction to 0 looks like, iykyk).

-XX:AllocatePrefetchLines=lines — Sets how many cache lines get loaded after the last object allocation, using the prefetch instructions that were generated in compiled code. 💫 The default value is 1 if the last allocated object was an instance, and 3 if it was an array. Because of course arrays get extra attention, main character energy fr.

Here's how you'd 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. 🦋 Stick the letter k or K on the end to mean kilobytes, m or M for megabytes, g or G for gigabytes. By default, the step size is set to 16 bytes:

-XX:AllocatePrefetchStepSize=16

-XX:AllocatePrefetchStyle=style — Sets the generated code style for prefetch instructions. 🎀 The style argument is an integer from 0 to 3, and tbh each one is giving a different vibe:
- 0 — Don't generate prefetch instructions. (literally doing nothing, we love a rest era)
- 1 — Execute prefetch instructions after each allocation. This is the default setting. 👑
- 2 — Use the thread-local allocation block (TLAB) watermark pointer to determine when prefetch instructions are executed.
- 3 — Generate one prefetch instruction per cache line.
-XX:+BackgroundCompilation — Enables background compilation. ✨ This option is enabled by default. To disable background compilation, specify -XX:-BackgroundCompilation (which is equivalent to specifying -Xbatch). Same thing, different outfit.
-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 the memory available for compiled code. Here's how to set the number of threads to 2:

-XX:CICompilerCount=2

-XX:+UseDynamicNumberOfCompilerThreads — Dynamically creates compiler threads up to the limit specified by -XX:CICompilerCount. 🌈 This option is enabled by default. It's giving "we'll staff up as needed" energy.
-XX:CompileCommand=command,method[,option] — Specifies a command to perform on a method. 🫶 Okay this one is the big one, stay with me bestie. For example, to exclude the indexOf() method of the String class from being compiled, you'd use:

-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 that case you should enclose the arguments in quotation marks, because otherwise the shell treats the semicolon as a command end 😭 (the shell is so dramatic). For example, if you want to exclude only the indexOf(String) method of the String class from being compiled:

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

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

-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:

-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. 🤫

-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 at 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. (Yes, same key, same lock, no cap.)
-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. (zero = girl, bye 👋)
-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. ⚠️ Here's 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's set to 1000 bytes:

-XX:InlineSmallCode=1000

-XX:+LogCompilation — turns on logging of all the compilation activity, and it dumps it into a file called hotspot.log in whatever directory you're currently in. Want it somewhere else with a different name? Use -XX:LogFile for that. Default is off, so no logging happens unless you ask. Heads up though: -XX:+LogCompilation has to be paired with -XX:UnlockDiagnosticVMOptions, the option that unlocks the diagnostic JVM options — otherwise it's just not gonna work, bestie. Oh and if you also want a lil message printed to the console every single time a method gets compiled, that's -XX:+PrintCompilation for the verbose diagnostic output. 🎀
-XX:FreqInlineSize=size — sets the max bytecode size (in bytes) of a **hot method that's allowed to get inlined. C2 compiler only, no C1 gate-crashing. You can slap a k or K on the end for kilobytes, m or M for megabytes, or g or G for gigabytes. The default depends on your platform. Example, set to 325 bytes:

-XX:FreqInlineSize=325

-XX:MaxInlineSize=size — sets the max bytecode size (in bytes) of a **cold method that's allowed to get inlined. Again, C2 compiler only. Same suffix situation: k/K = kilobytes, m/M = megabytes, g/G = gigabytes. Default max bytecode size is 35 bytes:

-XX:MaxInlineSize=35

-XX:C1MaxInlineSize=size — same story, but for a **cold method and this time it's the C1 compiler. k/K kilobytes, m/M megabytes, g/G gigabytes. Default max bytecode size is 35 bytes:

-XX:MaxInlineSize=35

-XX:MaxTrivialSize=size — max bytecode size (in bytes) of a **trivial method that can be inlined. C2 compiler only. k/K, m/M, g/G — you know the drill by now. Default max bytecode size of a trivial method is 6 bytes:

-XX:MaxTrivialSize=6

-XX:C1MaxTrivialSize=size — max bytecode size (in bytes) of a **trivial method that can be inlined, but for the C1 compiler. Suffixes: k/K kilobytes, m/M megabytes, g/G gigabytes. Default max bytecode size of a trivial method is 6 bytes:

-XX:MaxTrivialSize=6

*-XX:MaxNodeLimit=nodes* — caps the max number of nodes used while compiling a single method. Default value depends on which features you've got enabled. Example where it's set to 100,000:

-XX:MaxNodeLimit=100000

-XX:NonNMethodCodeHeapSize=size — sets the size in bytes of the code segment holding nonmethod code. A nonmethod code segment is the one containing nonmethod code — think compiler buffers and the bytecode interpreter. This code type stays in the code cache **forever, no eviction, it's giving permanent resident. Only used if -XX:SegmentedCodeCache is enabled. ✨
*-XX:NonProfiledCodeHeapSize=size* — sets the size in bytes of the code segment holding nonprofiled methods. Only used if -XX:SegmentedCodeCache is enabled.
-XX:+OptimizeStringConcat — enables optimization of String concatenation operations. Enabled by default, slay. Want it off? Specify -XX:-OptimizeStringConcat.
-XX:+PrintAssembly — enables printing of assembly code for bytecoded and native methods, using the external hsdis-<arch>.so or .dll library. For a 64-bit VM on Windows that's hsdis-amd64.dll. This lets you actually see the generated code, which can help you diagnose performance issues. 💅 Default: disabled, no assembly printed. And just like logging, -XX:+PrintAssembly has to be used together with -XX:UnlockDiagnosticVMOptions — the option that unlocks diagnostic JVM options.
*-XX:ProfiledCodeHeapSize=size* — sets the size in bytes of the code segment holding profiled methods. Only used if -XX:SegmentedCodeCache is enabled.
-XX:+PrintCompilation — enables verbose diagnostic output from the JVM by printing a message to the console every time a method is compiled. This is the one that lets you see which methods actually get compiled, iykyk. Default: disabled, no diagnostic output. You can also log compilation activity to a file with -XX:+LogCompilation.
-XX:+PrintInlining — enables printing of inlining decisions, so you can see which methods are getting inlined. Default: disabled, no inlining info printed. Needs to be used together with -XX:+UnlockDiagnosticVMOptions (the option that unlocks diagnostic JVM options) — no solo runs, bestie. 🦋
-XX:ReservedCodeCacheSize=size — sets the **maximum code cache size (in bytes) for JIT-compiled code. Suffixes: k/K kilobytes, m/M megabytes, g/G gigabytes. Default max code cache size is 240 MB; but if you disable tiered compilation with -XX:-TieredCompilation, the default drops to 48 MB. This option has a hard limit of 2 GB — go over and you get an error, no cap. Also, the max code cache size shouldn't be less than the initial code cache size; see -XX:InitialCodeCacheSize.

Lowkey important: max < initial = bad vibes. Don't do it.

-XX:+SegmentedCodeCache — enables segmentation of the code cache. Without it, the code cache is just one large segment. With -XX:+SegmentedCodeCache, you get separate segments for non-method code, profiled method code, and non-profiled method code. These segments are not resized at runtime, so plan accordingly. The advantages are genuinely iconic: better control of the memory footprint, reduced code fragmentation, and better CPU iTLB (instruction translation lookaside buffer) and instruction cache behavior thanks to improved locality. 🔥

This 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 once only the given percentage of the code cache is free. Default value is 10%.
-XX:-TieredCompilation — disables tiered compilation. By default this option is enabled (yes, the minus sign is doing the work here, don't come for me).
*-XX:UseSSE=version* — enables the use of the SSE instruction set of a specified version. 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. Set by default to the highest supported version available (x86 only).
-XX:+UseAES — enables hardware-based AES intrinsics on hardware that supports it. On by default on hardware with the necessary instructions. -XX:+UseAES is used in conjunction with UseAESIntrinsics. Heads up: flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseAESIntrinsics — enables AES intrinsics. Specifying -XX:+UseAESIntrinsics is equivalent to also enabling -XX:+UseAES. Want to disable hardware-based AES intrinsics? Specify -XX:-UseAES -XX:-UseAESIntrinsics. Example — to enable hardware AES, use these flags:

-XX:+UseAES -XX:+UseAESIntrinsics

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

-XX:+UseAESCTRIntrinsics — analogous to -XX:+UseAESIntrinsics, but it enables AES/CTR intrinsics. 🫶
-XX:+UseGHASHIntrinsics — controls the use of GHASH intrinsics. Enabled by default on platforms that support the corresponding instructions. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseChaCha20Intrinsics — enables ChaCha20 intrinsics. On by default for supported platforms. To disable ChaCha20 intrinsics, specify -XX:-UseChaCha20Intrinsics. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UsePoly1305Intrinsics — enables Poly1305 intrinsics. On by default for supported platforms. To disable Poly1305 intrinsics, specify -XX:-UsePoly1305Intrinsics. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseBASE64Intrinsics — controls the use of accelerated BASE64 encoding routines for java.util.Base64. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseAdler32Intrinsics — controls the use of the Adler32 checksum algorithm intrinsic for java.util.zip.Adler32. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseCRC32Intrinsics — controls the use of CRC32 intrinsics for java.util.zip.CRC32. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseCRC32CIntrinsics — controls the use of CRC32C intrinsics for java.util.zip.CRC32C. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions. 🌈
-XX:+UseSHA — turns on hardware-based intrinsics for SHA crypto hash functions on hardware that supports them 💅. It's a package deal with UseSHA1Intrinsics, UseSHA256Intrinsics, and UseSHA512Intrinsics. The UseSHA and UseSHA*Intrinsics flags are enabled by default on machines that support the corresponding instructions. Heads up though: this feature is only applicable when you're using the sun.security.provider.Sun provider for SHA operations. Also, flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions — no cap, that's the new rule 🔑. Wanna kill all the hardware-based SHA intrinsics? Specify -XX:-UseSHA. Wanna kill just one specific intrinsic? Use the matching option, like for example: -XX:-UseSHA256Intrinsics.
-XX:+UseSHA1Intrinsics — enables intrinsics for the SHA-1 crypto hash function 🍓. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseSHA256Intrinsics — enables intrinsics for the SHA-224 and SHA-256 crypto hash functions ✨. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseSHA512Intrinsics — enables intrinsics for the SHA-384 and SHA-512 crypto hash functions 🌸. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMathExactIntrinsics — enables intrinsification of various java.lang.Math.*Exact() functions 🧮. Enabled by default. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMultiplyToLenIntrinsic — enables intrinsification of BigInteger.multiplyToLen() ✖️. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseSquareToLenIntrinsic — enables intrinsification of BigInteger.squareToLen() 🔲. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMulAddIntrinsic — enables intrinsification of BigInteger.mulAdd() ➕. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMontgomeryMultiplyIntrinsic — enables intrinsification of BigInteger.montgomeryMultiply() 🔥. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMontgomerySquareIntrinsic — enables intrinsification of BigInteger.montgomerySquare() 💜. Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseCMoveUnconditionally — generates CMove (scalar and vector) instructions regardless of profitability analysis 😤. It just does it. No second-guessing.
-XX:+UseCodeCacheFlushing — enables flushing of the code cache before shutting down the compiler 🧹. Enabled by default. Want to disable flushing of the code cache before shutting down the compiler? Specify -XX:-UseCodeCacheFlushing.
-XX:+UseCondCardMark — enables checking if the card is already marked before updating the card table 🃏. Disabled by default. It should be used only on machines with multiple sockets, where it increases the performance of Java applications that rely on concurrent operations.
-XX:+UseCountedLoopSafepoints — keeps safepoints in counted loops 🔁. Its default value depends on whether the selected garbage collector requires low latency safepoints.
-XX:LoopStripMiningIter=number_of_iterations — controls the number of iterations in the inner strip mined loop ⛏️. Strip mining transforms counted loops into two level nested loops. Safepoints are kept in the outer loop while the inner loop can execute at full speed 🏎️. This option controls the maximum number of iterations in the inner loop. The default value is 1,000.
-XX:LoopStripMiningIterShortLoop=number_of_iterations — controls loop strip mining optimization 🎀. Loops with the number of iterations less than specified will not have safepoints in them. Default value is 1/10th of -XX:LoopStripMiningIter.
-XX:+UseFMA — enables hardware-based FMA intrinsics for hardware where FMA instructions are available (such as, Intel and ARM64) 💻. FMA intrinsics are generated for the java.lang.Math.fma(a, b, c) methods that calculate the value of ( a * b + c ) expressions.
-XX:+UseSuperWord — enables the transformation of scalar operations into superword operations 🦋. Superword is a vectorization optimization. Enabled by default. To disable the transformation of scalar operations into superword operations, specify -XX:-UseSuperWord.

🔍 ADVANCED SERVICEABILITY OPTIONS FOR JAVA 🔍

These java options provide the ability to gather system information and perform extensive debugging 🕵️‍♀️. Basically the "what is going ON in there" toolkit, bestie.

-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 ⚠️. Keep your versions matched, iykyk.

-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. Important detail: 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). So don't come for it when it doesn't catch those, fr fr.
-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. This example shows how to set the default file explicitly (%p represents the current process identifier):

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

Non-Windows: this 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: this 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: this example shows how to set the log file to /var/log/java/hotspot.log:

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

Windows: this 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 🧹💖. The cleanup crew, basically.

-XX:+AggressiveHeap — enables Java heap optimization 🔥. This sets various parameters to be optimal for long-running jobs with intensive memory allocation, based on the configuration of the computer (RAM and CPU). By default, the option is disabled and the heap sizes are configured less aggressively 😌.
-XX:+AlwaysPreTouch — asks the VM to touch every page on the Java heap after requesting it from the operating system and before handing memory out to the app. By default this is disabled, and all pages are committed as the application uses the heap space. 💅
-XX:ConcGCThreads=threads — sets the number of threads used for concurrent GC. It 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 this 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 calls to System.gc() are processed. If processing of calls to System.gc() is disabled, then the JVM still performs GC when necessary. 🫶
-XX:+ExplicitGCInvokesConcurrent — enables invoking 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 marking cycles used to gather samples until G1 adaptively determines the optimum value of -XX:InitiatingHeapOccupancyPercent. Before that, G1 uses the value of -XX:InitiatingHeapOccupancyPercent directly for this purpose. The default value is 3.
-XX:G1HeapRegionSize=size — sets the size of the regions into which the Java heap is subdivided when using the garbage-first (G1) collector. The value is a power of 2 and can range from 1 MB to 32 MB. The default region size is determined ergonomically based on the heap size with a goal of approximately 2048 regions. The following example sets the size of the subdivisions to 16 MB:

-XX:G1HeapRegionSize=16m

-XX:G1HeapWastePercent=percent — sets the percentage of heap that you're willing to waste. The Java HotSpot VM doesn't initiate the mixed garbage collection cycle when the reclaimable percentage is less than the heap waste percentage. The default is 5 percent.
-XX:G1MaxNewSizePercent=percent — sets the percentage of the heap size to use as the maximum for the young generation size. The default value is 60 percent of your Java heap. This is an experimental flag. This setting replaces the -XX:DefaultMaxNewGenPercent setting.
-XX:G1MixedGCCountTarget=number — sets the target number of mixed garbage collections after a marking cycle to collect old regions with at most G1MixedGCLIveThresholdPercent live data. The default is 8 mixed garbage collections. The goal for mixed collections is to be within this target number.
-XX:G1MixedGCLiveThresholdPercent=percent — sets the occupancy threshold for an old region to be included in a mixed garbage collection cycle. The default occupancy is 85 percent. This is an experimental flag. This setting replaces the -XX:G1OldCSetRegionLiveThresholdPercent setting.
-XX:G1NewSizePercent=percent — sets the percentage of the heap to use as the minimum for the young generation size. The default value is 5 percent of your Java heap. This is an experimental flag. This setting replaces the -XX:DefaultMinNewGenPercent setting.
-XX:G1OldCSetRegionThresholdPercent=percent — sets an upper limit on the number of old regions to be collected during a mixed garbage collection cycle. The default is 10 percent of the Java heap.
-XX:G1ReservePercent=percent — sets the percentage of the heap (0 to 50) that's reserved as a false ceiling to reduce the possibility of promotion failure for the G1 collector. When you increase or decrease the percentage, ensure that you adjust the total Java heap by the same amount. By default, this option is set to 10%. The following example sets the reserved heap to 20%:

-XX:G1ReservePercent=20

-XX:+G1UseAdaptiveIHOP — controls adaptive calculation of the old generation occupancy to start background work preparing for an old generation collection. If enabled, G1 uses -XX:InitiatingHeapOccupancyPercent 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:InitiatingHeapOccupancyPercent. The default is enabled.
-XX:InitialHeapSize=size — sets the initial size (in bytes) of the memory allocation pool. This value must be either 0, or a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value is selected at run time based on the system configuration. The following examples show how to set the size of allocated memory to 6 MB using various units:

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

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

-XX:InitialRAMPercentage=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 maximum amount determined as described in the -XX:MaxRAM option. The default value is 1.5625 percent. 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:InitiatingHeapOccupancyPercent=percent — sets the percentage of the old generation occupancy (0 to 100) at which to start the first few concurrent marking cycles for the G1 garbage collector. By default, the initiating value is set to 45%. A value of 0 implies nonstop concurrent GC cycles from the beginning until G1 adaptively sets this value. See also the -XX:G1UseAdaptiveIHOP and -XX:G1AdaptiveIHOPNumInitialSamples options. The following example shows how to set the initiating heap occupancy to 75%:

-XX:InitiatingHeapOccupancyPercent=75

-XX:MaxGCPauseMillis=time — sets a target for the maximum GC pause time (in milliseconds). This is a soft goal, and the JVM will make its best effort to achieve it. The specified value doesn't adapt to your heap size. By default, for G1 the maximum pause time target is 200 milliseconds. The other generational collectors do not use a pause time goal by default. 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. periodt. 💅

-XX:MaxHeapFreeRatio=percent — sets the max allowed percentage of free heap space (0 to 100) after a GC event. If free heap space balloons above this value, the heap gets shrunk. Default is 70%. Want a smol heap? Lower both MaxHeapFreeRatio (default 70%) and MinHeapFreeRatio (default 40%) using the command-line options -XX:MaxHeapFreeRatio and -XX:MinHeapFreeRatio. Dropping MaxHeapFreeRatio as low as 10% and MinHeapFreeRatio to 5% has successfully reduced heap size without too much performance regression — but ngl, results may vary greatly depending on your app. So experiment with different values until they're as low as possible while still keeping acceptable performance, okay? 💅

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

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

-XX:MaxMetaspaceSize=size — sets the maximum amount of native memory that can be allocated for class metadata. By default, the size isn't limited at all — no cap, live your truth ✨. The amount of metadata for an app depends on the app itself, other running applications, and the amount of memory available on the system.

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

-XX:MaxMetaspaceSize=256m

-XX:MaxNewSize=size — sets the maximum size (in bytes) of the heap for the young generation (nursery). The default value is set ergonomically. 🍼
-XX:MaxRAM=size — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics. Default value is the max amount of available memory to the JVM process or 128 GB, whichever is lower.

The maximum amount of available memory to the JVM process is the minimum of the machine's physical memory and any constraints set by the environment (e.g. container). Heads up: specifying this option disables automatic use of compressed oops if the combined result of this and other options influencing the maximum amount of memory is larger than the range of memory addressable by compressed oops. See -XX:UseCompressedOops for further information about compressed oops. Here's how to set the max amount of available memory for sizing the Java heap to 2 GB:

-XX:MaxRAM=2G

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

Same warning as above, bestie: specifying this option disables automatic use of compressed oops if the combined result of this and other options influencing the maximum amount of memory is larger than the range of memory addressable by compressed oops. See -XX:UseCompressedOops for further information about compressed oops. Setting the percentage of the max amount of memory used for the Java heap goes like this:

-XX:MaxRAMPercentage=75

-XX:MinRAMPercentage=percent — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics, as a percentage of the maximum amount determined as described in the -XX:MaxRAM option — but for small heaps. A small heap is a heap of approximately 125 MB. Default value is 50 percent.

Here's how to set the percentage of the max amount of memory used for the Java heap for small heaps:

-XX:MinRAMPercentage=75

-XX:MaxTenuringThreshold=threshold — sets the maximum tenuring threshold for use in adaptive GC sizing. The largest value is 15. The default value is 15 for the parallel (throughput) collector. Set it to 10 like this:

-XX:MaxTenuringThreshold=10

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

Minimize Java heap size by lowering the values of 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 the heap size without too much performance regression — however, results may vary greatly depending on your application. Try different values for these parameters until they're as low as possible, yet still retain acceptable performance. ✨

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

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

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

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

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

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

-XX:NewRatio=ratio — sets the ratio between young and old generation sizes. Default is 2. Set the young-to-old ratio to 1 like so:

-XX:NewRatio=1

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

The young generation region of the heap is used for new objects — it's giving fresh-start energy 🌱. GC is performed in this region more often than in other regions. If the size for the young generation is too low, then a large number of minor GCs are performed. If the size is too high, then only full GCs are performed, which can take a long time to complete. It's recommended that you keep the size for the young generation greater than 25% and less than 50% of the overall heap size. These examples show how to set the initial size of the young generation to 256 MB using various units:

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

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

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

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

-XX:ParallelGCThreads=2

-XX:+ParallelRefProcEnabled — enables parallel reference processing. By default, 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 employing multiple threads always perform reference processing in parallel. 🔥
-XX:+PrintAdaptiveSizePolicy — enables printing of information about adaptive-generation sizing. By default, this option is disabled. (Turn it on if you're nosy about your generations 💀)
-XX:SoftRefLRUPolicyMSPerMB=time — sets the amount of time (in milliseconds) a softly reachable object is kept active on the heap after the last time it was referenced. Default value is one second of lifetime per free megabyte in the heap. The -XX:SoftRefLRUPolicyMSPerMB option accepts integer values representing milliseconds per one megabyte of the current heap size (for Java HotSpot Client VM) or the maximum possible heap size (for Java HotSpot Server VM).

This difference means that the Client VM tends to flush soft references rather than grow the heap, whereas the Server VM tends to grow the heap rather than flush soft references. In the latter case, the value of the -Xmx option has a significant effect on how quickly soft references are garbage collected.

Here's how to set the value to 2.5 seconds:

-XX:SoftRefLRUPolicyMSPerMB=2500

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

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

-XX:StringDeduplicationAgeThreshold=threshold — tells the JVM which String objects are old enough to be considered for deduplication. An object's age is literally just how many garbage collections it's survived (people also call this tenuring, iykyk ✨).

Heads up bestie: any String that got promoted to an old heap region before reaching this age is always still considered a dedup candidate — no skipping. Default value is 3. Also go look at -XX:+UseStringDeduplication.

-XX:SurvivorRatio=ratio — sets the ratio between eden space size and survivor space size. Default is 8. Wanna flex it to 4? Here's how:

-XX:SurvivorRatio=4

-XX:TargetSurvivorRatio=percent — sets the desired percentage of survivor space (0 to 100) that gets used after a young garbage collection. Default is 50%. Setting it to 30% looks like this:

-XX:TargetSurvivorRatio=30

-XX:TLABSize=size — sets the initial size (in bytes) of a thread-local allocation buffer (TLAB 🫶). You can tack on k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. Set it to 0 and the JVM just picks the starting size for you, no stress. Example — 512 KB:

-XX:TLABSize=512k

-XX:+UseAdaptiveSizePolicy — turns on adaptive generation sizing. It's enabled by default, main character behaviour 💅. Want it off? Specify -XX:-UseAdaptiveSizePolicy and set the size of the memory allocation pool explicitly. Peep -XX:SurvivorRatio while you're at it.
-XX:+UseG1GC — enables the garbage-first (G1) garbage collector. It's a server-style collector, built for multiprocessor machines with a lot of RAM. It hits GC pause time goals with high probability while keeping throughput good. G1 is the recommended pick for apps that need big heaps (like around 6 GB or larger) with limited GC latency needs — meaning a stable, predictable pause time under 0.5 seconds. And plot twist: this is enabled by default, G1 is the default collector 👑.
-XX:+UseGCOverheadLimit — enables a policy that caps how much time the JVM is allowed to spend on GC before it throws an OutOfMemoryError. Enabled by default, and here's the receipt: the parallel GC throws OutOfMemoryError if more than 98% of total time goes to garbage collection and less than 2% of the heap gets recovered. When the heap is small, this stops apps from just vibing for ages making zero progress 😭. Turn it off with -XX:-UseGCOverheadLimit.
-XX:+UseNUMA — performance glow-up for apps running on a machine with nonuniform memory architecture (NUMA), by making the app use lower-latency memory more. The default value depends on the garbage collector, so no single answer, sorry 🎀.
-XX:+UseParallelGC — enables the parallel scavenge garbage collector (aka the throughput collector) to boost performance by leveraging multiple processors. Disabled by default — the default collector gets used instead.
-XX:+UseSerialGC — enables the serial garbage collector. Honestly the best pick for small, simple apps that don't need any special GC functionality. Disabled by default — default collector, again.
-XX:+UseStringDeduplication — enables string deduplication. Disabled by default, and lowkey important: to use it you must enable the garbage-first (G1) garbage collector first. What it does: shrinks the memory footprint of String objects on the Java heap by exploiting the fact that tons of Strings are identical. Instead of every String pointing at its own character array, identical ones point to and share the same character array. Slay, shared memory 🌸.
-XX:+UseTLAB — enables thread-local allocation blocks (TLABs) in the young generation space. Enabled by default. Disable with -XX:-UseTLAB.
-XX:+UseZGC — enables the Z garbage collector (ZGC). Low latency, max pause times of just a few milliseconds, at some throughput cost. Pause times don't care what heap size you're on — independent, periodt. Supports heap sizes from 8MB to 16TB 🔥.
-XX:ZAllocationSpikeTolerance=factor — sets the allocation spike tolerance for ZGC. Default is 2.0. This factor says how big a spike you expect. A factor of 3.0, for example, means the current allocation rate could be expected to triple at any moment.
-XX:ZCollectionInterval=seconds — sets the maximum interval (in seconds) between two GC cycles when using ZGC. Default is 0, which means disabled.
-XX:ZFragmentationLimit=percent — sets the maximum acceptable heap fragmentation (in percent) for ZGC. Default is 25. Go lower and the heap gets compacted more aggressively to reclaim more memory — at the cost of more CPU time. Trade-off vibes 💀.
-XX:+ZProactive — enables proactive GC cycles when using ZGC. Enabled by default. ZGC kicks off a proactive cycle if it expects minimal impact on the running app. Super useful if your app is mostly idle or barely allocates, but you still wanna keep the heap size down and let reference processing happen even when there's lots of free space on the heap ✨.
-XX:+ZUncommit — enables uncommitting of unused heap memory when using ZGC. Enabled by default. Uncommitting 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 has to sit unused before it gets uncommitted. Default is 300 (5 minutes). Committing and uncommitting memory are relatively expensive operations, so going lower uncommits earlier — at the risk of having to commit it right back 😭.
-XX:+UseShenandoahGC — enables the Shenandoah garbage collector. Low pause time, concurrent, and its pause times are not proportional to heap size. Shenandoah can work with compressed pointers — see -XX:UseCompressedOops for more on those.
-XX:ShenandoahGCMode=mode — sets the GC mode for Shenandoah. Default is satb. Among other things, this defines which barriers are in use. Possible values:
- satb — snapshot-at-the-beginning concurrent GC (three pass mark-evac-update). It's a single generation GC.
- generational — also snapshot-at-the-beginning and concurrent, but generational. Check out JEP 404 (https://openjdk.org/jeps/404) and JEP 521 (https://openjdk.org/jeps/521) for the advantages and the risks.
-XX:ShenandoahGCHeuristics=heuristics — sets the heuristics Shenandoah uses. Default is adaptive. This fine-tunes the GC mode you picked — deciding when to start GC, how much to process each cycle, and what other features to auto-enable. And when -XX:ShenandoahGCMode is generational, the only supported option is the default, adaptive. Possible heuristics:
- adaptive — keeps the given amount of free heap at all times, even mid-GC-cycle.
- static — triggers GC when free heap drops below a specified threshold.
- compact — runs GC more frequently with deeper targets to free up more memory.

💀 DEPRECATED JAVA OPTIONS 💀

These java options are deprecated and might get removed in a future JDK release. They're still accepted and still actually do the thing, but you get a warning the moment you use them — the JVM is like "bestie… are you sure" 🙈.

-Xloggc:filename — sets the file that verbose GC event info gets redirected to for logging. If both are given on the same java command, -Xloggc overrides -verbose:gc. This one's been replaced by -Xlog:gc:filename — see Enable Logging with the JVM Unified Logging Framework. Example:

-Xlog:gc:garbage-collection.log

-XX:+FlightRecorder — enables Java Flight Recorder (JFR) while the app runs. Since JDK 8u40 this option hasn't been required to use JFR at all, so it's just been sitting there 💅.

🧟 OBSOLETE JAVA OPTIONS 🧟

These java options are still accepted but completely ignored — and yeah, a warning gets issued when you use them.

--illegal-access=parameter — controlled relaxed strong encapsulation, as defined in JEP 261 (https://openjdk.org/jeps/261#Relaxed-strong-encapsulation). It 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). It's giving retired icon 🌈.

💀 REMOVED JAVA OPTIONS 💀

okay bestie, listen up — these ones are GONE. like, removed in JDK 25 removed. if you try to use any of them you get hit with this little error message, no cap:

Unrecognized VM option option-name

so yeah. rip bozo. but here's the archive of what they were, iykyk 🫶

the fallen soldiers 🎀

-XX:RTMAbortRatio=abort_ratio — sets the RTM abort ratio, and it's given as a percentage (%) of all the RTM transactions that executed. if the number of aborted transactions climbs higher than this ratio, the compiled code gets deoptimized. this ratio only does anything when -XX:+UseRTMDeopt is enabled. default is 50, which literally means: if 50% of all transactions get aborted, the compiled code gets deoptimized. 💅
-XX:RTMRetryCount=number_of_retries — how many times the RTM locking code gets retried when it's aborted or busy, before it gives up and falls back to the normal locking mechanism. default value is 5. heads up: -XX:UseRTMLocking must be enabled for this to apply. ✨
-XX:+UseRTMDeopt — autotunes RTM locking based on the abort ratio (the ratio comes from the -XX:RTMAbortRatio option, fyi). if the number of aborted transactions goes over that abort ratio, the method that contains the lock gets deoptimized and recompiled with all its locks as normal locks. disabled by default. also: -XX:+UseRTMLocking must be enabled. 🔥
-XX:+UseRTMLocking — generates Restricted Transactional Memory (RTM) locking code for all inflated locks, with the normal locking mechanism chilling there as the fallback handler. disabled by default. RTM-related options only exist on x86 CPUs that support Transactional Synchronization Extensions (TSX). 🦋

okay so let me actually explain this one because it's lowkey a whole saga:

RTM is part of Intel's TSX, which is an x86 instruction set extension and it facilitates the creation of multithreaded applications. RTM introduces the new instructions XBEGIN, XABORT, XEND, and XTEST. the XBEGIN and XEND instructions enclose a set of instructions to run as a transaction. if no conflict is found when running the transaction, then the memory and register modifications are committed together at the XEND instruction. the XABORT instruction can be used to explicitly abort a transaction, and the XTEST instruction checks if a set of instructions is being run in a transaction. 💀

a lock on a transaction gets inflated when another thread tries to access the same transaction, which thereby blocks the thread that didn't originally request access to the transaction. RTM requires that a fallback set of operations be specified in case a transaction aborts or fails. an RTM lock is a lock that has been delegated to the TSX's system. 🌈

RTM improves performance for highly contended locks with low conflict in a critical region (which is code that must not be accessed by more than one thread concurrently). RTM also improves the performance of coarse-grain locking, which typically doesn't perform well in multithreaded applications. (coarse-grain locking is the strategy of holding locks for long periods to minimize the overhead of taking and releasing locks, while fine-grained locking is the strategy of trying to achieve maximum parallelism by locking only when necessary and unlocking as soon as possible.) 👑

also, for lightly contended locks that are used by different threads, RTM can reduce false cache line sharing, also known as cache line ping-pong. this occurs when multiple threads from different processors are accessing different resources, but the resources share the same cache line. as a result, the processors repeatedly invalidate the cache lines of other processors, which forces them to read from main memory instead of their cache. so much drama for one cache line fr fr 😭

🕰️ the nostalgia corner — where to find older removals

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

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

bestie if you're still passing any of these flags in your startup script, that's your sign to go clean them up. the JVM is not gonna be nice about it — it's giving instant error, periodt. 🙈

🎀 JAVA COMMAND-LINE ARGUMENT FILES 🎀

Okay bestie, so you know how java commands can get absurdly long, right? Like, mile-long. Enter the glow-up: @ argument files. You can shorten or simplify the java command by using @ argument files to specify one or more text files that contain arguments — options, class names, all of it — which are then passed to the java command. This lets you create java commands of any length on any operating system. 💅

In the command line, use the at sign (@) prefix to identify an argument file that contains java options and class names. When the java command encounters a file beginning with the at sign (@), it expands the contents of that file into an argument list just as they would be specified on the command line. Lowkey magic. ✨

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

The following items describe the syntax of java argument files — read them, they're giving rules:

The argument file must contain only ASCII characters or characters in system default encoding that's ASCII friendly, such as UTF-8.
The argument file size must not exceed MAXINT (2,147,483,647) bytes.
The launcher doesn't expand wildcards that are present within an argument file. That means an asterisk (*) is passed on as-is to the starting VM. For example *.java stays *.java and is not expanded to Foo.java Bar.java ..., as would happen with some command line shells.
Use white space or new line characters to separate arguments included in the file.
White space includes a white space character, \t, \n, \r, and \f.

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

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

🫶 Yes, that last one means no inception-style @-inside-@ recursion. Not supported. Periodt.

🦋 Example of Open or Partial Quotes in an Argument File

In the argument file,

-cp "lib/
cool/
app/
jars

this is interpreted as:

-cp lib/cool/app/jars

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

To output the following:

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

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

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

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

In the argument file,

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

This is interpreted as:

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

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

In the argument file,

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

This is interpreted as:

-cp /lib/cool app/jars

👑 Examples of Using Single Argument File

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

java @myargumentfile

🌈 Examples of Using Argument Files with Paths

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

java @path1/options @path2/classes

🔥 CODE HEAP STATE ANALYTICS 🔥

💜 Overview

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

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

To provide this insight, a code heap state analytics feature has been implemented that enables on-the-fly analysis of the code heap. The analytics process is divided into two parts. The first part examines the entire code heap and aggregates all information that is believed to be useful or important. The second part consists of several independent steps that print the collected information with an emphasis on different aspects of the data. Data collection and printing are done on an "on request" basis. Iconic behaviour, honestly. ✨

🎀 Syntax

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

jcmd pid Compiler.CodeHeap_Analytics [function] [granularity]

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

-Xlog:codecache=Trace

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

-Xlog:codecache=Debug

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

⭐ ENABLE LOGGING WITH THE JVM UNIFIED LOGGING FRAMEWORK ⭐

You use the -Xlog option to configure or enable logging with the Java Virtual Machine (JVM) unified logging framework. One option to rule them all, iykyk. 💅

🌟 Synopsis

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

-Xlog:directive

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

💖 Description

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

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

-Xlog — Enables JVM logging on an info level.
-Xlog:help — Prints -Xlog usage syntax and available tags, levels, and decorators along with example command lines with explanations.
-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

🫶 Controlling Logging at Runtime

okay so logging isn't a set-it-and-forget-it thing, bestie 💅 — you can also control it at run time through Diagnostic Commands (with the jcmd utility). Literally everything that can be specified on the command line can also be specified dynamically with the VM.log command. And because the diagnostic commands are automatically exposed as MBeans, you can use JMX to change logging configuration at run time too. icon behavior honestly 👑

✨ -Xlog Tags and Levels ✨

Every single log message rolls up with a level and a tag set attached to it — the level 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 down below, and mapping legacy runtime logging flags to the corresponding Xlog configuration is described in Convert Runtime Logging Flags to Xlog. iykyk 🦋

Available log levels, in order:

off
trace
debug
info
warning
error

Available log tags:

There are literally dozens of log tags, and in the right combinations they'll enable a whole range of logging output. The full set of available log tags can be seen using -Xlog:help. And specifying all instead of a tag combination matches all tag combinations — main character move, fr fr 💜

🌸 -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. 🎀

Heads up bestie: 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. And there's a possibility of the pre-existing log file getting overwritten. 😭

💅 -Xlog Output Mode 💅

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

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

Write all logging asynchronously.

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

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.

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

Logging Messages Decorations

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

🌈 Convert GC Logging Flags to Xlog 🌈

Legacy GC Logging Flags to Xlog Configuration Mapping — here's the glow-up chart, bestie 💖 Each legacy Garbage Collection (GC) flag, what it maps to, and the lil comment:

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

🎀 Convert Runtime Logging Flags to Xlog 🎀

These legacy flags are no longer recognized and will cause an error if used directly. Use their unified logging equivalent instead, bestie — trust, the old way is so over 💀

🌈 Runtime Logging Flags to Xlog Configuration Mapping 🌈

okay bestie, this is the legacy-flag-to--Xlog glow-up cheat sheet. One bullet per row, no details dropped, periodt.

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

✨ -Xlog Usage Examples ✨

These are the -Xlog examples, bestie. Copy the commands exactly, okay?

-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

-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,safepoint

-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+ref=debug

-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=debug:file=gc.txt:none

-Xlog:gc=trace:file=gctrace.txt:uptimemillis,pid:filecount=5,filesize=1024 — Logs messages tagged with the gc tag using the trace level to a rotating file set with 5 files with size 1 MB with the base name gctrace.txt and uses decorations uptimemillis and pid. The default configuration for all other messages at level warning is still in effect.

-Xlog:gc=trace:file=gctrace.txt:uptimemillis,pid:filecount=5,filesize=1024

-Xlog:gc::uptime,tid — Logs messages tagged with the gc tag using the default 'info' level to the default output stdout and uses decorations uptime and tid. The default configuration for all other messages at level warning is still in effect.

-Xlog:gc::uptime,tid

-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:gc*=info,safepoint*=off

-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.

-Xlog:disable -Xlog:safepoint=trace:safepointtrace.txt

🧚 Complex -Xlog Usage Examples 🧚

These are a few complex examples of using the -Xlog option. stay with me, they’re iconic.

-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+class*=debug

-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,class*=off:file=gcmetatrace.txt

-Xlog:gc+meta=trace — Logs messages tagged with exactly the gc and meta tags using the trace level to stdout. The default configuration for all other messages at level warning is still be in effect.

-Xlog:gc+meta=trace

-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.

-Xlog:gc+class+heap*=debug,meta*=warning,threads*=off

💅 VALIDATE JAVA VIRTUAL MACHINE FLAG ARGUMENTS 💅

okay so you use the 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. validation era ✨

Whether they’re set ergonomically, in a command line, by an input tool, or through the APIs (for example, classes contained in the package java.lang.management), the values provided to all Java Virtual Machine (JVM) command-line flags are validated. Ergonomics are described in Java Platform, Standard Edition HotSpot Virtual Machine Garbage Collection Tuning Guide.

Range and constraints are validated either when all flags have their values set during JVM initialization or a flag’s value is changed during runtime (for example using the jcmd tool). The JVM is terminated if a value violates either the range or constraint check and an appropriate error message is printed on the error stream. no cap, the JVM will really exit.

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. 🫶

💖 LARGE PAGES 💖

Okay bestie, so large pages — also called huge pages — are memory pages that are way bigger than the standard memory page size (which varies depending on the processor and operating system, no cap). Large pages optimize processor Translation-Lookaside Buffers. ✨

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

But — and this is the important part — using large pages can negatively affect system performance. For example, when a large amount of memory is pinned by an application, it may create a shortage of regular memory and cause excessive paging in other applications and slow down the entire system. Also, a system that has been up for a long time could produce excessive fragmentation, which could make it impossible to reserve enough large page memory. When this happens, either the OS or JVM reverts to using regular pages. so like… big pages are iconic but not free, bestie. 😭

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. go little Huge fields go!! 🌈

Newer kernels have support for multiple large page sizes. To list the supported page sizes, run this:

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

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

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

It is always recommended to check the value of nr_hugepages after the request to make sure the kernel was able to allocate the requested number of large pages. like pls verify, bestie, don't just trust it. 🫶

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

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

Code cache
Marking bitmaps

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

🪟 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!! the admin is not exempt!! 😭

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

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

📦 Using CDS Archives 📦

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

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. iykyk, dynamic is still cute tho. 💅

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. a little test-drive first, obviously. ✨

🧱 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. only JAR files, bestie, don't sneak anything else in there. 💀

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. deterministic!! bit-for-bit!! iconic behavior!! 🌈

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. speed run mode, activated. 🔥

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

Advantages of dynamic CDS archives are:

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

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

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

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

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

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

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

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

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

To run the application using this dynamic CDS archive:

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

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

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

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

🪄 Creating CDS Archive Files with `jcmd` 🪄

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

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

🌸 Creating a Dynamic CDS Archive File with `jcmd <pid> VM.cds dynamic_dump` (part two, the dynamic part 🎀)

Okay so quick note before we go: to use jcmd <pid> VM.cds dynamic_dump, the JVM process identified by <pid> has to be started with -XX:+RecordDynamicDumpInfo. And bestie, you can also pass that to the application start-up script with the exact same technique:

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

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

Honestly? -XX:+AutoCreateSharedArchive is the convenient way of creating/using CDS archives. Like, the manual CDS archive creation methods in the previous section are cute, but with -XX:+AutoCreateSharedArchive you no longer need a separate trial run. Nope! Instead you just always run the application with the same command-line and get the CDS benefits automatically. Main character energy fr fr ✨

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

So here's the tea on how it behaves:

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's ignored at VM startup.
At VM exit, if the specified archive file does not exist, it will be created. If it exists but was created with a different (but post JDK 19) version of the JDK, then it will be replaced. In both cases the archive will be ready to be loaded the next time the JVM is launched with the same command line. Slay. 💅
If the specified archive file exists but was created by a JDK version prior to JDK 19, then it will be ignored: neither loaded at startup, nor replaced at exit. 🫠

Developers should note (hi devs, ily 🫶) that the contents of the CDS archive file are specific to each build of the JDK. So 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

Not everything is allowed, bestie. Here's the vibe check:

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, so breathe 😮‍💨)
The CDS archive cannot be loaded if any JAR files in the class path or module path are modified after the archive is generated. Don't touch them, periodt.

These are the module related options that CDS supports: --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's a mismatch of any of these options, the CDS archive may be partially or completely disabled — which means lower performance. Not iconic. 😭

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

And one more thing 👑 — if any of the VM options --upgrade-module-path, --patch-module or --limit-modules are specified, CDS is disabled. That means the JVM will execute without loading any CDS archives. Plus, if you try to create a CDS archive with any of these 3 options specified, the JVM will report an error. So like, don't do that. 💀

💖 AHEAD-OF-TIME CACHE 💖

The JDK supports ahead-of-time (AOT) optimizations that can be performed before an application is executed. One example is Class Data Sharing (CDS), as described above, that 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.

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

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

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

-XX:AOTCache=cachefile — Specifies the location of the AOT cache. The standard extension for cachefile is .aot. This option cannot be used together with -XX:AOTCacheOutput. This option is compatible with AOTMode settings of on, create, or auto (the default). The cachefile is read in AOT modes on and auto, and is ignored by mode off. The cachefile is written by AOT mode create. In that case, this option is equivalent to -XX:AOTCacheOutput=cachefile.
-XX:AOTCacheOutput=cachefile — Specifies the location of the AOT cache to write. The standard extension for cachefile is .aot. This option cannot be used together with -XX:AOTCache. This option is compatible with AOTMode settings of record, create, or auto (the default).
-XX:AOTConfiguration=configfile — Specifies the AOT Configuration file for the JVM to write to or read from. The standard extension for configfile is .aotconfig. This option is compatible with AOTMode settings of record, create, or auto (the default). The configfile is read by AOT mode create, and written by the other applicable modes. If the AOT mode is auto, then AOTCacheOutput must also be present.
-XX:AOTMode=mode — Specifies the AOT Mode for this run. mode must be one of the following: auto, off, record, create, or on.
- 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, on, 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 on (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.
on: Execute the application in the Production phase. If -XX:AOTCache=cachefile is specified, the JVM tries to load cachefile as the AOT cache. Otherwise, the JVM tries to load a default CDS archive from the JDK installation directory 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 is on, 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.

-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.

🚀 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 manual, bestie — source/version: JDK 25.0.5.0.3, date: 2026, page: JAVA(1). Periodt. 💖

📦 where this page comes from

arch rolling: jdk25-openjdk 25.0.5.0.3.u1-1 · LicenseRef-Java
arch rolling: jre25-openjdk 25.0.5.0.3.u1-1 · LicenseRef-Java
arch rolling: jre25-openjdk-headless 25.0.5.0.3.u1-1 · LicenseRef-Java
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms

JAVA(1)

🎀 JAVA(1) 🎀

🌸 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

🔍 ADVANCED SERVICEABILITY OPTIONS FOR JAVA 🔍

🗑️ ADVANCED GARBAGE COLLECTION OPTIONS FOR JAVA 🗑️

💀 DEPRECATED JAVA OPTIONS 💀

🧟 OBSOLETE JAVA OPTIONS 🧟

💀 REMOVED JAVA OPTIONS 💀

the fallen soldiers 🎀

🕰️ the nostalgia corner — where to find older removals

🎀 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 🌈

🎀 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 a Dynamic CDS Archive File with jcmd <pid> VM.cds dynamic_dump (part two, the dynamic part 🎀)

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

🎀 Restrictions on Class Path and Module Path

🌈 Module related options

💖 AHEAD-OF-TIME CACHE 💖

🔥 PERFORMANCE TUNING EXAMPLES 🔥

🚀 Tuning for Higher Throughput

🕰️ Tuning for Lower Response Time

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

✨ EXIT STATUS ✨

`-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`

🧱 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 a Dynamic CDS Archive File with `jcmd <pid> VM.cds dynamic_dump` (part two, the dynamic part 🎀)

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