JAVA(1)

🌸 NAME

java — launch a Java application. And hiii bestie, this is the JAVA(1) page from JDK Commands — the manual for the java launcher — and we're gonna make it make sense. 💖

✨ 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 source-file program:

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

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

💅 DESCRIPTION

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

public static void main(String[] args)

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

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

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

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

🪟 javaw

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

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

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

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

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

Note: The valid values of N change for each release, with new values added and old values removed. You'll get an error message if you use a value of N that is no longer supported. The supported values of N are the current Java SE release (23) and a limited number of previous releases, detailed in the command-line help for javac, under the --source and --release options.

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

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

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

java HelloWorld.java

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

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

where <source-root> is computed

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

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

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

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

In source-file mode, compilation proceeds as follows:

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

In source-file mode, execution proceeds as follows:

• The class to be executed is the first top-level class found in the source file. It must contain a declaration of an entry main method.
• The compiled classes are loaded by a custom class loader, that delegates to the application class loader. This implies that classes appearing on the application class path cannot refer to any classes declared in source files.
• If a module-info.java file exists in the <source-root> directory, then all the classes compiled from .java files in the source tree will be in that module, which will serve as the root module for the execution of the program. If module-info.java does not exist, the compiled classes are executed in the context of an unnamed module, as though --add-modules=ALL-DEFAULT is in effect. This is in addition to any other --add-module options that may be have been specified on the command line.
• Any arguments appearing after the name of the file on the command line are passed to the main method in the obvious way.
• It is an error if there is a class on the application class path whose name is the same as that of the class to be executed.

See JEP 458: Launch Multi-File Source-Code Programs — https://openjdk.org/jeps/458 — for complete details.

🌸 USING THE JDK_JAVA_OPTIONS LAUNCHER ENVIRONMENT VARIABLE 🌸

okay bestie, so JDK_JAVA_OPTIONS is like this little env var that prepends its whole vibe to the options the launcher already parsed from the command line. The content of the environment variable is literally 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 exactly the same as the java command line on the system, and the content gets treated in the same manner as whatever you specified right there on the command line. no cap 💅

You can use single (') or double (") quotes to enclose arguments that contain whitespace characters. Everything between the open quote and the first matching close quote is preserved — the launcher just removes the pair of quotes and keeps the good stuff. But if a matching quote is not found? the launcher will abort with an error message. 😭 @-files are supported just like they are on the command line, however — same as with @-files — use of a wildcard is not supported.

🫶 so here's the tea: to mitigate potential misuse of JDK_JAVA_OPTIONS behavior, options that specify the main class (such as -jar) or that cause the java launcher to exit without executing the main class (such as -h) are disallowed in the environment variable. If any of these options appear in the environment variable, the launcher will abort with an error message. And whenever JDK_JAVA_OPTIONS is set, the launcher prints a message to stderr as a reminder. considerate king behavior honestly.

Example, because we love a demo:

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

is equivalent to the command line:

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

see how it just slid right on in front? iconic ✨

---

🌈 OVERVIEW OF JAVA OPTIONS 🌈

The java command supports a wide range of options, and they live in these 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 — checking the version of the JRE, setting the class path, enabling verbose output, and so on. the everyday essentials 💖
• 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 they're subject to change. These options start with -X.

so, the advanced options? not recommended for casual use, bestie. These are developer options used for tuning specific areas of the Java HotSpot Virtual Machine operation, and they 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 equals yes, minus equals no, we love a binary 💅

For options that require an argument, the argument may be separated from the option name by a space, a colon (:), or an equal sign (=), or the argument may directly follow the option (the exact syntax differs for each option — read the fine print, babe). 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, honestly 🍓

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

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

---

👑 STANDARD OPTIONS FOR JAVA 👑

These are the most commonly used options supported by all implementations of the JVM. the whole squad, no notes.

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

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

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

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

• -agentpath:pathname[=options] — loads the native agent library specified by the absolute path name. This option is equivalent to -agentlib but uses the full path and file name of the library. same energy, just with the whole address 🫶
• --class-path classpath, -classpath classpath, or -cp classpath — specifies a list of directories, JAR files, and ZIP archives to search for class files.

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

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

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

• --disable-@files — can be used anywhere on the command line, including in an argument file, to prevent further @filename expansion. This option stops expanding @-argfiles after the option. bye bestie, no more files 👋
• --enable-preview — allows classes to depend on preview features [https://docs.oracle.com/en/java/javase/12/language/index.html#JSLAN-GUID-5A82FE0E-0CA4-4F1F-B075-564874FE2823] of the release. sneak peek mode ✨
• --enable-native-access module[,module...] — native access involves access to code or data outside the Java runtime. This is generally unsafe and, if done incorrectly, might crash the JVM or result in memory corruption. Methods that provide native access are restricted, and by default their use causes warnings. This option allows code in the specified modules to use restricted methods without warnings. module can be ALL-UNNAMED to indicate code on the class path. When this option is present, any use of restricted methods by code outside the specified modules causes an IllegalCallerException. 😭 handle with care, fr.
• --finalization=value — controls whether the JVM performs finalization of objects. Valid values are "enabled" and "disabled". Finalization is enabled by default, so the value "enabled" does nothing. The value "disabled" disables finalization, so that no finalizers are invoked. 💀
• --module-path modulepath... or -p modulepath — specifies where to find application modules with a list of path elements. The elements of a module path can be a file path to a module or a directory containing modules. Each module is either a modular JAR or an exploded-module directory.

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

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

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

• --add-modules module[,module...] — specifies the root modules to resolve in addition to the initial module. module can also be ALL-DEFAULT, ALL-SYSTEM, and ALL-MODULE-PATH. collect them all 🎀
• --list-modules — lists the observable modules and then exits. little show-and-tell moment.
• -d module_name or --describe-module module_name — describes a specified module and then exits. tell me everything 👑
• --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. all dressed up, nowhere to run 💅
• --validate-modules — checks all the modules and then dips. Super handy for sniffing out conflicts and other messy errors with the modules chillin' on the module path.
• -Dproperty=value — sets a system property, classic. The property bit is a string with no spaces and it's the property's name. The value bit is a string holding the property's value. If your value has spaces in it, wrap that baby in quotation marks, like -Dfoo="foo bar". No spaces, or quote it — those are the rules, bestie.
• -disableassertions[:[packagename]...|:classname] or -da[:[packagename]...|:classname] — turns assertions off. By default, assertions are already disabled in, like, everything — all packages and classes. With no arguments, -disableassertions (-da) disables assertions in all packages and classes. If you pass a packagename argument that ends in ..., the switch disables assertions in that package and any subpackages. And if the argument is literally just ..., then the switch disables assertions in the unnamed package in the current working directory. With a classname argument, it disables assertions in that specific class. 💅

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

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

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

The -enableassertions (-ea) option applies to all class loaders and to system classes (which don't have a class loader). One exception, and it's the same one as before: if you provide the option with no arguments, then it doesn't apply to system classes. That makes it easy to enable assertions in all classes except system classes. The -enablesystemassertions option gives you a separate switch to enable assertions in all system classes. To explicitly disable assertions in specific packages or classes, use the -disableassertions (-da) option. And here's the tea: if a single command contains multiple instances of these switches, then they're processed in order, before loading any classes. So order matters, periodt. 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 this:

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

• -enablesystemassertions or -esa — enables assertions in all system classes. Slay. 🌸
• -help, -h, or -? — prints the help message to the error stream. (Yes, the error stream. Don't ask, it's giving legacy.)
• --help — prints the help message to the output stream. Now that's the main character version. 👑
• -javaagent:jarpath[=options] — loads the specified Java programming language agent. See java.lang.instrument for the whole story.
• --show-version — prints the product version to the output stream and then continues running. Not a quitter. 💖
• -showversion — prints the product version to the error stream and continues. Same energy, different stream.
• --show-module-resolution — shows module resolution output during startup. Peek behind the curtain, bestie. 🦋
• -splash:imagepath — shows the splash screen with the image specified by imagepath. HiDPI scaled images are automatically supported and used if they're available. The unscaled image file name, such as image.ext, should always be passed as the argument to the -splash option. The most appropriate scaled image that's provided gets picked up automatically. Ngl, kind of iconic.

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

-splash:images/splash.gif

See the SplashScreen API documentation for more info.

• -verbose:class — displays information about each loaded class. All of them. Every single one. 😭
• -verbose:gc — displays information about each garbage collection (GC) event.
• -verbose:jni — displays information about the use of native methods and other Java Native Interface (JNI) activity.
• -verbose:module — displays information about the modules in use.
• --version — prints the product version to the output stream and exits. Bye. 💅
• -version — prints the product version to the error stream and exits.
• -X — prints the help on extra options to the error stream. Foreshadowing, much? 👀
• --help-extra — prints the help on extra options to the output stream.
• @argfile — specifies one or more argument files prefixed by @ that the java command uses. Real talk: it is not uncommon for the java command line to be extremely long because of the .jar files needed in the classpath. The @argfile option overcomes command-line length limitations by letting the launcher expand the contents of argument files after shell expansion, but before argument processing. Contents in the argument files are expanded because otherwise, they'd be specified on the command line until the --disable-@files option was encountered.

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

java @argfile

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

---

🔥 EXTRA OPTIONS FOR JAVA 🔥

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

• -Xbatch — disables background compilation. By default, the JVM compiles the method as a background task, running the method in interpreter mode until the background compilation is finished. The -Xbatch flag disables background compilation so that compilation of all methods proceeds as a foreground task until it's completed. This option is equivalent to -XX:-BackgroundCompilation. Lowkey a control freak option and I respect it.
• -Xbootclasspath/a:directories|zip|JAR-files — specifies a list of directories, JAR files, and ZIP archives to append to the end of the default bootstrap class path.

On Windows, semicolons (;) separate entities in this list; on other platforms it's a colon (:). Platform differences, classic.

• -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

Heads up: expect a performance degradation when this option is used. Not free, bestie. 😭

• -Xcomp — testing mode to exercise JIT compilers. This option should not be used in production environments. Fr fr, do not ship this. 🙈
• -Xdebug — does nothing; deprecated for removal in a future release. It's literally just standing there. 💀
• -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. Fast to start, slow to go, iykyk.
• -Xinternalversion — displays more detailed JVM version information than the -version option, and then exits. Extra detailed. Extra done. ✨
• -Xlog:option — configure or enable logging with the Java Virtual Machine (JVM) unified logging framework. See Enable Logging with the JVM Unified Logging Framework.
• -Xmixed — executes all bytecode by the interpreter except for hot methods, which are compiled to native code. On by default. Use -Xint to switch off. 🔥
• -Xmn size — sets both the initial and maximum size (in bytes) of the heap for the young generation (aka the nursery 🍼) in the generational collectors. Pop a k or K on the end for kilobytes, m or M for megabytes, or g or G for gigabytes. The young-gen region is where brand-new objects live, and GC happens here way more often than in other regions. Too small? Then you get a whole bunch of minor garbage collections, like, constantly 😭. Too large? Then only full garbage collections run, and those can take forever to finish. So the recommendation is: don't set the young-gen size for the G1 collector at all, and for the other collectors keep it greater than 25% and less than 50% of the overall heap size. The following examples show how to set the initial and maximum size of young generation to 256 MB using various units:

-Xmn256m
-Xmn262144k
-Xmn268435456

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

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

-Xms6291456
-Xms6144k
-Xms6m

If you don't set this option, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation. The initial size of the heap for the young generation can be set using the -Xmn option or the -XX:NewSize option.

Ngl, watch out: the -XX:InitialHeapSize option can also be used to set the initial heap size. And if it appears after -Xms on the command line, then the initial heap size gets set to the value specified with -XX:InitialHeapSize. Order matters, bestie 💅

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

-Xmx83886080
-Xmx81920k
-Xmx80m

The -Xmx option is equivalent to -XX:MaxHeapSize. same vibe, different name 🫶

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

There are two consequences of specifying -Xrs:

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

User code is responsible for causing shutdown hooks to run, for example, by calling System.exit() when the JVM is to be terminated. you're the main character of cleanup now 💀

---

🦋 -Xshare and the show-settings girlies 🦋

• -Xshare: mode — sets the class data sharing (CDS) mode. Possible mode arguments for this option include the following:
  • auto — use shared class data if possible (default). very go-with-the-flow of her 🌸
  • on — require using shared class data, otherwise fail.

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

• 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 for this option include the following:
  • all — shows all categories of settings in verbose detail.
  • locale — shows settings related to locale.
  • properties — shows settings related to system properties.
  • security — shows all settings related to security. Sub-category arguments for security include the following:
    • security:all : shows all security settings
    • security:properties : shows security properties
    • security:providers : shows static security provider settings
    • security:tls : shows TLS related security settings
  • vm — shows the settings of the JVM.
  • system — Linux only: shows host system or container configuration and continues.
• -Xss size — sets the thread stack size (in bytes). Append 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 (the OS said "rounding is self-care" 💅). The default value depends on the platform. For example:
  • Linux/x64: 1024 KB
  • Linux/Aarch64: 2048 KB
  • macOS/x64: 1024 KB
  • macOS/Aarch64: 2048 KB
  • Windows: the default value depends on virtual memory

The following examples set the thread stack size to 1024 KB in different units:

-Xss1m
-Xss1024k
-Xss1048576

This option is similar to -XX:ThreadStackSize. icon behavior 🫶

---

🔥 module options, the friend-group of flags 🔥

• --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. no gatekeeping allowed 🔓
• --add-exports module/package=target-module(,target-module)* — updates *module* to export *package* to *target-module*, regardless of module declaration. *target-module* can be ALL-UNNAMED` to export to all unnamed modules.
• --add-opens module/package=target-module(,target-module`)* — updates module to open package to target-module, regardless of module declaration. opening up = vulnerability but make it intentional 🦋
• --limit-modules module[,module...] — specifies the limit of the universe of observable modules. the observable universe but for modules, obsessed 🌌
• --patch-module module=file(;file)* — overrides or augments a module with classes and resources in JAR files or directories.
• --source version — sets the version of the source in source-file mode.
• --sun-misc-unsafe-memory-access= value — allow or deny usage of the unsupported API sun.misc.Unsafe. value is one of:
  • allow — allow use of the memory-access methods with no warnings at run time.
  • warn — allow use of the memory-access methods, but issues a warning on the first occasion that any memory-access method is used. At most one warning is issued.
  • debug — allow use of the memory-access methods, but issue a one-line warning and a stack trace when any memory-access method is used.
  • deny — disallow use of the memory-access methods by throwing an UnsupportedOperationException on every usage.

The default value when the option is not specified is allow. iykyk, it's the "unsafe" one for a reason 😭

---

🍎 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. main character energy, literally ✨
• -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. aesthetic customization, we love to see it 🎀

🔓 ADVANCED OPTIONS FOR JAVA 🔓

Okay bestie, these java options are the keys that unlock other options. Very meta, very main-character. 🔑

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

⚠️ Real talk, bestie: command line options that are enabled with the use of this option are not supported. If you encounter issues while using any of these options, it is very likely that you will be required to reproduce the problem without using any of these unsupported options before Oracle Support can assist with an investigation. It is also possible that any of these options may be removed or their behavior changed without any warning. 💀 So like… use at your own risk, no cap.

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

---

⚙️ ADVANCED RUNTIME OPTIONS FOR JAVA ⚙️

These java options control the runtime behavior of the Java HotSpot VM. This is where the actual ~vibes~ of your JVM get tuned. 🎛️

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

• -XX:+ExtensiveErrorReports — Enables the reporting of more extensive error information in the ErrorFile. This option can be turned on in environments where maximal information is desired — even if the resulting logs may be quite large and/or contain information that might be considered sensitive. The information can vary from release to release, and across different platforms. By default this option is disabled. 🔍

🙈 Heads up that "might be considered sensitive" part — this flag is giving oversharer energy and you should know that going in.

• -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. 🍎

💡 That's the whole macOS tea, fr fr — 10240 is literally less than what the system could do on its own, so flipping this off is the move there.

• -XX:NativeMemoryTracking=mode — sets the mode for tracking JVM native memory usage. Possible mode arguments for this option include:
  • 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 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 — enables the "Client VM emulation" mode, which only uses the C1 JIT compiler, a 32Mb CodeCache and the Serial GC. The maximum amount of memory that 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. -XX:StartFlightRecording:help prints available options and example command lines. You can set the following parameter=value entries when starting a JFR recording:
  • delay=time — specifies the delay between the Java application launch time and the start of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 10m means 10 minutes). By default, there's no delay, and this parameter is set to 0.
  • disk={true|false} — specifies whether to write data to disk while recording. By default, this parameter is enabled.
  • dumponexit={true|false} — specifies if the running recording is dumped when the JVM shuts down. If enabled and a filename 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. The filename may also be a directory in which case, the filename is generated from the PID and the current date in the specified directory.

• name=identifier — takes both the name and the identifier of a recording.
• maxage=time — specifies the maximum age of disk data to keep for the recording. This parameter is valid only when the disk parameter is set to true. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 30s means 30 seconds). By default, the maximum age isn't limited, and this parameter is set to 0s.

• maxsize=size — sets the maximum size, in bytes, of disk data to keep for the recording 💾. This one is valid only when the disk parameter is set to true. The value must not be less than the value for the maxchunksize parameter set with -XX:FlightRecorderOptions. Append m or M for megabytes, or g or G for gigabytes. By default, the maximum size of disk data isn’t limited, and this parameter is set to 0.
• path-to-gc-roots={true|false} — says 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 — gives the path and name of the event settings file, of type JFC 🎀. By default, the default.jfc file is used, and it lives in JAVA_HOME/lib/jfr. This default settings file collects a predefined set of information with low overhead, so it has minimal impact on performance and can be used with recordings that run continuously.

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

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

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

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

To only see warnings and errors from JFR during startup set -Xlog:jfr+startup=warning.

• -XX:ThreadStackSize=size — sets the Java thread stack size, in kilobytes 🧵. Use of a scaling suffix, such as k, results in the scaling of the kilobytes value so that -XX:ThreadStackSize=1k sets the Java thread stack size to 1024*1024 bytes or 1 megabyte. The default value depends on the platform. For example:
  • Linux/x64: 1024 KB
  • Linux/Aarch64: 2048 KB
  • macOS/x64: 1024 KB
  • macOS/Aarch64: 2048 KB
  • Windows: The default value depends on virtual memory

The following examples show how to set the thread stack size to 1 megabyte in different units:

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

This option is similar to -Xss.

• -XX:-UseCompressedOops — disables the use of compressed pointers 🙈. By default, this option is enabled, and compressed pointers are used. This will automatically limit the maximum ergonomically determined Java heap size to the maximum amount of memory that can be covered by compressed pointers. By default this range is 32 GB.

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

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

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

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

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

• -XX:+UseLargePages — enables the use of large page memory 📄. By default, this option is disabled and large page memory isn’t used.

See Large Pages.

• -XX:+UseTransparentHugePages — Linux only: enables the use of large pages that can dynamically grow or shrink 🦋. This option is disabled by default. You may encounter performance problems with transparent huge pages as the OS moves other pages around to create huge pages; this option is made available for experimentation.
• -XX:+AllowUserSignalHandlers — Non-Windows: enables installation of signal handlers by the application ⚡. By default, this option is disabled and the application isn’t allowed to install signal handlers.
• -XX:VMOptionsFile=filename — allows user to specify VM options in a file, for example, java -XX:VMOptionsFile=/var/my_vm_options HelloWorld.
• -XX:UseBranchProtection=mode — Linux AArch64 only: specifies the branch protection mode 🛡️. All options other than none require the VM to have been built with branch protection enabled. In addition, for full protection, any native libraries provided by applications should be compiled with the same level of protection.

Possible mode arguments for this option include the following:

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

⚡ ADVANCED JIT COMPILER OPTIONS FOR JAVA ⚡

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

• -XX:AllocateInstancePrefetchLines=lines — sets the number of lines to prefetch ahead of the instance allocation pointer. By default, the number of lines to prefetch is set to 1:

-XX:AllocateInstancePrefetchLines=1

• -XX:AllocatePrefetchDistance=size — sets the size, in bytes, of the prefetch distance for object allocation 📏. Memory about to be written with the value of new objects is prefetched up to this distance starting from the address of the last allocated object. Each Java thread has its own allocation point.

Negative values denote that prefetch distance is chosen based on the platform. Positive values are bytes to prefetch. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default value is set to -1.

The following example shows how to set the prefetch distance to 1024 bytes:

-XX:AllocatePrefetchDistance=1024

• -XX:AllocatePrefetchInstr=instruction — sets the prefetch instruction to prefetch ahead of the allocation pointer 🧚. Possible values are from 0 to 3. The actual instructions behind the values depend on the platform. By default, the prefetch instruction is set to 0:

-XX:AllocatePrefetchInstr=0

• -XX:AllocatePrefetchLines=lines — sets the number of cache lines to load after the last object allocation by using the prefetch instructions generated in compiled code 🌸. The default value is 1 if the last allocated object was an instance, and 3 if it was an array.

The following example shows how to set the number of loaded cache lines to 5:

-XX:AllocatePrefetchLines=5

• -XX:AllocatePrefetchStepSize=size — sets the step size, in bytes, for sequential prefetch instructions 👟. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the step size is set to 16 bytes:

-XX:AllocatePrefetchStepSize=16

• -XX:AllocatePrefetchStyle=style — sets the generated code style for prefetch instructions 🎨. The style argument is an integer from 0 to 3:
  • 0 — Don’t generate prefetch instructions.
  • 1 — Execute prefetch instructions after each allocation. This is the default setting.
  • 2 — Use the thread-local allocation block (TLAB) watermark pointer to determine when prefetch instructions are executed.
  • 3 — Generate one prefetch instruction per cache line.
• -XX:+BackgroundCompilation — enables background compilation 🌈. This option is enabled by default. To disable background compilation, specify -XX:-BackgroundCompilation (this is equivalent to specifying -Xbatch).
• -XX:CICompilerCount=threads — sets the number of compiler threads to use for compilation 🧵. By default, the number of compiler threads is selected automatically depending on the number of CPUs and memory available for compiled code. The following example shows how to set the number of threads to 2:

-XX:CICompilerCount=2

• -XX:+UseDynamicNumberOfCompilerThreads — compiler threads get spawned on demand up to whatever cap -XX:CICompilerCount sets. Growth mindset fr fr 🌱. This one is enabled by default, so you're already living that life.
• -XX:CompileCommand=command,method[,option] — tells the JIT compiler to run a specific command on a specific method, with an optional extra option tacked on. Basically micromanaging the compiler, but make it fashion 💅.

Say you want the indexOf() method of the String class to never get compiled — honestly valid:

-XX:CompileCommand=exclude,java/lang/String.indexOf

⚠️ heads up bestie: the full class name goes in there, including all packages and subpackages, separated by a slash (/). And because cut-and-paste is a lifestyle, you can also use the method-name format that -XX:+PrintCompilation and -XX:+LogCompilation produce:

-XX:CompileCommand=exclude,java.lang.String::indexOf

If you name the method without a signature, the command applies to all methods with that name. But you can nail down the signature in class-file format — just wrap the args in quotation marks, because otherwise your shell sees the semicolon and goes "cool, command over" and everything falls apart 😭. Example: exclude only the indexOf(String) method of String:

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

The asterisk (*) is your wildcard for class and method names too ✨. Banish every indexOf() in every class:

-XX:CompileCommand=exclude,*.indexOf

Commas and periods are literally aliases for spaces, which makes shoving compiler commands through a shell so much easier. So you can pass args with spaces as separators by quoting the whole thing:

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

📌 important order-of-operations moment: after the JIT compiler parses the commands you passed on the command line with -XX:CompileCommand, it then reads commands from the .hotspot_compiler file. You can add commands to that file, or point it at a different file with -XX:CompileCommandFile.

To add several commands, either specify -XX:CompileCommand multiple times, or separate each argument with the newline separator (\n). Here's the whole menu of commands, no cap:

• break — drops a breakpoint when you're debugging the JVM, so it stops at the very beginning of compilation of the specified method 🛑.
• compileonly — excludes all methods from compilation except the one you named. There's also -XX:CompileOnly, which lets you list several methods instead.
• dontinline — blocks inlining of the specified method. Not today, inliner 🙅.
• exclude — kicks the specified method out of compilation entirely.
• help — prints a help message for the -XX:CompileCommand option. We love a self-aware icon.
• inline — attempts to inline the specified method.
• log — excludes compilation logging (the -XX:+LogCompilation kind) for all methods except the specified one. By default, logging happens for all compiled methods.
• option — passes a JIT compilation option to the specified method in place of the last argument (option). The compilation option goes at the end, after the method name. So to enable the BlockLayoutByFrequency option for the append() method of the StringBuffer class:

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

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

• print — prints the generated assembler code after the specified method gets compiled. Look at the little bytes go 🎀.
• quiet — says "don't print the compile commands." By default the commands you specify with -XX:CompileCommand are printed; e.g. if you exclude the indexOf() method of String from compilation, you get this on standard output:

CompilerOracle: exclude java/lang/String.indexOf

You can suppress it by putting -XX:CompileCommand=quiet before your other -XX:CompileCommand options.

• -XX:CompileCommandFile=filename — sets the file that JIT compiler commands get read from. Default is the .hotspot_compiler file, which stores the commands performed by the JIT compiler.

Each line in the command file is a command, a class name, and a method name — that's the whole vibe. For example, this line prints assembly code for the toString() method of the String class:

print java/lang/String toString

If you're writing commands for the JIT compiler to perform on methods, go check out the -XX:CompileCommand option 💖.

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

🚨 the -XX:CompilerDirectivesFile option has to be used together with the -XX:UnlockDiagnosticVMOptions option, the one that unlocks diagnostic JVM options. No unlock, no entry.

• -XX:+CompilerDirectivesPrint — prints the directives stack when the program starts, or whenever a new directive gets added. Live updates, we love to see it 🌸.

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

• -XX:CompileOnly=methods — sets the list of methods (comma-separated, obviously) that compilation is restricted to. Only the methods you name get compiled. Main character methods only 👑.

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

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

• -XX:CompileThresholdScaling=scale — the unified control knob for first compilation, handling both the tiered and nontiered modes of operation. The CompileThresholdScaling option takes a floating point value between 0 and +Inf and scales the thresholds corresponding to the current mode of operation (both tiered and nontiered). Set CompileThresholdScaling below 1.0 and compilation happens earlier; above 1.0 and it gets delayed. Setting CompileThresholdScaling to 0 is equivalent to disabling compilation. One knob, all the power ⚡.
• -XX:+DoEscapeAnalysis — enables escape analysis. Enabled by default 💜. To turn it off, specify -XX:-DoEscapeAnalysis.
• -XX:InitialCodeCacheSize=size — sets the initial code cache size, in bytes. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. Default value depends on the platform. Ngl the initial code cache size shouldn't be smaller than the system's minimal memory page size. Setting it to 32 KB looks like this:

-XX:InitialCodeCacheSize=32k

• -XX:+Inline — enables method inlining. Enabled by default, because performance is the moment 🔥. To disable method inlining, specify -XX:-Inline.
• -XX:InlineSmallCode=size — sets the max code size (in bytes) for already compiled methods that are allowed to be inlined. This flag only applies to the C2 compiler, fyi. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. The default depends on the platform and on whether tiered compilation is enabled. In this example it's set to 1000 bytes:

-XX:InlineSmallCode=1000

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

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

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

• -XX:FreqInlineSize=size — sets the max bytecode size (in bytes) of a hot method that's allowed to be inlined 🔥. This flag only applies to the C2 compiler. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. The default depends on the platform. Here it's set to 325 bytes:

-XX:FreqInlineSize=325

• -XX:MaxInlineSize=size — sets the max bytecode size (in bytes) of a cold method that's allowed to be inlined 🧊. This flag only applies to the C2 compiler. Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. By default the max bytecode size is set to 35 bytes:

-XX:MaxInlineSize=35

• -XX:C1MaxInlineSize=size — sets the max bytecode size (in bytes) of a cold method that's allowed to be inlined ☁️. This flag only applies to the C1 compiler (yes, different compiler than the two above, keep up bestie). Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. By default the max bytecode size is set to 35 bytes:

-XX:MaxInlineSize=35

---

• -XX:MaxTrivialSize=size — sets the max bytecode size (in bytes) of a trivial method that's allowed to get inlined. C2 compiler only, bestie — C1 girlies this one's not for you. You can slap a k or K on the end for kilobytes, m or M for megabytes, or g or G for gigabytes. Default is a modest little 6 bytes:

-XX:MaxTrivialSize=6

• -XX:C1MaxTrivialSize=size — same vibe, different compiler: max bytecode size (in bytes) of a trivial method up for inlining, and this one is C1 only. Again, k/K for kilobytes, m/M for megabytes, g/G for gigabytes. The default is 6 bytes:

-XX:MaxTrivialSize=6

• -XX:MaxNodeLimit=nodes — caps how many nodes can be used during a single method compilation. What the default actually is depends on which features you've got enabled — very "it depends" of it. Example time, capping it at 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 holds nonmethod code, e.g. compiler buffers and the bytecode interpreter. This code type lives in the code cache forever, no eviction, it's not leaving. This flag only does anything if -XX:SegmentedCodeCache is enabled.

• -XX:NonProfiledCodeHeapSize=size — sets the size in bytes of the code segment holding nonprofiled methods. Again: only used if -XX:SegmentedCodeCache is enabled. 🫶
• -XX:+OptimizeStringConcat — turns on optimization of String concatenation operations. On by default, slay. Wanna turn it off? Specify -XX:-OptimizeStringConcat.
• -XX:+PrintAssembly — prints assembly code for bytecoded and native methods, using the external hsdis-<arch>.so or .dll library. For a 64-bit VM on Windows, that's hsdis-amd64.dll. This lets you actually see the generated code, which can help you diagnose performance issues. main character debugging energy

By default it's disabled and no assembly gets printed. And heads up: -XX:+PrintAssembly has to be used together with -XX:UnlockDiagnosticVMOptions, the option that unlocks diagnostic JVM options. No unlock, no assembly. 💅

• -XX:ProfiledCodeHeapSize=size — sets the size in bytes of the code segment holding profiled methods. Yep, you guessed it: only used if -XX:SegmentedCodeCache is enabled.
• -XX:+PrintCompilation — enables verbose diagnostic output from the JVM, printing a message to the console every single time a method gets compiled. This lets you see which methods actually get compiled. Disabled by default, so no diagnostic output unless you ask. It's giving "who's getting compiled" reality show.

You can also log compilation activity to a file with -XX:+LogCompilation.

• -XX:+PrintInlining — prints out inlining decisions, so you can see which methods are getting inlined. Receipts for the inliner 🎀.

Disabled by default — no inlining info printed. And -XX:+PrintInlining has to be used together with -XX:+UnlockDiagnosticVMOptions, the option that unlocks diagnostic JVM options. Unlock first, bestie.

• -XX:ReservedCodeCacheSize=size — sets the max code cache size (in bytes) for JIT-compiled code. k/K = kilobytes, m/M = megabytes, g/G = gigabytes, you know the drill. Default max code cache size is 240 MB; if you disable tiered compilation with -XX:-TieredCompilation, the default drops to 48 MB. This option has a limit of 2 GB — go over and you get an error, periodt. Also, the max code cache size shouldn't be less than the initial code cache size; see -XX:InitialCodeCacheSize.
• -XX:+SegmentedCodeCache — enables segmentation of the code cache. Without it, the code cache is just one big segment. With -XX:+SegmentedCodeCache, you get separate segments for non-method, profiled method, and non-profiled method code. The segments do NOT get resized at runtime — they're set and that's that.

The glow-up? Better control of the memory footprint, reduced code fragmentation, and better CPU iTLB (instruction translation lookaside buffer) and instruction cache behavior thanks to improved locality. ✨

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

• -XX:StartAggressiveSweepingAt=percent — forces stack scanning of active methods to aggressively remove unused code once only that given percentage of the code cache is free. Default value is 10%.
• -XX:-TieredCompilation — disables tiered compilation. By default, this option is enabled. (Yes, the flag is written with a minus because you're turning the thing off. Iconic formatting, no notes.)
• -XX:UseSSE=version — enables use of the SSE instruction set at the version you specify. Default is set to the highest supported version available (x86 only).
• -XX:UseAVX=version — enables use of the AVX instruction set at the version you specify. Default is set to the highest supported version available (x86 only).
• -XX:+UseAES — enables hardware-based AES intrinsics for hardware that supports it. On by default on hardware with the necessary instructions. -XX:+UseAES is used in conjunction with UseAESIntrinsics. Note: flags that control intrinsics now require the -XX:+UnlockDiagnosticVMOptions option. 🔐
• -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. Example — to enable hardware AES:

-XX:+UseAES -XX:+UseAESIntrinsics

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

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

UseSHA and the 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 -XX:+UnlockDiagnosticVMOptions option.

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

• -XX:+UseSHA1Intrinsics — enables intrinsics for the SHA-1 crypto hash function. Flags that control intrinsics now require the -XX:+UnlockDiagnosticVMOptions option.
• -XX:+UseSHA256Intrinsics — enables intrinsics for the SHA-224 and SHA-256 crypto hash functions. Flags that control intrinsics now require the -XX:+UnlockDiagnosticVMOptions option.
• -XX:+UseSHA512Intrinsics — enables intrinsics for the SHA-384 and SHA-512 crypto hash functions. Flags that control intrinsics now require the -XX:+UnlockDiagnosticVMOptions option.
• -XX:+UseMathExactIntrinsics — enables intrinsification of various java.lang.Math.*Exact() functions. Enabled by default. Flags that control intrinsics now require the -XX:+UnlockDiagnosticVMOptions option.
• -XX:+UseMultiplyToLenIntrinsic — enables intrinsification of BigInteger.multiplyToLen(). Enabled by default on platforms that support it. Flags that control intrinsics now require the -XX:+UnlockDiagnosticVMOptions option.
• -XX:+UseSquareToLenIntrinsic — enables intrinsification of BigInteger.squareToLen(). Enabled by default on platforms that support it. Flags that control intrinsics now require the -XX:+UnlockDiagnosticVMOptions option. 🌈
• -XX:+UseMulAddIntrinsic — turns on intrinsification of BigInteger.mulAdd(). It's enabled by default on platforms that support it. Also, quick heads up: flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions. 💅
• -XX:+UseMontgomeryMultiplyIntrinsic — turns on intrinsification of BigInteger.montgomeryMultiply(). Enabled by default on platforms that support it. Same dealio as above: flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.
• -XX:+UseMontgomerySquareIntrinsic — turns on intrinsification of BigInteger.montgomerySquare(). Enabled by default on platforms that support it. And yes, flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions. We love consistency 🎀
• -XX:+UseCMoveUnconditionally — generates CMove (scalar and vector) instructions regardless of profitability analysis. No vibe check on whether it's worth it, it just does it ✨
• -XX:+UseCodeCacheFlushing — enables flushing of the code cache before shutting down the compiler. This one is enabled by default, bestie. If you want to disable flushing of the code cache before shutting down the compiler, you specify -XX:-UseCodeCacheFlushing.
• -XX:+UseCondCardMark — enables checking if the card is already marked before updating the card table. Disabled by default. It should be used only on machines with multiple sockets, where it increases the performance of Java applications that rely on concurrent operations. So like, don't just turn this on for funsies 🙈
• -XX:+UseCountedLoopSafepoints — keeps safepoints in counted loops. Its default value depends on whether the selected garbage collector requires low latency safepoints.
• -XX:LoopStripMiningIter=number_of_iterations — controls the number of iterations in the inner strip mined loop. Okay so strip mining transforms counted loops into two level nested loops — safepoints are kept in the outer loop while the inner loop can execute at full speed 🏎️ This option controls the maximum number of iterations in the inner loop. Default value is 1,000.
• -XX:LoopStripMiningIterShortLoop=number_of_iterations — controls loop strip mining optimization. Loops with the number of iterations less than specified will not have safepoints in them. Default value is 1/10th of -XX:LoopStripMiningIter.
• -XX:+UseFMA — enables hardware-based FMA intrinsics for hardware where FMA instructions are available (such as, Intel and ARM64). FMA intrinsics are generated for the java.lang.Math.fma(a, b, c) methods that calculate the value of ( a * b + c ) expressions. Math but make it hardware 💖
• -XX:+UseSuperWord — enables the transformation of scalar operations into superword operations. Superword is a vectorization optimization, and this option is enabled by default. To disable the transformation of scalar operations into superword operations, specify -XX:-UseSuperWord.

---

✨ ADVANCED SERVICEABILITY OPTIONS FOR JAVA ✨

These java options provide the ability to gather system information and perform extensive debugging. Detective mode: activated 🕵️‍♀️

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

Note: The tools such as jcmd, jinfo, jmap, and jstack shipped with the JDK aren't supported when using the tools from one JDK version to troubleshoot a different JDK version. Like, version mixing is not the vibe. Keep them matched 💀

• -XX:+DTraceAllocProbes — Linux and macOS: enable dtrace tool probes for object allocation.
• -XX:+DTraceMethodProbes — Linux and macOS: enable dtrace tool probes for method-entry and method-exit.
• -XX:+DTraceMonitorProbes — Linux and macOS: enable dtrace tool probes for monitor events.
• -XX:+HeapDumpOnOutOfMemoryError — enables the dumping of the Java heap to a file in the current directory by using the heap profiler (HPROF) when a java.lang.OutOfMemoryError exception is thrown. You can explicitly set the heap dump file path and name using the -XX:HeapDumpPath option. By default this option is disabled and the heap isn't dumped when an OutOfMemoryError exception is thrown. 😭 Literally the "save the evidence" button.
• -XX:HeapDumpPath=path — sets the path and file name for writing the heap dump provided by the heap profiler (HPROF) when the -XX:+HeapDumpOnOutOfMemoryError option is set. By default, the file is created in the current working directory, and it's named java_pid<pid>.hprof where <pid> is the identifier of the process that caused the error.

The following example shows how to set the default file explicitly (%p represents the current process identifier):

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

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

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

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

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

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

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

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

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

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

By default, this option is disabled.

Setting this option is equivalent to running the jmap -histo command, or the jcmd pid GC.class_histogram command, where pid is the current Java process identifier. Same energy, different route 💅

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

By default, this option is disabled.

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

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

jstat -class file:///path/hsperfdata_pid

jstat -gc file:///path/hsperfdata_pid

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

---

🌈 ADVANCED GARBAGE COLLECTION OPTIONS FOR JAVA 🌈

These java options control how garbage collection (GC) is performed by the Java HotSpot VM. Cleanup crew, but make it highkey optimized 🧹✨

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

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

-XX:ConcGCThreads=2

• -XX:+DisableExplicitGC — enables the option that disables processing of calls to the System.gc() method. This option is disabled by default, meaning that calls to System.gc() are processed. If processing of calls to System.gc() is disabled, then the JVM still performs GC when necessary. So like, you're not cancelling GC entirely, bestie — the JVM still does its thing when it has to 💅
• -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, G1 uses the value of -XX:InitiatingHeapOccupancyPercent directly for this purpose. The default value is 3.
• -XX:G1HeapRegionSize=size — sets the size of the regions into which the Java heap is subdivided when using the garbage-first (G1) collector. The value is a power of 2 and can range from 1 MB to 32 MB. The default region size is determined ergonomically based on the heap size with a goal of approximately 2048 regions.

The following example sets the size of the subdivisions to 16 MB:

-XX:G1HeapRegionSize=16m

• -XX:G1HeapWastePercent=percent — sets the percentage of heap that you're willing to waste. The Java HotSpot VM doesn't initiate the mixed garbage collection cycle when the reclaimable percentage is less than the heap waste percentage. The default is 5 percent. 🗑️ A little waste is fine, honestly.
• -XX:G1MaxNewSizePercent=percent — sets the percentage of the heap size that's the maximum for the young generation size. Default? 60 percent of your Java heap. This is an experimental flag, and it replaces the -XX:DefaultMaxNewGenPercent setting.
• -XX:G1MixedGCCountTarget=number — sets the target number of mixed garbage collections after a marking cycle, so old regions with at most G1MixedGCLiveThresholdPercent live data get collected. Default is 8 mixed garbage collections, and the goal for mixed collections is to stay within this target number. 🎯
• -XX:G1MixedGCLiveThresholdPercent=percent — sets the occupancy threshold for an old region to be included in a mixed garbage collection cycle. Default occupancy is 85 percent. This one is also an experimental flag, and it replaces the -XX:G1OldCSetRegionLiveThresholdPercent setting.
• -XX:G1NewSizePercent=percent — sets the percentage of the heap that's the minimum for the young generation size. Default is 5 percent of your Java heap. Also experimental, and it replaces the -XX:DefaultMinNewGenPercent setting.
• -XX:G1OldCSetRegionThresholdPercent=percent — sets an upper limit on how many old regions get collected during a mixed garbage collection cycle. Default is 10 percent of the Java heap.
• -XX:G1ReservePercent=percent — sets the percentage of the heap (0 to 50) reserved as a false ceiling, lowkey reducing the possibility of promotion failure for the G1 collector. When you increase or decrease this percentage, make sure you adjust the total Java heap by the same amount. Default is 10%.

Example that sets the reserved heap to 20%:

-XX:G1ReservePercent=20

• -XX:+G1UseAdaptiveIHOP — controls adaptive calculation of the old generation occupancy that starts background work preparing for an old generation collection. If enabled, G1 uses -XX:InitiatingHeapOccupancyPercent for the first few times as specified by the value of -XX:G1AdaptiveIHOPNumInitialSamples, and after that it adaptively calculates a new optimum value for the initiating occupancy automatically. Otherwise, the old generation collection process always starts at the old generation occupancy determined by -XX:InitiatingHeapOccupancyPercent. Default is enabled. ✨
• -XX:InitialHeapSize=size — sets the initial size (in bytes) of the memory allocation pool. This value must be either 0, or a multiple of 1024 and greater than 1 MB. Append the letter k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. Default is picked at run time based on the system configuration.

Here's how to set 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 young generation heap size can be set with -XX:NewSize. Note that -Xms sets both the minimum and the initial heap size of the heap. If -Xms appears after -XX:InitialHeapSize on the command line, then the initial heap size gets set to the value specified with -Xms. 💅

• -XX:InitialRAMPercentage=percent — sets the initial amount of memory 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. Default is 1.5625 percent.

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

-XX:InitialRAMPercentage=5

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

Formula to calculate initial survivor space size (S) from young generation size (Y) and initial survivor space ratio (R):

S=Y/(R+2)

The 2 in the equation means two survivor spaces, bestie. The larger the initial survivor space ratio, the smaller the initial survivor space size. Default ratio is 8. If the default young generation space size is used (2 MB), then the initial survivor space size is 0.2 MB.

Example setting the initial survivor space ratio to 4:

-XX:InitialSurvivorRatio=4

• -XX:InitiatingHeapOccupancyPercent=percent — sets the percentage of old generation occupancy (0 to 100) at which to start the first few concurrent marking cycles for the G1 garbage collector. Default initiating value is 45%. A value of 0 implies nonstop concurrent GC cycles from the beginning until G1 adaptively sets this value. See also -XX:G1UseAdaptiveIHOP and -XX:G1AdaptiveIHOPNumInitialSamples.

Example setting initiating heap occupancy to 75%:

-XX:InitiatingHeapOccupancyPercent=75

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

Example setting the maximum target pause time to 500 ms:

-XX:MaxGCPauseMillis=500

• -XX:MaxHeapSize=size — sets the maximum size of the memory allocation pool. (The manpage really says “byes” there, which is a whole mood 😭 — it means bytes.) 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. Default is picked at run time based on system configuration. For server deployments, -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value.

Here's 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. Default is 70%.

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

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

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

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

Example setting 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, aka the nursery. Default value is set ergonomically. 🌱
• -XX:MaxRAM=size — sets the maximum amount of memory the JVM may use for the Java heap before applying ergonomics heuristics. Default 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.

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

-XX:MaxRAM=2G

• -XX:MaxRAMPercentage=percent — caps how much memory the JVM is allowed to use for the Java heap before ergonomics heuristics kick in, as a percentage of the maximum amount determined by -XX:MaxRAM. Default is 25 percent. 💖

Heads up bestie: setting this disables automatic use of compressed oops if the combined result of this option and other options influencing max memory ends up larger than the range of memory addressable by compressed oops. Peep -XX:UseCompressedOops for the whole compressed-oops lore.

Wanna set that percentage to 75? Like this:

-XX:MaxRAMPercentage=75

• -XX:MinRAMPercentage=percent — same idea as above but for small heaps: sets the maximum amount of memory the JVM may use for the Java heap before applying ergonomics heuristics, as a percentage of the max determined by -XX:MaxRAM. A small heap is a heap of approximately 125 MB. Default value is 50 percent. ✨

Example, setting it to 75:

-XX:MinRAMPercentage=75

• -XX:MaxTenuringThreshold=threshold — sets the max tenuring threshold used in adaptive GC sizing. The largest value is 15, and the default is 15 for the parallel (throughput) collector. Tenuring is literally just "how many GCs did u survive," iconic behavior tbh.

Setting it to 10:

-XX:MaxTenuringThreshold=10

• -XX:MetaspaceSize=size — sets the size of the allocated class metadata space that triggers a garbage collection the first time it's exceeded. After that, the GC threshold gets bumped up or down depending on how much metadata is actually being used. Default size depends on the platform (mysterious, we love that for her). 🦋
• -XX:MinHeapFreeRatio=percent — sets the minimum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space drops below this, the heap expands. Default is 40%.

Want a tiny heap? Lower both MaxHeapFreeRatio (default 70%) and MinHeapFreeRatio (default 40%) using the command-line options -XX:MaxHeapFreeRatio and -XX:MinHeapFreeRatio. Dropping MaxHeapFreeRatio as low as 10% and MinHeapFreeRatio to 5% has successfully reduced heap size without too much performance regression — but fr fr results vary greatly depending on your app. Try different values until they're as low as possible while still keeping acceptable performance. 🫶

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

Also, if you're trying to keep the heap small, add the option -XX:-ShrinkHeapInSteps. See Performance Tuning Examples for the whole story on using this option to keep the Java heap small by reducing the dynamic footprint for embedded applications.

• -XX:MinHeapSize=size — sets the minimum size (in bytes) of the memory allocation pool. Gotta 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. Default is picked at run time based on your system config. ✨

Three ways to say "6 MB", all valid:

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

If you set this to 0, the minimum size gets set to the same value as the initial size.

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

-XX:NewRatio=1

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

The young generation region is where new objects live, and GC happens there way more often than in other regions. Too small? Then you get a whole bunch of minor GCs. Too big? Then you only get full GCs, which can take forever to finish. Recommendation: keep the young generation size greater than 25% and less than 50% of the overall heap size.

Setting the initial young gen size to 256 MB, three ways:

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

Oh and btw, -XX:NewSize is equivalent to -Xmn.

• -XX:ParallelGCThreads=threads — sets the number of stop-the-world (STW) worker threads. Default depends on the number of CPUs available to the JVM and which garbage collector you picked. 💅

For example, setting the thread count for G1 GC to 2:

-XX:ParallelGCThreads=2

• -XX:+ParallelRefProcEnabled — turns on parallel reference processing. Disabled by default. 🔥
• -XX:+PrintAdaptiveSizePolicy — enables printing of info about adaptive-generation sizing. Disabled by default.
• -XX:SoftRefLRUPolicyMSPerMB=time — sets how long (in milliseconds) a softly reachable object stays active on the heap after the last time it was referenced. Default is one second of lifetime per free megabyte in the heap.

Real talk, this 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 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 Server VM case, the value of -Xmx has a significant effect on how quickly soft references get garbage collected. 😭

Setting it to 2.5 seconds:

-XX:SoftRefLRUPolicyMSPerMB=2500

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

⚠️ You will likely encounter performance degradation when this option is disabled. Bestie, consider yourself warned.

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 that reach the specified age and are considered candidates for deduplication. An object's age = how many times it has survived garbage collection. People sometimes call this tenuring. 🎀

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

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

-XX:SurvivorRatio=4

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

Setting the target survivor space ratio to 30%:

-XX:TargetSurvivorRatio=30

• -XX:TLABSize=size — sets the initial size (in bytes) of a thread-local allocation buffer (TLAB). Append k or K for kilobytes, m or M for megabytes, or g or G for gigabytes. Set it to 0 and the JVM picks the initial size automatically for you, king behavior. 👑

Setting the initial TLAB size to 512 KB:

-XX:TLABSize=512k

• -XX:+UseAdaptiveSizePolicy — enables the use of adaptive generation sizing. Enabled by default! To disable it, specify -XX:-UseAdaptiveSizePolicy and set the size of the memory allocation pool explicitly — see the -XX:SurvivorRatio option.
• -XX:+UseG1GC — enables the garbage-first (G1) garbage collector. She's a server-style collector, targeted at multiprocessor machines with a large amount of RAM. This option meets GC pause time goals with high probability while maintaining good throughput. G1 is recommended for applications that need large heaps (sizes of around 6 GB or larger) with limited GC latency requirements — like a stable and predictable pause time below 0.5 seconds. By default this option is enabled, and G1 is the default garbage collector. 👑💖
• -XX:+UseGCOverheadLimit — turns on the policy that limits how much of the JVM’s time can go to GC before an OutOfMemoryError exception gets thrown. This one is enabled by default, and the parallel GC will throw OutOfMemoryError if more than 98% of total time is spent on garbage collection and less than 2% of the heap is recovered. When the heap is small, this can keep apps from running for long stretches with little or no progress. To disable it, specify -XX:-UseGCOverheadLimit. 💅
• -XX:+UseNUMA — enables performance optimization for an app on a machine with nonuniform memory architecture (NUMA) by increasing the app’s use of lower latency memory. By default this option is disabled and no optimization for NUMA is made. It’s available only when the parallel garbage collector is used (-XX:+UseParallelGC). 🦋
• -XX:+UseParallelGC — enables the parallel scavenge garbage collector (also known as the throughput collector) to improve your app’s performance by leveraging multiple processors.

By default this option is disabled and the default collector is used. ⭐

• -XX:+UseSerialGC — enables the serial garbage collector. This is generally the best choice for small and simple apps that don’t require any special functionality from garbage collection. By default this option is disabled and the default collector is used. 🎀
• -XX:+UseStringDeduplication — enables string deduplication. By default this option is disabled. To use it, you must enable the garbage-first (G1) garbage collector.

String deduplication reduces the memory footprint of String objects on the Java heap by taking advantage of the fact that many String objects are identical. Instead of each String object pointing to its own character array, identical String objects can point to and share the same character array. 💖

• -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 -XX:-UseTLAB. ✨
• -XX:+UseZGC — enables the use of the Z garbage collector (ZGC). This is a low latency garbage collector, providing max pause times of a few milliseconds, at some throughput cost. Pause times are independent of what heap size is used. Supports heap sizes from 8MB to 16TB. 🔥
• -XX:ZAllocationSpikeTolerance=factor — sets the allocation spike tolerance for ZGC. By default this option is set to 2.0. This factor describes the level of allocation spikes to expect. For example, using a factor of 3.0 means the current allocation rate can be expected to triple at any time. 🌀
• -XX:ZCollectionInterval=seconds — sets the maximum interval (in seconds) between two GC cycles when using ZGC. By default this option is set to 0 (disabled). ⏳
• -XX:ZFragmentationLimit=percent — sets the maximum acceptable heap fragmentation (in percent) for ZGC. By default this option is set to 25. Using a lower value will cause the heap to be compacted more aggressively, to reclaim more memory at the cost of using more CPU time. 🧚
• -XX:+ZProactive — enables proactive GC cycles when using ZGC. By default this option is enabled. ZGC will start a proactive GC cycle if doing so is expected to have minimal impact on the running app. This is useful if the app is mostly idle or allocates very few objects, but you still want to keep the heap size down and allow reference processing to happen even when there are a lot of free space on the heap. 🌈
• -XX:+ZUncommit — enables uncommitting of unused heap memory when using ZGC. By default this option is enabled. Uncommitting unused heap memory will lower the memory footprint of the JVM, and make that memory available for other processes to use. 🫶
• -XX:ZUncommitDelay=seconds — sets the amount of time (in seconds) that heap memory must have been unused before being uncommitted. By default this option is set to 300 (5 minutes). Committing and uncommitting memory are relatively expensive operations. Using a lower value will cause heap memory to be uncommitted earlier, at the risk of soon having to commit it again. 💀

💀 DEPRECATED JAVA OPTIONS 💀

Okay so these java options are deprecated and might be removed in a future JDK release. They’re still accepted and acted upon, but a warning is issued when they’re used. 🙈

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

Example:

-Xlog:gc:garbage-collection.log

• -XX:+FlightRecorder — enables the use of Java Flight Recorder (JFR) during the runtime of the app. Since JDK 8u40 this option has not been required to use JFR. ✈️
• -XX:RTMAbortRatio=abort_ratio — specifies the RTM abort ratio as a percentage (%) of all executed RTM transactions. If a 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. The default value of this option is 50. This means that the compiled code is deoptimized if 50% of all transactions are aborted. 💥
• -XX:RTMRetryCount=number_of_retries — specifies the number of times that the RTM locking code is retried, when it is aborted or busy, before falling back to the normal locking mechanism. The default value for this option is 5. The -XX:UseRTMLocking option must be enabled. 🔁
• -XX:+UseRTMDeopt — autotunes RTM locking depending on the abort ratio. This ratio is specified by the -XX:RTMAbortRatio option. If the number of aborted transactions exceeds the abort ratio, then the method containing the lock is deoptimized and recompiled with all locks as normal locks. This option is disabled by default. The -XX:+UseRTMLocking option must be enabled. 🎛️
• -XX:+UseRTMLocking — generates Restricted Transactional Memory (RTM) locking code for all inflated locks, with the normal locking mechanism as the fallback handler. This option is disabled by default. Options related to RTM are available only on x86 CPUs that support Transactional Synchronization Extensions (TSX).

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

A lock on a transaction 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 apps. (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. 🧠

🌸 OBSOLETE JAVA OPTIONS 🌸

Okay so these java options are still accepted but they literally do nothing — the JVM just ignores them and throws a warning at you when you use them. It's giving "we keep you on the roster but you're not playing" energy 💀

• --illegal-access=parameter — this one controlled relaxed strong encapsulation, as defined in JEP 261 [https://openjdk.org/jeps/261#Relaxed-strong-encapsulation]. It got deprecated in JDK 16 by JEP 396 [https://openjdk.org/jeps/396] and then made fully obsolete in JDK 17 by JEP 403 [https://openjdk.org/jeps/403]. RIP bestie 🫶
• -XX:+ScavengeBeforeFullGC — turns on GC of the young generation before each full GC. It's enabled by default already, and the manpage is like "please don't turn this off" — 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. If you truly must disable GC of the young generation before each full GC, you'd specify -XX:-ScavengeBeforeFullGC. (Note: still obsolete. Still ignored. Just so you know 💅)
• -XX:InitialRAMFraction=ratio — sets the initial amount of memory 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. Default value is 64. 👉 Use the option -XX:InitialRAMPercentage instead, bestie.
• -XX:MaxRAMFraction=ratio — sets the maximum amount of memory 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. Default value is 4.

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

👉 Use the option -XX:MaxRAMPercentage instead.

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

👉 Use the option -XX:MinRAMPercentage instead.

---

💀 REMOVED JAVA OPTIONS 💀

These java options got removed in JDK 23, and using them results in an error of:

Unrecognized VM option option-name

Like, it's not "ignored with a warning" anymore — it's a whole error. Not me getting yelled at by the launcher 😭

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

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

• The java Command, Release 22 [https://docs.oracle.com/en/java/javase/22/docs/specs/man/java.html]
• The java Command, Release 21 [https://docs.oracle.com/en/java/javase/21/docs/specs/man/java.html]
• The java Command, Release 20 [https://docs.oracle.com/en/java/javase/20/docs/specs/man/java.html]
• The java Command, Release 19 [https://docs.oracle.com/en/java/javase/19/docs/specs/man/java.html]
• The java Command, Release 18 [https://docs.oracle.com/en/java/javase/18/docs/specs/man/java.html]
• The java Command, Release 17 [https://docs.oracle.com/en/java/javase/17/docs/specs/man/java.html]
• The java Command, Release 16 [https://docs.oracle.com/en/java/javase/16/docs/specs/man/java.html]
• The java Command, Release 15 [https://docs.oracle.com/en/java/javase/15/docs/specs/man/java.html]
• The java Command, Release 14 [https://docs.oracle.com/en/java/javase/14/docs/specs/man/java.html]
• The java Command, Release 13 [https://docs.oracle.com/en/java/javase/13/docs/specs/man/java.html]
• Java Platform, Standard Edition Tools Reference, Release 12 [https://docs.oracle.com/en/java/javase/12/tools/java.html#GUID-3B1CE181-CD30-4178-9602-230B800D4FAE]
• Java Platform, Standard Edition Tools Reference, Release 11 [https://docs.oracle.com/en/java/javase/11/tools/java.html#GUID-741FC470-AA3E-494A-8D2B-1B1FE4A990D1]
• Java Platform, Standard Edition Tools Reference, Release 10 [https://docs.oracle.com/javase/10/tools/java.htm#JSWOR624]
• Java Platform, Standard Edition Tools Reference, Release 9 [https://docs.oracle.com/javase/9/tools/java.htm#JSWOR624]
• Java Platform, Standard Edition Tools Reference, Release 8 for Oracle JDK on Windows [https://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html#BGBCIEFC]
• Java Platform, Standard Edition Tools Reference, Release 8 for Oracle JDK on Solaris, Linux, and macOS [https://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html#BGBCIEFC]

---

🎀 JAVA COMMAND-LINE ARGUMENT FILES 🎀

Okay so this is the glow-up tip you've been waiting for: you can 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 are passed to the java command. This lets you create java commands of any length on any operating system. Periodt. 👑

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 in an argument file — to stop @ argument files expansion. 🛑

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's 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. This one trips so many people, ngl.
• 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. No recursion bestie, we're not doing inception today 🙈

✨ 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

Lowkey the single most important thing on this whole page: relative paths inside an argument file are relative to where you ran the command, not where the file lives. Write it on your mirror. 🪞💖

🧚 CODE HEAP STATE ANALYTICS 💅

✨ Overview ✨

ok so sometimes you literally need to peek inside the current state of the JVM code heap to answer questions like:

• 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? 🙈

and to give you that insight, a code heap state analytics feature is implemented that lets you analyze the code heap on the fly. The analytics process has two parts: part one examines the entire code heap and aggregates all info believed to be useful or important, and part two is several independent steps that print the collected info with an emphasis on different aspects of the data. Data collection and printing happen on an "on request" basis. periodt.

🌸 Syntax 🌸

Real-time, on-the-fly analysis requests go like this:

jcmd pid Compiler.CodeHeap_Analytics [function] [granularity]

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

-Xlog:codecache=Trace

And to see the code heap state when a "CodeCache full" condition exists, start the VM with:

-Xlog:codecache=Debug

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

---

🌈 ENABLE LOGGING WITH THE JVM UNIFIED LOGGING FRAMEWORK 🎀

Use the -Xlog option to configure or enable logging with the Java Virtual Machine (JVM) unified logging framework. iconic behavior tbh.

🦋 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 gives you one common logging system for all components of the JVM. GC logging for the JVM has been changed to use this new logging framework — the mapping of old GC flags to the corresponding new Xlog configuration is described in Convert GC Logging Flags to Xlog. Runtime logging also switched over to the JVM unified logging framework, and the mapping of legacy runtime logging flags to the corresponding new Xlog configuration is described in Convert Runtime Logging Flags to Xlog. no cap.

Quick reference for the -Xlog command and its option syntax:

• -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, and without the wildcard only exact matches of the specified tag sets are selected.

output-options is:

filecount=file-count filesize=file size with optional K, M or G suffix foldmultilines=<true|false>

When foldmultilines is true, a log event that consists of multiple lines gets folded into a single line by replacing newline characters with the sequence '\' and 'n' in the output. Existing single backslash characters are also replaced with a sequence of two backslashes so 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. It logs all messages with a level that matches either warning or error, regardless of what tags the message is associated with. This default configuration is equivalent to entering the following on the command line:

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

⭐ Controlling Logging at Runtime ⭐

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

🔥 -Xlog Tags and Levels 🔥

Each 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, and mapping legacy runtime logging flags to the corresponding Xlog configuration is described in Convert Runtime Logging Flags to Xlog.

Available log levels:

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

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

🌸 -Xlog Output 🌸

The -Xlog option supports the following types of outputs:

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

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

🌈 -Xlog Output Mode 🌈

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

• -Xlog:async — write all logging asynchronously.

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

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. slay.

🎀 Decorations 🎀

Logging messages are decorated with information about the message, and 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 (which is now a cute list, bestie). You can configure the decorations to be used at run time. Decorations are prepended to the log message, for example:

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

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

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

💖 Logging Messages Decorations 💖

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

✨ Convert GC Logging Flags to Xlog ✨

okay bestie, this is the glow-up chart for old GC logging flags → new -Xlog config. I’m keeping every single mapping because future-you will absolutely need the receipts. 💅

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. 🫶

Runtime Logging Flags to Xlog Configuration Mapping

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

---

✨ -Xlog Usage Examples ✨

The following are -Xlog examples. Obsessed with how flexible this is, honestly. 🦋

• -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. This is where it gets lowkey iconic. 🔥

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

💅 VALIDATE JAVA VIRTUAL MACHINE FLAG ARGUMENTS 💅

Okay so bestie, here’s the vibe: every value you hand to any Java Virtual Machine (JVM) command-line flag gets validated, and if the input value is invalid or out-of-range, you get an appropriate error message. No silent chaos. We love that for us. ✨

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

Range and constraints are validated either when all flags have their values set during JVM initialization or a flag’s value is changed during runtime (for example using the jcmd tool). The JVM is terminated if a value violates either the range or constraint check, and an appropriate error message is printed on the error stream. like, it will literally shut the whole thing down 💀

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

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

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

For example,

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

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

size_t NewSize         [   ...                  ] {product}

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

---

🌸 LARGE PAGES 🌸

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

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

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

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 do the vibe check after asking the kernel for pages 💅

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

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. budget already spent, bestie 😭

🪟 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.

Important: 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 needs the extra permission glow-up 👑

💖 APPLICATION CLASS DATA SHARING 💖

okay bestie, AppCDS is literally the moment where your app’s classes get stored in an archive file so they load so much faster than classes sitting in a JAR. It can improve start-up time, and it can also reduce runtime memory footprint by sharing parts of those classes across multiple processes. 👑

The classes in the CDS archive are stored in an optimized format that’s about 2 to 5 times larger than classes stored in JAR files or the JDK runtime image. So yeah, it’s a good idea to archive only those classes your app actually uses — usually that’s just a small portion of all available classes. Like, your app may use only a few APIs provided by a large library. ✨

---

🎀 Using CDS Archives 🎀

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

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.)

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. 🫶

🌸 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.

2. 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

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

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

4. Optional: Verify that the test.Hello application is using the class contained in the hello.jsa shared archive:

java -XX:SharedArchiveFile=hello.jsa -cp hello.jar -Xlog:class+load test.Hello

The output of this command should contain the following text:

[info][class,load] test.Hello source: shared objects file

By default, when the -Xshare:dump option is used, the JVM runs in interpreter-only mode (as if the -Xint option were specified). This is required for generating deterministic output in the shared archive file. I.e., the exact same archive will be generated, bit-for-bit, every time you dump it. However, if deterministic output is not needed, and you have a large classlist, you can explicitly add -Xmixed to the command-line to enable the JIT compiler. This will speed up the archive creation. ⚡

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

Advantages of dynamic CDS archives are:

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

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

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

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

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

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

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

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

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

To run the application using this dynamic CDS archive:

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

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

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

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

🧚 Creating CDS Archive Files with jcmd 🧚

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

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

jcmd <pid> VM.cds static_dump my_static_archive.jsa

or a dynamic archive:

jcmd <pid> VM.cds dynamic_dump my_dynamic_archive.jsa

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

env JAVA_TOOL_OPTIONS=-XX:SharedArchiveFile=my_static_archive.jsa bash app_start.sh

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

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

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

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

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

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

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

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

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

🌈 Restrictions on Class Path and Module Path 🌈

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

🚀 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

Okay bestie, so when the launcher dips out, it usually hands back one of these exit values — and lowkey this happens when the launcher got called with the wrong arguments, or something seriously went wrong, or the JVM threw exceptions. 💀

But like, plot twist: a Java application can honestly choose to return any value it wants by using the API call System.exit(exitValue). So the exit code you see isn't always the launcher's story — sometimes it's the app's whole personality. 🙈

Here's the vibe check on the values:

• 0: Successful completion ✨ — everything ate, no crumbs, we love to see it 💅
• >0: An error occurred 😭 — anything bigger than zero means something went sideways

---

And that's the whole page, bestie! 🎀 You just read the entire JAVA(1) manual — every last flag, every deprecated ghost, every logging framework detail. Genuinely iconic of you. 👑

This lil masterpiece comes to you from JDK 23, dated 2024. So when you cite it in the group chat, that's the source and the version, fr fr. 🦋 Now go compile something beautiful. 💖