JAVA(1)

📦 version 21.0.12.1.1, in centos-stream 9

okay so bestie, this is JAVA(1) from the JDK Commands manual — literally the page for the java launcher, and it's kind of iconic. Let's get into it 💅

✨ NAME ✨

java — launch a Java application

🎀 SYNOPSIS 🎀

To launch a class file:

java [options] mainclass [args ...]

To launch the main class in a JAR file:

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

To launch the main class in a module:

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

or

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

To launch a single source-file program:

java [options] source-file [args ...]
  • options — Optional: specifies command-line options separated by spaces. See Overview of Java Options for a description of the available options.
  • mainclass — Specifies the name of the class to be launched. Command-line entries following classname are the arguments for the main method.
  • -jar jarfile — Executes a program encapsulated in a JAR file. The jarfile argument is the name of a JAR file with a manifest that contains a line in the form Main-Class:classname that defines the class with the public static void main(String[] args) method that serves as your application's starting point. When you use -jar, the specified JAR file is the source of all user classes, and other class path settings are ignored. If you're using JAR files, then see jar.
  • -m or --module module[/mainclass] — Executes the main class in a module specified by mainclass if it is given, or, if it is not given, the value in the module. In other words, mainclass can be used when it is not specified by the module, or to override the value when it is specified.

See Standard Options for Java.

  • source-file — Only used to launch a single source-file program. Specifies the source file that contains the main class when using source-file mode. See Using Source-File Mode to Launch Single-File Source-Code Programs.
  • args ... — Optional: arguments following mainclass, source-file, -jar jarfile, and -m or --module module/mainclass are passed as arguments to the main class.

🦋 DESCRIPTION 🦋

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

public static void main(String[] args)

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

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

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

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

🪟 javaw

Windows: The javaw command is identical to java, except that with javaw there's no associated console window. Use javaw when you don't want a command prompt window to appear. The javaw launcher will, however, display a dialog box with error information if a launch fails.

🌸 USING SOURCE-FILE MODE TO LAUNCH SINGLE-FILE SOURCE-CODE PROGRAMS 🌸

To launch a class declared in a source file, run the java launcher in source-file mode. Entering source-file mode is determined by two items on the java command line:

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

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

Note: The valid values of N change for each release, with new values added and old values removed. You'll get an error message if you use a value of N that is no longer supported. The supported values of N are the current Java SE release (21) and a limited number of previous releases, detailed in the command-line help for javac, under the --source and --release options.

If the file does not have the .java extension, the --source option must be used to tell the java command to use the source-file mode. The --source option is used for cases when the source file is a "script" to be executed and the name of the source file does not follow the normal naming conventions for Java source files.

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

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

java HelloWorld.java

The example illustrates that the class can be in a named package, and does not need to be in the unnamed package. This use of source-file mode is informally equivalent to using the following two commands where hello.World is the name of the class in the package:

javac -d <memory> HelloWorld.java
java -cp <memory> hello.World

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

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

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

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

In source-file mode, compilation proceeds as follows:

  • Any command-line options that are relevant to the compilation environment are taken into account.
  • No other source files are found and compiled, as if the source path is set to an empty value.
  • Annotation processing is disabled, as if -proc:none is in effect.
  • If a version is specified, via the --source option, the value is used as the argument for an implicit --release option for the compilation. This sets both the source version accepted by compiler and the system API that may be used by the code in the source file.
  • The source file is compiled in the context of an unnamed module.
  • The source file should contain one or more top-level classes, the first of which is taken as the class to be executed.
  • 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 the 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 the standard public static void main(String[]) 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 the source file.
  • 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 standard main method in the obvious way.
  • It is an error if there is a class on the application class path whose name is the same as that of the class to be executed.

See JEP 330: Launch Single-File Source-Code Programs at https://openjdk.org/jeps/330 for complete details.

💅 USING THE JDK_JAVA_OPTIONS LAUNCHER ENVIRONMENT VARIABLE 💅

Okay so real talk, JDK_JAVA_OPTIONS is that girl who shows up early and prepends her whole vibe to the options parsed from the command line. The content of the JDK_JAVA_OPTIONS environment variable is a list of arguments separated by white-space characters (as determined by isspace()), and those get prepended to the command line arguments passed to the java launcher. The encoding requirement for the environment variable is the same as the java command line on the system, and JDK_JAVA_OPTIONS environment variable content is treated in the same manner as that specified in the command line. Consistency queen 💅

Single (') or double (") quotes can be used to enclose arguments that contain whitespace characters. All content between the open quote and the first matching close quote are preserved by simply removing the pair of quotes. But bestie — if a matching quote is not found, the launcher will abort with an error message. No half-quoted chaos allowed 🙅‍♀️

@-files are supported as they are specified in the command line. However, as in @-files, use of a wildcard is not supported. And listen, 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. Also when JDK_JAVA_OPTIONS is set, the launcher prints a message to stderr as a reminder. She's just checking in on you 🫶

Example, because we love a visual aid:

$ 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

Literally the same thing, just with extra steps and more drama ✨

🌈 OVERVIEW OF JAVA OPTIONS 🌈

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

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

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

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

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

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%. Math ate, no cap 🔥

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.

Three stages of a glow-down, honestly 😭

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

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

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

She's listening on port 8000 and she's suspending the JVM before the main class loads — main character energy fr 🧚

  • -agentpath:pathname[=options] — Loads the native agent library specified by the absolute path name. This option is equivalent to -agentlib but uses the full path and file name of the library. Same energy, more specific about where she lives 🌸
  • --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 (:). Know your separator, bestie 💅

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

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

  • --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. The "okay we're DONE expanding" button 🙈
  • --enable-preview — Allows classes to depend on preview features [https://docs.oracle.com/en/java/javase/12/language/index.html#JSLAN-GUID-5A82FE0E-0CA4-4F1F-B075-564874FE2823] of the release. Preview features are giving early-access sneak peek, use responsibly 👑
  • --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. So like, "enabled" is literally just there to be there 😭
  • --module-path modulepath... or -p modulepath — Specifies where to find application modules with a list of path elements. The elements of a module path can be a file path to a module or a directory containing modules. Each module is either a modular JAR or an exploded-module directory.

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

  • --upgrade-module-path modulepath... — Specifies where to find module replacements of upgradeable modules in the runtime image with a list of path elements. The elements of a module path can be a file path to a module or a directory containing modules. Each module is either a modular JAR or an exploded-module directory.

On Windows, semicolons (;) separate path elements in this list; on other platforms it is a colon (:). Yeah she said it twice, because it matters twice 💖

  • --add-modules module[,module...] — Specifies the root modules to resolve in addition to the initial module. module also can be ALL-DEFAULT, ALL-SYSTEM, and ALL-MODULE-PATH. Pick your faves and add them to the group chat 🫶
  • --list-modules — Lists the observable modules and then exits. Just a little show-and-tell, then she's out ✨
  • -d module_name or --describe-module module_name — Describes a specified module and then exits. Spilling the tea about a module, then leaving the party early 🎀
  • --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. It's the rehearsal, not the performance 🌈
  • --validate-modules — Validates all modules and exit. This option is helpful for finding conflicts and other errors with modules on the module path. Vibe check for your modules — and honestly, some of them need it 💀
  • -Dproperty=value — Sets a system property value. The property variable is a string with no spaces that represents the name of the property. The value variable is a string that represents the value of the property. If value is a string with spaces, then enclose it in quotation marks (for example -Dfoo="foo bar"). Spaces in the value? Quote her. Periodt 👑
  • -disableassertions[:[packagename]...|:classname] or -da[:[packagename]...|:classname] — this one turns assertions off. And ngl, they're already off by default in all packages and classes, so this is you being extra explicit 💅. No arguments? Then -disableassertions (-da) disables assertions in every package and class. Pass a packagename that ends in ... and the switch disables assertions in that package and any subpackages. If the argument is literally just ..., then it disables assertions in the unnamed package in the current working directory. Pass a classname and it disables assertions in that specific class, periodt.

The -disableassertions (-da) option applies to all class loaders and to system classes (which don't have a class loader). There's exactly one exception to this rule: if the option is provided with no arguments, then it doesn't apply to system classes. That's lowkey the whole point — it makes it easy to disable assertions in all classes except the system classes. The -disablesystemassertions option is what lets you disable assertions in all system classes. And if you want to explicitly enable assertions in specific packages or classes, that's the -enableassertions (-ea) option's job. Both options can be used at the same time, no drama. For example, to run the MyClass application with assertions enabled in the package com.wombat.fruitbat (and any subpackages) but disabled in the class com.wombat.fruitbat.Brickbat, you'd use this command:

java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass
  • -disablesystemassertions or -dsa — disables assertions in all system classes. That's it, that's the tweet 🎀.
  • -enableassertions[:[packagename]...|:classname] or -ea[:[packagename]...|:classname] — the glow-up version: this enables assertions ✨. Reminder that by default assertions are disabled in all packages and classes, so this is you opting in. No arguments? Then -enableassertions (-ea) enables assertions in all packages and classes. Pass a packagename ending in ... and 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. Pass a classname and the switch enables assertions in that 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 — so order matters, bestie, don't play yourself 🙈. For example, to run the MyClass application with assertions enabled only in the package com.wombat.fruitbat (and any subpackages) but disabled in the class com.wombat.fruitbat.Brickbat, use the following command:

java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass
  • -enablesystemassertions or -esa — enables assertions in all system classes. Little icon 👑.
  • -help, -h, or -? — prints the help message to the error stream. Yes, the error stream. It's giving chaotic but that's the vibe 🤷‍♀️.
  • --help — prints the help message to the output stream. The well-behaved sibling 🫶.
  • -javaagent:jarpath[=options] — loads the specified Java programming language agent. See java.lang.instrument for the whole lore.
  • --show-version — prints the product version to the output stream and then continues running. Not an exit, just a flex.
  • -showversion — prints the product version to the error stream and continues. Same energy, different stream 😭.
  • --show-module-resolution — shows module resolution output during startup. Great for when you're nosy about modules (respectfully).
  • -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, and the most appropriate scaled image provided is picked up automatically. Iconic behavior honestly 🌸.

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

-splash:images/splash.gif

See the SplashScreen API documentation for more information.

  • -verbose:class — displays information about each loaded class. Every single one. It's a lot and it's iconic.
  • -verbose:gc — displays information about each garbage collection (GC) event. Garbage in, gossip out 🗑️✨.
  • -verbose:jni — displays information about the use of native methods and other Java Native Interface (JNI) activity.
  • -verbose:module — displays information about the modules in use.
  • --version — prints the product version to the output stream and exits. Bye bestie 👋.
  • -version — prints the product version to the error stream and exits. Again: error stream. That's just how it is 💀.
  • -X — prints the help on extra options to the error stream.
  • --help-extra — prints the help on extra options to the output stream.
  • @argfile — specifies one or more argument files prefixed by @ used by the java command. Real talk: it isn't uncommon for the java command line to get very long because of the .jar files needed in the classpath. The @argfile option overcomes those command-line length limitations by enabling the launcher to expand the contents of argument files after shell expansion, but before argument processing. The contents in the argument files are expanded because otherwise, they would be specified on the command line until the --disable-@files option was encountered. Yes, that is a real sentence, and yes, it matters 😌.

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

java @argfile

See java Command-Line Argument Files for a description and examples of using @-argfiles. This is the self-care option for anyone with a 4,000-character classpath, fr fr 🧚.

🔥 EXTRA OPTIONS FOR JAVA 🔥

The following java options are general purpose options that are specific to the Java HotSpot Virtual Machine. These are the deep-magic ones 👑

  • -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. Main character energy: it refuses to do anything quietly in the background 💅.
  • -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 (:). Platform lore, don't forget it 🪟.

  • -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. It's thorough, not fast 😭.
  • -Xcomp — testing mode to exercise JIT compilers.
This option should not be used in production environments. Seriously, bestie, don't 🙈.
  • -Xdebug — does nothing. Provided for backward compatibility. The most useless icon, and yet we keep it 🎀.
  • -Xdiag — shows additional diagnostic messages.
  • -Xint — runs the application in interpreted-only mode. Compilation to native code is disabled, and all bytecode is executed by the interpreter. The performance benefits offered by the just-in-time (JIT) compiler aren't present in this mode. It's slow on purpose and honestly kind of brave 💀.
  • -Xinternalversion — displays more detailed JVM version information than the -version option, and then exits. It's the lore-deep version, and then it 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.
  • -Xmixed — executes all bytecode by the interpreter except for hot methods, which are compiled to native code. On by default. Use -Xint to switch off. The default! The main character! ⭐
  • -Xmn size — this one sets both the initial and max size (in bytes) of the heap for the young generation (the nursery, aka where all the shiny new baby objects hang out) in the generational collectors. Slap a k or K on the end for kilobytes, m or M for megabytes, or g or G for gigabytes 💖. The young-generation region is literally the zone for new objects, and GC happens there way more often than in the other regions. Too small a young gen? Then you get a ton of minor garbage collections. Too big? Then you get only full garbage collections, which can take forever to finish 😭. Recommendation, straight from the manpage: don't set the young-gen size for the G1 collector, and for the other collectors keep the young-gen size greater than 25% and less than 50% of the overall heap size. Here's how you'd set the initial and max young-gen size to 256 MB in different units:
-Xmn256m
-Xmn262144k
-Xmn268435456

Instead of using -Xmn to set both the initial and max young-gen heap size, you can use -XX:NewSize for the initial size and -XX:MaxNewSize for the maximum size. Options, options, options 🎀

  • -Xms size — sets the minimum and the initial size (in bytes) of the heap. This value must be a multiple of 1024 and greater than 1 MB — no cap, that's the rule 🫶. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. Setting allocated memory to 6 MB in various units, iconically:
-Xms6291456
-Xms6144k
-Xms6m

If you don't set this option, then 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 young-gen heap can be set with the -Xmn option or the -XX:NewSize option ✨

Heads up bestie: the -XX:InitialHeapSize option can also be used to set the initial heap size. If it shows up after -Xms on the command line, then the initial heap size gets set to the value specified with -XX:InitialHeapSize. Order matters, iykyk 💅
  • -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, so it's giving ✨main character energy✨ (it decides for itself). For server deployments, -Xms and -Xmx are often set to the same value. Setting the max allowed allocated memory to 80 MB, various units:
-Xmx83886080
-Xmx81920k
-Xmx80m

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

  • -Xnoclassgc — disables garbage collection (GC) of classes. This can save some GC time, which shortens interruptions while your app is running 🏃‍♀️. When you specify -Xnoclassgc at startup, the class objects in the application are left untouched during GC and are always considered live. Lowkey risky though: this can result in more memory being permanently occupied which, if not used carefully, throws an out-of-memory exception 💀
  • -Xrs — reduces the use of operating system signals by the JVM. Shutdown hooks enable the orderly shutdown of a Java application by running user cleanup code (like closing database connections) at shutdown, even if the JVM terminates abruptly. Two operating-system flavoured storylines:
    • 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 exists 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 that it doesn't watch for or process CTRL_C_EVENT, CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, or CTRL_SHUTDOWN_EVENT.

There are two consequences of specifying -Xrs, and they are:

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

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

  • -Xshare:mode — sets the class data sharing (CDS) mode 🦋. Possible mode arguments include:
    • auto — use shared class data if possible (default). Chill, low-stakes.
    • on — require using shared class data, otherwise fail. Very "all or nothing" of it.
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. Like, seriously, don't.
  • off — do not attempt to use shared class data.
  • -XshowSettings — shows all settings and then continues. That's it, that's the whole vibe 👑
  • -XshowSettings:category — shows settings and continues. Possible category arguments:
    • all — shows all categories of settings. This is the default value.
    • locale — shows settings related to locale 🌸
    • properties — shows settings related to system properties.
    • vm — shows the settings of the JVM.
    • system — Linux only: shows host system or container configuration and continues.
  • -Xss size — sets the thread stack size (in bytes). Append k or K for KB, m or M for MB, or g or G for GB. The actual size may be rounded up to a multiple of the system page size, as required by the operating system (not us, blame the OS 😤). 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

Setting the thread stack size to 1024 KB in different units:

-Xss1m
-Xss1024k
-Xss1048576

This option is similar to -XX:ThreadStackSize. Siblings, basically 🫶

  • *--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. Giving "you will be friends" energy ✨
  • *--add-exports *module/package=target-module(,*target-module*)* ** — updates *module* to export *package* to *target-module*, regardless of module declaration. The *target-module* can be all unnamed` to export to all unnamed modules.
  • *--add-opens *module/package=target-module(,`target-module)* ** — updates module to open package to target-module, regardless of module declaration. Boundaries? Never heard of her 💅
  • --limit-modules module[,module...] — specifies the limit of the universe of observable modules 🌌 (yes, "universe", they really said that, iconic)
  • *--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.

🍎 EXTRA OPTIONS FOR MACOS 🍎

The following extra options are macOS specific. Apple girlies, this one's for you 🌈

  • -XstartOnFirstThread — runs the main() method on the first (AppKit) thread. Very "first is the worst, second is the best"? No, here first is where you want to be 🧚
  • -Xdock:name=application_name — overrides the default application name displayed in dock.
  • -Xdock:icon=path_to_icon_file — overrides the default icon displayed in dock. Cute icon era 🎀

🔓 ADVANCED OPTIONS FOR JAVA 🔓

These java options can be used to enable other advanced options. Unlocking the DLC, basically 🎮

  • -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's very likely you'll be required to reproduce the problem without using any of these unsupported options before Oracle Support can assist with an investigation. It's also possible that any of these options may be removed or their behavior changed without any warning. So like... proceed with caution, bestie.
  • -XX:+UnlockExperimentalVMOptions — unlocks the options that provide experimental features in the JVM. By default, this option is disabled and experimental features aren't available. It's giving "lab rat chic" 🧪

⚡ ADVANCED RUNTIME OPTIONS FOR JAVA ⚡

These java options control the runtime behavior of the Java HotSpot VM.

  • -XX:ActiveProcessorCount=x — overrides the number of CPUs that the VM will use to calculate the size of thread pools it'll use for various operations such as Garbage Collection and ForkJoinPool. Lying to the VM about how many CPUs you have, but make it fashion 💅

The VM normally figures out how many processors are available by asking the operating system, lowkey. This flag is useful for partitioning CPU resources when you’re running multiple Java processes in docker containers — and it’s honored even if UseContainerSupport isn’t enabled, bestie. Wanna know how to turn container support on/off? See -XX:-UseContainerSupport for the deets. 💅

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

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

-XX:MaxDirectMemorySize=1m
-XX:MaxDirectMemorySize=1024k
-XX:MaxDirectMemorySize=1048576
  • -XX:-MaxFDLimit — disables the attempt to set the soft limit for the number of open file descriptors to the hard limit. By default, this option is enabled on all platforms, but is ignored on Windows. The only time that you may need to disable this is on macOS, where its use imposes a maximum of 10240, which is lower than the actual system maximum.
  • -XX:NativeMemoryTracking=mode — specifies the mode for tracking JVM native memory usage. Possible mode arguments for this option include the following:
    • off — instructs not to track JVM native memory usage. This is the default behavior if you don’t specify the -XX:NativeMemoryTracking option.
    • summary — tracks memory usage only by JVM subsystems, such as Java heap, class, code, and thread.
    • detail — in addition to tracking memory usage by JVM subsystems, track memory usage by individual CallSite, individual virtual memory region and its committed regions.
  • -XX:TrimNativeHeapInterval=millis — interval, in ms, at which the JVM will trim the native heap. Lower values will reclaim memory more eagerly at the cost of higher overhead. A value of 0 (default) disables native heap trimming. Native heap trimming is performed in a dedicated thread.

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

  • -XX:+NeverActAsServerClassMachine — turns on 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" is 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 the following 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 specified value should be a power of 2, and must be within the range of 8 and 256 (inclusive). This option makes it possible to use compressed pointers with large Java heap sizes.

The heap size limit in bytes is calculated as:

4GB * ObjectAlignmentInBytes
Note: as the alignment value increases, the unused space between objects also increases. As a result, you may not realize any benefits from using compressed pointers with large Java heap sizes.
  • -XX:OnError=string — sets a custom command or a series of semicolon-separated commands to run when an irrecoverable error occurs. If the string contains spaces, then it must be enclosed in quotation marks.
    • Non-Windows: the following example shows how the -XX:OnError option can be used to run the gcore command to create a core image, and start the gdb debugger to attach to the process in case of an irrecoverable error (the %p designates the current process identifier):
-XX:OnError="gcore %p;gdb -p %p"
  • Windows: the following example shows how the -XX:OnError option can be used to run the userdump.exe utility to obtain a crash dump in case of an irrecoverable error (the %p designates the current process identifier). This example assumes that the path to the userdump.exe utility is specified in the PATH environment variable:
-XX:OnError="userdump.exe %p"
  • -XX:OnOutOfMemoryError=string — sets a custom command or a series of semicolon-separated commands to run when an OutOfMemoryError exception is first thrown. 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.
  • -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.
  • -XX:+PrintNMTStatistics — enables printing of collected native memory tracking data at JVM exit when native memory tracking is enabled (see -XX:NativeMemoryTracking). By default, this option is disabled and native memory tracking data isn't printed.
  • -XX:SharedArchiveFile=path — specifies the path and name of the class data sharing (CDS) archive file.

See Application Class Data Sharing.

  • -XX:+VerifySharedSpaces — if this option is specified, the JVM will load a CDS archive file only if it passes an integrity check based on CRC32 checksums. The purpose of this flag is to check for unintentional damage to CDS archive files in transmission or storage. To guarantee the security and proper operation of CDS, the user must ensure that the CDS archive files used by Java applications cannot be modified without proper authorization.
  • -XX:SharedArchiveConfigFile=shared_config_file — specifies additional shared data added to the archive file.
  • -XX:SharedClassListFile=file_name — specifies the text file that contains the names of the classes to store in the class data sharing (CDS) archive. This file contains the full name of one class per line, except slashes (/) replace dots (.). For example, to specify the classes java.lang.Object and hello.Main, create a text file that contains the following two lines:
java/lang/Object
hello/Main

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

See Application Class Data Sharing.

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

If %p and/or %t is specified in the filename, it expands to the JVM's PID and the current timestamp, respectively.

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

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

  • settings=path — tells the JVM where your event settings file lives (it's a JFC-type file, fancy) and what it's called. Default is the default.jfc file, chilling in JAVA_HOME/lib/jfr 🫶. That default one collects a predefined set of info with low overhead, so it barely touches performance and honestly? you can leave it running on recordings that run continuously, no stress. ✨

There's also a second settings file, profile.jfc, which serves up more data than the default config — but it can have more overhead and it will impact performance 😭. Use that one for short periods of time when you need more info. Balance, bestie.

You can set values for multiple parameters by separating them with a comma. Event settings and .jfc options get specified with this syntax:

  • option=value — sets the option value you wanna modify. Wanna see what options exist? Use the JAVA_HOME/bin/jfr tool 💅.
  • event-setting=value — sets the event setting value you wanna modify. Format goes like this: <event-name>#<setting-name>=<value>. And to add a brand new event setting, prefix the event name with +. Iconic little +.

You can also set values for multiple event settings and .jfc options by separating them with commas. Now the drama: if a parameter and a .jfc option are in conflict, the parameter wins 👑. Also the whitespace delimiter can be omitted for timespan values, i.e. 20ms. For more info about the settings syntax, see the Javadoc of the jdk.jfr package.

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

Here's how you'd set that thread stack size to 1 megabyte in different units:

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

This option is similar to -Xss. Sibling energy.

  • -XX:-UseCompressedOops — disables compressed pointers. By default this option is enabled and compressed pointers are used, which automatically limits the max ergonomically determined Java heap size to the max amount of memory compressed pointers can cover. 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 boosts performance when you're running the app with Java heap sizes smaller than the compressed oops pointer range 💖. Heads up: this option works only for 64-bit JVMs.

And yes, it's possible to use compressed pointers with Java heap sizes greater than 32 GB — see the -XX:ObjectAlignmentInBytes option. iykyk.

  • -XX:-UseContainerSupport — Linux only 🐧. The VM now ships automatic container detection support, which lets it figure out how much memory and how many processors are available to a Java process running in docker containers, and it uses that info to allocate system resources. The default for this flag is true, so container support is on by default. You can disable it 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:+UseHugeTLBFS — Linux only. This option is the equivalent of specifying -XX:+UseLargePages. It's disabled by default. And here's the catch: it pre-allocates all large pages up-front when memory is reserved, so consequently the JVM can't dynamically grow or shrink large pages memory areas 😭. See -XX:UseTransparentHugePages if you want that behavior instead.

See Large Pages.

  • -XX:+UseLargePages — enables the use of large page memory. By default this option is disabled, and large page memory isn't used. Simple as that 💅.

See Large Pages.

  • -XX:+UseTransparentHugePages — Linux only. Enables large pages that can dynamically grow or shrink 🦋. Disabled by default. Fair warning though: you may hit performance problems with transparent huge pages because the OS moves other pages around to create huge pages. This option is made available for experimentation, so like… tinker responsibly.
  • -XX:+AllowUserSignalHandlers — Non-Windows ✨. Enables installation of signal handlers by the application. By default this option is disabled, meaning the application isn't allowed to install signal handlers. Periodt.
  • -XX:VMOptionsFile=filename — lets the user specify VM options in a file, for example: java -XX:VMOptionsFile=/var/my_vm_options HelloWorld. Clean, organised, we love a tidy file 🎀.
  • -XX:UseBranchProtection=mode — Linux AArch64 only. Specifies the branch protection mode. Every option other than none requires the VM to have been built with branch protection enabled — no cap. And for full protection, any native libraries provided by applications should be compiled with the same level of protection 🌈.

Possible mode arguments 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 ⚡. Very main character of it.

  • -XX:AllocateInstancePrefetchLines=lines — sets the number of lines to prefetch ahead of the instance allocation pointer. By default that number is set to 1:
-XX:AllocateInstancePrefetchLines=1
  • -XX:AllocatePrefetchDistance=size — sets the size (in bytes) of the prefetch distance for object allocation. Memory that's about to be written with the value of new objects gets prefetched up to this distance, starting from the address of the last allocated object. Every Java thread has its own allocation point, cute 💖.

Negative values mean the prefetch distance is chosen based on the platform. Positive values are bytes to prefetch. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. Default value is -1.

Setting the prefetch distance to 1024 bytes looks like:

-XX:AllocatePrefetchDistance=1024
  • -XX:AllocatePrefetchInstr=instruction — sets the prefetch instruction used to prefetch ahead of the allocation pointer. Possible values are from 0 to 3. The actual instructions behind those values depend on the platform 🌀. By default the prefetch instruction is set to 0:
-XX:AllocatePrefetchInstr=0
  • -XX:AllocatePrefetchLines=lines — sets the number of cache lines to load after the last object allocation, using the prefetch instructions generated in compiled code. Default is 1 if the last allocated object was an instance, and 3 if it was an array. Context matters 💅.

Setting the number of loaded cache lines to 5:

-XX:AllocatePrefetchLines=5
  • -XX:AllocatePrefetchStepSize=size — sets the step size (in bytes) for sequential prefetch instructions. Append k or K for kilobytes, m or M for megabytes, g or G for gigabytes. By default the step size is 16 bytes:
-XX:AllocatePrefetchStepSize=16
  • -XX:AllocatePrefetchStyle=style — sets the generated code style for prefetch instructions. The style argument is an integer from 0 to 3:
    • 0 — don't generate prefetch instructions.
    • 1 — execute prefetch instructions after each allocation. This is the default setting.
    • 2 — use the thread-local allocation block (TLAB) watermark pointer to determine when prefetch instructions are executed.
    • 3 — generate one prefetch instruction per cache line.
  • -XX:+BackgroundCompilation — enables background compilation. It's enabled by default ⭐. To disable background compilation, specify -XX:-BackgroundCompilation — which, fun fact, is equivalent to specifying -Xbatch.
  • -XX:CICompilerCount=threads — sets the number of compiler threads used for compilation. By default that number is selected automatically, depending on the number of CPUs and the memory available for compiled code 🧚. Setting it to 2 threads:
-XX:CICompilerCount=2
  • -XX:+UseDynamicNumberOfCompilerThreads — dynamically creates compiler threads up to the limit specified by -XX:CICompilerCount. This option is enabled by default 💜.
  • -XX:CompileCommand=command,method[,option] — specifies a command to perform on a method. For example, to exclude the indexOf() method of the String class from being compiled, you'd use:
-XX:CompileCommand=exclude,java/lang/String.indexOf

Note that you specify the full class name, including all packages and subpackages separated by a slash (/). And 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 you name a method without its signature, the command hits every method with that name, no cap. But bestie, you can also spell out the signature in class file format 💅 — and when you do, wrap the args in quotation marks, because otherwise the shell sees that semicolon and thinks the command ended. Rude of it, honestly. So if you only wanna exclude the indexOf(String) method of the String class from being compiled, it's giving:

-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 ✨. Like, to exclude all the indexOf() methods in all classes from being compiled:

-XX:CompileCommand=exclude,*.indexOf

The commas and periods are literally just aliases for spaces, which makes passing compiler commands through a shell way easier 🫶. So 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"

Okay so heads up 👀 — after it parses the commands you passed on the command line with the -XX:CompileCommand options, the JIT compiler then reads commands from the .hotspot_compiler file. You can add commands to that file, or point it at a totally different file with the -XX:CompileCommandFile option.

Wanna add several commands? Either drop the -XX:CompileCommand option multiple times, or separate each argument with the new line separator (\n). These are the commands available, bestie:

  • break — Sets a breakpoint when debugging the JVM, so it stops at the beginning of compilation of the specified method.
  • compileonly — Excludes all methods from compilation except 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 the specified method. By default, logging is performed for all compiled methods.
  • option — Passes a JIT compilation option to the specified method in place of the last argument (option). The compilation option is set at the end, after the method name. For example, to enable the BlockLayoutByFrequency option for the append() method of the StringBuffer class, use this:
-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 you specify with the -XX:CompileCommand option are printed — like, if you exclude the indexOf() method of the String class from compilation, then this gets printed to standard output:
CompilerOracle: exclude java/lang/String.indexOf

You can suppress this by specifying the -XX:CompileCommand=quiet option before the 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 go look at the -XX:CompileCommand option 👑.

Fr fr, the -XX:CompilerDirectivesFile option has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options. No unlock, no vibe check.
  • -XX:+CompilerDirectivesPrint — Prints the directives stack when the program starts, or when a new directive is added.
Same deal, bestie: the -XX:+CompilerDirectivesPrint option has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.
  • -XX:CompileOnly=methods — Sets the list of methods (separated by commas) to which compilation should be restricted. Only the specified methods are compiled. Specify each method with the full class name (including the packages and subpackages). For example, to compile only the length() method of the String class and the size() method of the List class:
-XX:CompileOnly=java/lang/String.length,java/util/List.size

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:CompileOnly=java.lang.String::length,java.util.List::size

Although wildcards aren't supported, you can specify only the class or package name to compile all methods in that class or package, and you can also specify just the method to compile methods with this name in any class:

-XX:CompileOnly=java/lang/String
-XX:CompileOnly=java/lang
-XX:CompileOnly=.length
  • -XX:CompileThresholdScaling=scale — Provides unified control of first compilation. This option controls when methods are first compiled for both the tiered and the nontiered modes of operation. The CompileThresholdScaling option has a floating point value between 0 and +Inf and scales the thresholds corresponding to the current mode of operation (both tiered and nontiered). Setting CompileThresholdScaling to a value less than 1.0 results in earlier compilation, while values greater than 1.0 delay compilation. Setting CompileThresholdScaling to 0 is equivalent to disabling compilation 💀.
  • -XX:+DoEscapeAnalysis — Enables the use of escape analysis. This option is enabled by default. To disable the use of escape analysis, specify -XX:-DoEscapeAnalysis.
  • -XX:InitialCodeCacheSize=size — Sets the initial code cache size (in bytes). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value depends on the platform. The initial code cache size shouldn't be less than the system's minimal memory page size. Here's how you 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 — Enables logging of compilation activity to a file named hotspot.log in the current working directory. You can specify a different log file path and name using the -XX:LogFile option.

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

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

  • -XX:FreqInlineSize=size — Sets the maximum bytecode size (in bytes) of a hot method to be inlined 🥵. This flag only applies to the C2 compiler. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value depends on the platform. In the following example it's set to 325 bytes:
-XX:FreqInlineSize=325
  • -XX:MaxInlineSize=size — Sets the maximum bytecode size (in bytes) of a cold method to be inlined 🧊. This flag only applies to the C2 compiler. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. By default, the maximum bytecode size is set to 35 bytes:
-XX:MaxInlineSize=35
  • -XX:C1MaxInlineSize=size — Sets the maximum bytecode size (in bytes) of a cold method to be inlined. This flag only applies to the C1 compiler. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. By default, the maximum bytecode size is set to 35 bytes:
-XX:MaxInlineSize=35
  • -XX:MaxTrivialSize=size — sets the max bytecode size (in bytes) a trivial method can have and still get inlined 💅 this flag is C2-only, so don't come at C1 with it. You can slap a k or K on the end for kilobytes, m or M for megabytes, or g or G for gigabytes. Default max trivial-method bytecode size is 6 bytes:
-XX:MaxTrivialSize=6
  • -XX:C1MaxTrivialSize=size — basically the same vibe as above, but for the C1 compiler specifically ✨ max bytecode size (in bytes) of a trivial method to be inlined, k/K = kilobytes, m/M = megabytes, g/G = gigabytes, and yeah the default is also 6 bytes:
-XX:MaxTrivialSize=6
  • -XX:MaxNodeLimit=nodes — caps the max number of nodes used during a single method compilation 🔥 the default lowkey depends on whichever features you've got enabled. Example time — setting the max node count to 100,000:
-XX:MaxNodeLimit=100000
  • -XX:NonNMethodCodeHeapSize=size — sets the size in bytes of the code segment that holds nonmethod code 🫶 so like, a nonmethod code segment contains nonmethod code — compiler buffers, the bytecode interpreter, that whole situation. This type of code stays in the code cache forever (iconic commitment tbh). Heads up: this flag is only used if -XX:SegmentedCodeCache is enabled.
  • -XX:NonProfiledCodeHeapSize=size — sets the size in bytes of the code segment holding nonprofiled methods 💖 again, only used when -XX:SegmentedCodeCache is enabled.
  • -XX:+OptimizeStringConcat — enables optimization of String concatenation operations ⚡ this one's enabled by default, so you're already winning. Wanna turn it off? Specify -XX:-OptimizeStringConcat and watch the vibes tank.
  • -XX:+PrintAssembly — enables printing of assembly code for bytecoded and native methods by using the external hsdis-<arch>.so or .dll library 🔍 for 64-bit VM on Windows, it's hsdis-amd64.dll. This lets you actually see the generated code, which may help you diagnose performance issues — very detective-main-character of you.

By default this option is disabled and assembly code isn't printed. Also, and this is the important part bestie: -XX:+PrintAssembly has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options. No unlock, no assembly. Periodt.

  • -XX:ProfiledCodeHeapSize=size — sets the size in bytes of the code segment containing profiled methods 🌸 only used if -XX:SegmentedCodeCache is enabled. See the pattern? It's giving segmented code cache gang.
  • -XX:+PrintCompilation — enables verbose diagnostic output from the JVM by printing a message to the console every time a method is compiled 📣 lets you see which methods actually get compiled. By default it's disabled and no diagnostic output is printed.

You can also log compilation activity to a file by using the -XX:+LogCompilation option. Multitasking queen.

  • -XX:+PrintInlining — enables printing of inlining decisions 👀 so you can see which methods are getting inlined. By default this is disabled and inlining info isn't printed. And yes, -XX:+PrintInlining has to be used together with the -XX:+UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.
  • -XX:ReservedCodeCacheSize=size — sets the max code cache size (in bytes) for JIT-compiled code 🧚 append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. Default max code cache size is 240 MB; but if you disable tiered compilation with -XX:-TieredCompilation, then the default size is 48 MB. This option has a limit of 2 GB — go over and an error gets generated, no cap. Also, the max code cache size shouldn't be less than the initial code cache size; see the option -XX:InitialCodeCacheSize.
  • -XX:RTMAbortRatio=abort_ratio — specifies the RTM abort ratio as a percentage (%) of all executed RTM transactions 💀 if the number of aborted transactions becomes greater than this ratio, then the compiled code is deoptimized. This ratio is used when the -XX:+UseRTMDeopt option is enabled. Default value is 50 — meaning the compiled code gets deoptimized if 50% of all transactions are aborted.
  • -XX:RTMRetryCount=number_of_retries — specifies how many times the RTM locking code is retried when it's aborted or busy, before falling back to the normal locking mechanism 🔁 default is 5. Reminder: the -XX:UseRTMLocking option must be enabled.
  • -XX:+SegmentedCodeCache — enables segmentation of the code cache 🎀 without it, the code cache is just one large segment. With -XX:+SegmentedCodeCache, separate segments get used for non-method, profiled method, and non-profiled method code. The segments are not resized at runtime — set it and forget it, literally.

The advantages are better control of the memory footprint, reduced code fragmentation, and better CPU iTLB (instruction translation lookaside buffer) and instruction cache behavior due to improved locality. All that from one flag, we love efficiency.

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

  • -XX:StartAggressiveSweepingAt=percent — forces stack scanning of active methods to aggressively remove unused code when only the given percentage of the code cache is free 🧹 default value is 10%.
  • -XX:-TieredCompilation — disables the use of tiered compilation 🙈 by default, this option is enabled (so this flag turns it off — yes, the double-negative is real, stay with 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, bestie).
  • -XX:UseAVX=version — enables the use of the AVX instruction set of a specified version 🌈 also set by default to the highest supported version available (x86 only).
  • -XX:+UseAES — enables hardware-based AES intrinsics for hardware that supports it 🔐 on by default on hardware that has the necessary instructions. -XX:+UseAES is used in conjunction with UseAESIntrinsics. And ngl: flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.
  • -XX:+UseAESIntrinsics — enables AES intrinsics ✨ specifying -XX:+UseAESIntrinsics is equivalent to also enabling -XX:+UseAES. To disable hardware-based AES intrinsics, specify -XX:-UseAES -XX:-UseAESIntrinsics. For example, to enable hardware AES, use the following flags:
-XX:+UseAES -XX:+UseAESIntrinsics

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

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

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

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

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

  • -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 — turns on intrinsification for all those java.lang.Math.*Exact() girlies ✨. On by default, no notes. But heads up: flags that control intrinsics now require the -XX:+UnlockDiagnosticVMOptions option first, so like... unlock the door before you walk in 💅
  • -XX:+UseMultiplyToLenIntrinsic — enables intrinsification of BigInteger.multiplyToLen(). On by default on platforms that support it (read: not everywhere, check your vibe). And again — flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions 🫶
  • -XX:+UseSquareToLenIntrinsic — same energy but for BigInteger.squareToLen(). Enabled by default on platforms that support it. Yes, still need -XX:+UnlockDiagnosticVMOptions for the intrinsic-controlling flags 😌
  • -XX:+UseMulAddIntrinsic — intrinsicifies BigInteger.mulAdd(). On by default where the platform supports it. Same rule: intrinsic-controlling flags need -XX:+UnlockDiagnosticVMOptions 🔑
  • -XX:+UseMontgomeryMultiplyIntrinsic — intrinsicifies BigInteger.montgomeryMultiply(). Default-on for supported platforms, and yep, -XX:+UnlockDiagnosticVMOptions is the entry fee for the intrinsic flags 🎟️
  • -XX:+UseMontgomerySquareIntrinsic — last of the BigInteger crew, this one intrinsicifies BigInteger.montgomerySquare(). Enabled by default on platforms that support it. And it's still -XX:+UnlockDiagnosticVMOptions for anything controlling intrinsics, periodt 💅
  • -XX:+UseCMoveUnconditionally — generates CMove instructions (both scalar and vector, she does both 💁) no matter what the profitability analysis says. It's not asking permission, it's just doing it.
  • -XX:+UseCodeCacheFlushing — lets the code cache get flushed before the compiler shuts down. This one's enabled by default babes ✨. Wanna turn it off? Specify -XX:-UseCodeCacheFlushing instead. (Notice the minus, it's giving "no thank you" 🎀)
  • -XX:+UseCondCardMark — makes it check whether the card is already marked before updating the card table. Disabled by default. Use it only on machines with multiple sockets, where it increases performance of Java apps that rely on concurrent operations. Like, multi-socket only, iykyk 🖥️🖥️
  • -XX:+UseCountedLoopSafepoints — keeps safepoints in counted loops. The default value depends on whether the garbage collector you picked needs low latency safepoints. So it's giving "it depends on your GC's whole personality" 🌸
  • -XX:LoopStripMiningIter= number_of_iterations — controls how many iterations live in the inner strip mined loop. So strip mining turns counted loops into two-level nested loops — safepoints get kept in the outer loop while the inner loop just zooms at full speed 🔥. This option caps the max number of iterations in the inner loop. Default value is 1,000 ⭐
  • -XX:LoopStripMiningIterShortLoop= number_of_iterations — controls the loop strip mining optimization: loops with fewer iterations than the number you specify will not have safepoints in them. Default value is 1/10th of -XX:LoopStripMiningIter 🎀
  • -XX:+UseFMA — enables hardware-based FMA intrinsics on hardware where FMA instructions actually exist (like Intel and ARM64, bestie). FMA intrinsics get generated for the java.lang.Math.fma(a, b, c) methods that calculate the value of ( a * b + c ) expressions. Math but make it hardware 🧮✨
  • -XX:+UseRTMDeopt — autotunes RTM locking based on the abort ratio, which is specified by the -XX:RTMAbortRatio option. If the number of aborted transactions exceeds the abort ratio, then the method containing the lock gets deoptimized and recompiled with all locks as normal locks. Disabled by default. The -XX:+UseRTMLocking option must be enabled for this one to work — it's a package deal 🙈
  • -XX:+UseRTMLocking — generates Restricted Transactional Memory (RTM) locking code for all inflated locks, with the normal locking mechanism as the fallback handler. Disabled by default. Also: options related to RTM are available only on x86 CPUs that support Transactional Synchronization Extensions (TSX). No TSX, no RTM, sorry babe 💀

So RTM is part of Intel's TSX, which is an x86 instruction set extension and 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. Iconic instruction lineup honestly 👑

A lock on a transaction is inflated when another thread tries to access the same transaction, thereby blocking 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. Not the ping-pong we wanted but ok 😭

  • -XX:+UseSuperWord — enables the transformation of scalar operations into superword operations. Superword is a vectorization optimization. This option is enabled by default ✨. Wanna stop the transformation of scalar operations into superword operations? Specify -XX:-UseSuperWord and that's it, bye 👋

🩺 ADVANCED SERVICEABILITY OPTIONS FOR JAVA

Okay bestie, these java options are the whole "gather system info + do extensive debugging" situation. Detective mode: activated 🔍

  • -XX:+DisableAttachMechanism — Turns off the mechanism that lets tools attach to the JVM. By default this option is disabled, which means the attach mechanism is enabled and you can use diagnostics and troubleshooting tools like jcmd, jstack, jmap, and jinfo. 🫶
Note: the tools shipped with the JDK — think jcmd, jinfo, jmap, and jstack — aren't supported when you use tools from one JDK version to troubleshoot a different JDK version. No mixing, babe. 🙈
  • -XX:+DTraceAllocProbes — Linux and macOS: enables dtrace tool probes for object allocation. ✨
  • -XX:+DTraceMethodProbes — Linux and macOS: enables dtrace tool probes for method-entry and method-exit.
  • -XX:+DTraceMonitorProbes — Linux and macOS: enables dtrace tool probes for monitor events.
  • -XX:+HeapDumpOnOutOfMemoryError — Enables the dumping of the Java heap to a file in the current directory, using the heap profiler (HPROF), when a java.lang.OutOfMemoryError exception is thrown. You can explicitly set the heap dump file path and name with the -XX:HeapDumpPath option. By default this option is disabled, and the heap isn't dumped when an OutOfMemoryError exception is thrown. 💀
  • -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. Here's how to set that default file explicitly (%p represents the current process identifier):
-XX:HeapDumpPath=./java_pid%p.hprof
  • Non-Windows: this example sets the heap dump file to /var/log/java/java_heapdump.hprof:
-XX:HeapDumpPath=/var/log/java/java_heapdump.hprof
  • Windows: this example sets 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 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 sets the log file to /var/log/java/hotspot.log:
-XX:LogFile=/var/log/java/hotspot.log
  • Windows: this example sets 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 these events:
    • Non-Windows: Control+\ (SIGQUIT)
    • Windows: Control+C (SIGTERM)

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

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

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

  • -XX:+PrintFlagsRanges — Prints the range specified and allows automatic testing of the values. See Validate Java Virtual Machine Flag Arguments. 👑
  • -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 like so:
jstat -class file:///path/hsperfdata_pid
jstat -gc file:///path/hsperfdata_pid
  • -XX:+UsePerfData — Enables the perfdata feature. This option is enabled by default to allow JVM monitoring and performance testing. Disabling it suppresses the creation of the hsperfdata_userid directories. To disable the perfdata feature, specify -XX:-UsePerfData. 🌸

🗑️ ADVANCED GARBAGE COLLECTION OPTIONS FOR JAVA

These java options control how garbage collection (GC) is performed by the Java HotSpot VM. Yes, we're talking about the trash collectors, and they're honestly kind of iconic. 🧚

  • -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 — Requests the VM to touch every page on the Java heap after requesting it from the operating system and before handing memory out to the application. By default this option is disabled and all pages are committed as the application uses the heap space.
  • -XX:ConcGCThreads=threads — Sets the number of threads used for concurrent GC. 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 that calls to System.gc() are processed. If processing of calls to System.gc() is disabled, then the JVM still performs GC when necessary. Bestie, it does not skip the work — it just ignores your little request. 😌
  • -XX:+ExplicitGCInvokesConcurrent — Enables invoking of concurrent GC by using the System.gc() request. This option is disabled by default and can be enabled only with the -XX:+UseG1GC option.
  • -XX:G1AdaptiveIHOPNumInitialSamples=number — When -XX:UseAdaptiveIHOP is enabled, this option sets the number of completed 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.

This 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, make sure you adjust the total Java heap by the same amount. By default this option is set to 10%.

This 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. 🎀

...and yes, we're still in the -XX:InitialHeapSize glow-up, bestie — here are the receipts:

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

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

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

  • -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. iykyk ✨

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). Heads up: 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 byes — and yes, the manpage fr says "byes" 😭) of the memory allocation pool. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value is selected at run time based on the system configuration. For server deployments, the options -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value. matching set, cute 🎀

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.

  • -XX:MaxHeapFreeRatio=percent — sets the maximum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space expands above this value, then the heap is shrunk. By default, this value is set to 70%.

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

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

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

  • -XX:MaxMetaspaceSize=size — sets the maximum amount of native memory that can be allocated for class metadata. By default, the size isn't limited. The amount of metadata for an application depends on the application itself, other running applications, and the amount of memory available on the system.

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

-XX:MaxMetaspaceSize=256m
  • -XX:MaxNewSize=size — sets the maximum size (in bytes) of the heap for the young generation (nursery). The default value is set ergonomically. the JVM picks it for you, main character behavior ✨
  • -XX:MaxRAM=size — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics. The default value is the maximum amount of available memory to the JVM process or 128 GB, whichever is lower.

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

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. no cap, this one has consequences 💀

The following example shows how to set the maximum amount of available memory for sizing the Java heap to 2 GB:

-XX:MaxRAM=2G
  • -XX:MaxRAMPercentage=percent — sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics, as a percentage of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 25 percent.

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

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

-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. The default value is 50 percent. (Yes, it's MinRAMPercentage but the description says "maximum amount" — that's genuinely what the page says, so we're keeping it fr fr 👀)

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

-XX:MinRAMPercentage=75
  • -XX:MaxTenuringThreshold=threshold — sets the maximum tenuring threshold for use in adaptive GC sizing. The largest value is 15. The default value is 15 for the parallel (throughput) collector. max value AND default being 15? obsessed ⭐

The following example shows how to set the maximum tenuring threshold to 10:

-XX:MaxTenuringThreshold=10
  • -XX:MetaspaceSize=size — sets the size of the allocated class metadata space that triggers a garbage collection the first time it's exceeded. This threshold for a garbage collection is increased or decreased depending on the amount of metadata used. The default size depends on the platform. platform girlies, check your own vibes 🌸
  • -XX:MinHeapFreeRatio=percent — sets the minimum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space falls below this value, then the heap is expanded. By default, this value is set to 40%.

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

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

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. same tea, different cup 💚

okay so — the actual GC knobs. buckle up bestie, these are the ones that lowkey decide whether your app is ✨vibing✨ or fighting for its life 🎀

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

here's how you'd set the minimum allocated memory to 6 MB in the different units:

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

and if you set this option to 0, then the minimum size gets set to the same value as the initial size. cute, right? 💅

  • -XX:NewRatio=ratio — sets the ratio between the young and old generation sizes. by default this option is set to 2. so to set the young-to-old ratio to 1:
-XX:NewRatio=1
  • -XX:NewSize=size — sets the initial size (in bytes) of the heap for the young generation (the nursery 🍼). append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes.

the young generation region of the heap is where new objects live. GC happens in this region way more often than in the other regions. and here's the vibe check: if the young generation size is too low, you get a large number of minor GCs. if it's too high, you get only full GCs, which can take a long time to complete 😭. the recommendation is to keep the young generation size greater than 25% and less than 50% of the overall heap size.

setting the initial young generation size to 256 MB, three ways:

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

and btw, the -XX:NewSize option is equivalent to -Xmn. same energy, different name 💫

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

like, to set the number of threads for G1 GC to 2:

-XX:ParallelGCThreads=2
  • -XX:+ParallelRefProcEnabled — enables parallel reference processing. by default this option is disabled. turn it on and let the refs process in parallel, no cap 🫶
  • -XX:+PrintAdaptiveSizePolicy — enables printing of information about adaptive-generation sizing. by default this option is disabled.
  • -XX:+ScavengeBeforeFullGC — enables GC of the young generation before each full GC. this one is enabled by default, and the recommendation is that you don't disable it, because scavenging the young generation before a full GC can reduce the number of objects reachable from the old generation space into the young generation space. to disable GC of the young generation before each full GC, specify -XX:-ScavengeBeforeFullGC (note the minus, bestie, the minus is the whole plot twist 🙈).
  • -XX:SoftRefLRUPolicyMSPerMB=time — sets the amount of time (in milliseconds) a softly reachable object is kept active on the heap after the last time it was referenced. the default value is one second of lifetime per free megabyte in the heap. the -XX:SoftRefLRUPolicyMSPerMB option accepts integer values representing milliseconds per one megabyte of the current heap size (for Java HotSpot Client VM) or the maximum possible heap size (for Java HotSpot Server VM).

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

setting the value to 2.5 seconds:

-XX:SoftRefLRUPolicyMSPerMB=2500
  • -XX:-ShrinkHeapInSteps — incrementally reduces the Java heap to the target size, specified by the option -XX:MaxHeapFreeRatio. this option is enabled by default. if disabled, then it immediately reduces the Java heap to the target size instead of requiring multiple garbage collection cycles. disable this option if you want to minimize the Java heap size. heads up tho — you will likely encounter performance degradation when this option is disabled 💀

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

  • -XX:StringDeduplicationAgeThreshold=threshold — identifies String objects reaching the specified age that are considered candidates for deduplication. an object's age is a measure of how many times it has survived garbage collection. this is sometimes referred to as tenuring.
Note: String objects that are promoted to an old heap region before this age has been reached are always considered candidates for deduplication. the default value for this option is 3. see the -XX:+UseStringDeduplication option.
  • -XX:SurvivorRatio=ratio — sets the ratio between eden space size and survivor space size. by default this option is set to 8. setting the eden/survivor space ratio to 4:
-XX:SurvivorRatio=4
  • -XX:TargetSurvivorRatio=percent — sets the desired percentage of survivor space (0 to 100) used after young garbage collection. by default this option is set to 50%.

to set the target survivor space ratio to 30%:

-XX:TargetSurvivorRatio=30
  • -XX:TLABSize=size — sets the initial size (in bytes) of a thread-local allocation buffer (TLAB). append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. if this option is set to 0, then the JVM selects the initial size automatically 👑.

initial TLAB size of 512 KB:

-XX:TLABSize=512k
  • -XX:+UseAdaptiveSizePolicy — enables the use of adaptive generation sizing. this option is enabled by default. to disable adaptive generation sizing, specify -XX:-UseAdaptiveSizePolicy and set the size of the memory allocation pool explicitly. see the -XX:SurvivorRatio option.
  • -XX:+UseG1GC — enables the use of the garbage-first (G1) garbage collector. it's a server-style garbage collector, targeted for multiprocessor machines with a large amount of RAM. this option meets GC pause time goals with high probability, while maintaining good throughput ✨. the G1 collector is recommended for applications requiring large heaps (sizes of around 6 GB or larger) with limited GC latency requirements (a stable and predictable pause time below 0.5 seconds). by default this option is enabled and G1 is used as the default garbage collector — it's the main character fr 💅
  • -XX:+UseGCOverheadLimit — enables the use of a policy that limits the proportion of time spent by the JVM on GC before an OutOfMemoryError exception is thrown. this option is enabled by default, and the parallel GC will throw an OutOfMemoryError if more than 98% of the total time is spent on garbage collection and less than 2% of the heap is recovered. when the heap is small, this feature can be used to prevent applications from running for long periods of time with little or no progress 🫠. to disable this option, specify -XX:-UseGCOverheadLimit.
  • -XX:+UseNUMA — enables performance optimization of an application on a machine with nonuniform memory architecture (NUMA) by increasing the application's use of lower latency memory. by default this option is disabled and no optimization for NUMA is made. the option is available only when the parallel garbage collector is used (-XX:+UseParallelGC).
  • -XX:+UseParallelGC — enables the use of the parallel scavenge garbage collector (also known as the throughput collector) to improve the performance of your application by leveraging multiple processors 🔥.

by default this option is disabled and the default collector is used.

  • -XX:+UseSerialGC — enables the use of the serial garbage collector. this is generally the best choice for small and simple applications that don't require any special functionality from garbage collection — sometimes simple is iconic 🎀. by default this option is disabled and the default collector is used.
  • -XX:+UseSHM — Linux only: enables the JVM to use shared memory to set up large pages.

see Large Pages for setting up large pages.

  • -XX:+UseStringDeduplication — enables string deduplication. by default this option is disabled. to use this option, you must enable the garbage-first (G1) garbage collector.

string deduplication reduces the memory footprint of String objects on the Java heap by taking advantage of the fact that many String objects are identical. instead of each String object pointing to its own character array, identical String objects can point to and share the same character array 🦋 — besties sharing one closet, basically.

  • -XX:+UseTLAB — enables the use of thread-local allocation blocks (TLABs) in the young generation space. this option is enabled by default. to disable the use of TLABs, specify the option -XX:-UseTLAB.
  • -XX:+UseZGC — enables the use of the Z garbage collector (ZGC). this is a low latency garbage collector, providing max pause times of a few milliseconds, at some throughput cost. pause times are independent of what heap size is used. supports heap sizes from 8MB to 16TB 🤯 — yes, terabytes, she's not playing around.
  • -XX:ZAllocationSpikeTolerance=factor — sets the allocation spike tolerance for ZGC. by default this option is set to 2.0. this factor describes the level of allocation spikes to expect. like, using a factor of 3.0 means the current allocation rate can be expected to triple at any time 📈

ZGC keeps going and honestly? iconic. Here's the rest of the ZGC option lineup 💅

  • -XX:ZCollectionInterval=seconds — Sets the maximum interval (in seconds) between two GC cycles when using ZGC. By default, this option is set to 0 (disabled). So yeah, it's off unless you say otherwise.
  • -XX:ZFragmentationLimit=percent — Sets the maximum acceptable heap fragmentation (in percent) for ZGC. By default, this option is set to 25. Using a lower value will cause the heap to be compacted more aggressively, to reclaim more memory at the cost of using more CPU time. Aggressive glow-up, but it costs CPU 💀
  • -XX:+ZProactive — Enables proactive GC cycles when using ZGC. By default, this option is enabled. ZGC will start a proactive GC cycle if doing so is expected to have minimal impact on the running application. This is useful if the application is mostly idle or allocates very few objects, but you still want to keep the heap size down and allow reference processing to happen even when there is a lot of free space on the heap. Lowkey the "clean your room before it's a problem" option ✨
  • -XX:+ZUncommit — Enables uncommitting of unused heap memory when using ZGC. By default, this option is enabled. Uncommitting unused heap memory will lower the memory footprint of the JVM, and make that memory available for other processes to use. Sharing is caring, bestie 🫶
  • -XX:ZUncommitDelay=seconds — Sets the amount of time (in seconds) that heap memory must have been unused before being uncommitted. By default, this option is set to 300 (5 minutes). Committing and uncommitting memory are relatively expensive operations. Using a lower value will cause heap memory to be uncommitted earlier, at the risk of soon having to commit it again. So don't rush her 😭

💀 DEPRECATED JAVA OPTIONS 💀

These java options are deprecated and might be removed in a future JDK release. They're still accepted and acted upon, but a warning is issued when they're used. Basically: they still work, but they're on thin ice 🧊

  • -Xfuture — Enables strict class-file format checks that enforce close conformance to the class-file format specification. Developers should use this flag when developing new code. Stricter checks may become the default in future releases. Future-you will thank you, fr fr.
  • -Xloggc:filename — Sets the file to which verbose GC events information should be redirected for logging. The -Xloggc option overrides -verbose:gc if both are given with the same java command. -Xloggc:filename is replaced by -Xlog:gc:filename. See Enable Logging with the JVM Unified Logging Framework.

Example:

-Xlog:gc:garbage-collection.log
  • -XX:+FlightRecorder — Enables the use of Java Flight Recorder (JFR) during the runtime of the application. Since JDK 8u40 this option has not been required to use JFR. So she's giving legacy era, but we respect her 🎀
  • -XX:InitialRAMFraction=ratio — Sets the initial amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a ratio of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 64.

Use the option -XX:InitialRAMPercentage instead. That's the upgrade, bestie ✨

  • -XX:MaxRAMFraction=ratio — Sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a fraction of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 4.

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

Use the option -XX:MaxRAMPercentage instead. No cap, that's the move 💅

  • -XX:MinRAMFraction=ratio — Sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a fraction of the maximum amount determined as described in the -XX:MaxRAM option for small heaps. A small heap is a heap of approximately 125 MB. The default value is 2.

Use the option -XX:MinRAMPercentage instead. Please, she's begging 🥺

🫥 OBSOLETE JAVA OPTIONS 🫥

These java options are still accepted but ignored, and a warning is issued when they're used. So they're literally just standing there. Not helping. Just vibing 🧍‍♀️

🚫 REMOVED JAVA OPTIONS 🚫

These java options have been removed in JDK 21 and using them results in an error of:

Unrecognized VM option option-name
  • -XX:+ExtendedDTraceProbes — Linux and macOS: Enables additional dtrace tool probes that affect performance. By default, this option is disabled and dtrace performs only standard probes. Use the combination of these flags instead: -XX:+DTraceMethodProbes, -XX:+DTraceAllocProbes, -XX:+DTraceMonitorProbes. She's gone, but the trio ate 🔥

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

🌸 JAVA COMMAND-LINE ARGUMENT FILES 🌸

okay so bestie, you can literally shorten or simplify the java command by using @ argument files to specify one or more text files that contain arguments — like options and class names — which get passed to the java command. This lets you create java commands of any length on any operating system. iconic behavior honestly 💅

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.

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 inside an argument file — to stop @ argument files expansion. slay, an escape hatch 🫶

The following items describe the syntax of java argument files:

  • The argument file must contain only ASCII characters or characters in system default encoding that's ASCII friendly, such as UTF-8.
  • The argument file size must not exceed MAXINT (2,147,483,647) bytes.
  • The launcher doesn't expand wildcards that are present within an argument file.
  • Use white space or new line characters to separate arguments included in the file.
  • White space includes a white space character, \t, \n, \r, and \f.

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

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

✨ 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.

🧚 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. iykyk, the pdf is the lore drop 📄✨

💖 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.

🌸 Synopsis 🌸

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

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

💅 Description 💅

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

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

  • -Xlog — Enables JVM logging on an info level.
  • -Xlog:help — Prints -Xlog usage syntax and available tags, levels, and decorators along with example command lines with explanations.
  • -Xlog:disable — Turns off all logging and clears all configuration of the logging framework including the default configuration for warnings and errors.
  • -Xlog[:option] — 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. no cap 🔍

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

lowkey the most chill default ever, it just quietly handles warnings and errors and minds its own business 💜

💅 Controlling Logging at Runtime

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

🏷️ -Xlog Tags and Levels

Every log message has a level and a tag set associated with 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. Mapping legacy runtime logging flags to the corresponding Xlog configuration is described in Convert Runtime Logging Flags to Xlog.

Available log levels:

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

Available log tags:

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

📤 -Xlog Output

The -Xlog option supports the following types of outputs:

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

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

⚡ -Xlog Output Mode

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

  • -Xlog:async — Write all logging asynchronously.

In asynchronous logging mode, log sites enqueue all logging messages to an intermediate buffer and a standalone thread is responsible for flushing them to the corresponding outputs. The intermediate buffer is bounded and on buffer exhaustion the enqueuing message is discarded. Log entry write operations are guaranteed non-blocking.

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 below. You can configure the decorations to be used at run time. Decorations are prepended to the log message. For example:

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

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

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

Logging Messages Decorations

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

🔄 Convert GC Logging Flags to Xlog

Legacy GC Logging Flags to Xlog Configuration Mapping

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

🧾 Convert Runtime Logging Flags to Xlog

These legacy flags are no longer recognized and will cause an error if used directly. Use their unified logging equivalent instead. bestie, do not try it. 🙈

Runtime Logging Flags to Xlog Configuration Mapping 🗺️✨

Okay bestie, here's the cheat sheet — every legacy runtime logging flag on the left, the shiny -Xlog config that replaces it, and a lil note about it. 💖

  • 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 🧪✨

Here come the actual examples, bestie — copy-paste vibes only. 🫶

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

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

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

Complex -Xlog Usage Examples 🌈👑

The following describes a few complex examples of using the -Xlog option. Buckle up, these ones are spicy. 🌶️

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

🛡️ VALIDATE JAVA VIRTUAL MACHINE FLAG ARGUMENTS 🛡️

So here's the tea ☕: you use values provided to all Java Virtual Machine (JVM) command-line flags for validation, and if the input value is invalid or out-of-range, then an appropriate error message is displayed. No silent nonsense, periodt. 💅

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. 😭 brutal but fair.

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 known as huge pages — are memory pages that are significantly bigger than the standard memory page size, which, btw, varies depending on your processor and operating system. The whole point? 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. It’s giving “scarce system resource,” and a TLB miss can be costly because the processor then has to 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. That means less pressure on a TLB, and memory-intensive applications may have better performance. Iconic behavior, 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 that happens, either the OS or JVM reverts to using regular pages. So yeah, slay, but responsibly. 🫶

Linux and Windows support large pages.

🐧 Large Pages Support for Linux 🐧

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

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

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

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

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

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

It is always recommended to check the value of nr_hugepages after the request to make sure the kernel was able to allocate the requested number of large pages. Like, always. No cap. ✅

When using the option -XX:+UseSHM to enable large pages you also need to make sure the SHMMAX parameter is configured to allow large enough shared memory segments to be allocated. To allow a maximum shared segment of 8 GB, login as root and run:

# echo 8589934592 > /proc/sys/kernel/shmmax

In some environments this is not needed since the default value is large enough, but it is important to make sure the value is large enough to fit the amount of memory intended to be backed by large pages. Don’t skip the vibe check on that one. 😭

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

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

  • Code cache
  • Marking bitmaps

Consequently, if you configure the nr_hugepages parameter to the size of the Java heap, then the JVM can still fail to allocate the heap using large pages because other areas such as the code cache might already have used some of the configured large pages. It’s giving “budget already spent before the main character arrived.” 💀

🪟 Large Pages Support for Windows 🪟

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

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

Note that these steps are required even if it’s the administrator who’s running the application, because administrators by default don’t have the privilege to lock pages in memory. Yes, even the admin. The audacity. 🙈

🦋 APPLICATION CLASS DATA SHARING 🦋

Okay so bestie, Application Class Data Sharing (aka AppCDS) is literally iconic: it stores the classes your apps use in an archive file. Because those classes are stored in a format that can be loaded very quickly compared to classes stored in a JAR file, AppCDS can improve your app's start-up time. No cap. Plus, AppCDS can reduce the runtime memory footprint by sharing parts of these classes across multiple processes. Slay.

Classes in the CDS archive are stored in an optimized format that's about 2 to 5 times larger than classes stored in JAR files or the JDK runtime image. So it's a good idea to archive only those classes that are actually used by your application. Those 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. Keep it cute, keep it minimal. 🎀

✨ Using CDS Archives ✨

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

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

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

(The names "static" and "dynamic" are used for historical reasons. The only significance is that the "static" archive is loaded first and the "dynamic" archive is loaded second. It's giving lore, but make it archive.)

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. It's basically the vibe check before the main character moment. 💅

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

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

  1. Create a list of all classes used by the test.Hello application. The following command creates a file named hello.classlist that contains a list of all classes used by this application:
java -Xshare:off -XX:DumpLoadedClassList=hello.classlist -cp hello.jar test.Hello

The classpath specified by the -cp parameter must contain only JAR files.

  1. 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
  1. Run the application test.Hello with the archive hello.jsa:
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello
  1. 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

🎀 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.

  1. Create a dynamic CDS archive, named hello.jsa, that contains all the classes in hello.jar loaded by the application test.Hello:
java -XX:ArchiveClassesAtExit=hello.jsa -cp hello.jar Hello
  1. Run the application test.Hello with the shared archive hello.jsa:
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello
  1. 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. Honestly, relatable chaos.

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

jcmd <pid> VM.cds static_dump my_static_archive.jsa

or a dynamic archive:

jcmd <pid> VM.cds dynamic_dump my_dynamic_archive.jsa

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

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

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

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

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

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

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

If the specified archive file exists but was created by a JDK version prior to JDK 19, then it will be ignored: neither loaded at startup, nor replaced at exit. Not me being ignored by the JVM. 😭

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

🌈 Restrictions on Class Path and Module Path 🌈

  • Neither the class path (-classpath and -Xbootclasspath/a) nor the module path (--module-path) can contain non-empty directories.
  • Only modular JAR files are supported in --module-path. Exploded modules are not supported.
  • The class path used at archive creation time must be the same as (or a prefix of) the class path used at run time. (There's no such requirement for the module path.)
  • The CDS archive cannot be loaded if any JAR files in the class path or module path are modified after the archive is generated.
  • If any of the VM options --upgrade-module-path, --patch-module or --limit-modules are specified, CDS is disabled. This means that the JVM will execute without loading any CDS archives. In addition, if you try to create a CDS archive with any of these 3 options specified, the JVM will report an error.

🔥 PERFORMANCE TUNING EXAMPLES 🔥

You can use the Java advanced runtime options to optimize the performance of your applications. Let's make it fast, bestie. 🏎️✨

👑 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 💀

And that's the whole page, bestie — source/version JDK 21, dated 2023, manual page JAVA(1). Periodt. 🎀💅🫶

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