Section: User Commands (1)
Updated: 18 Jul 2013
java - the Java application launcher
''' java' [ options ] class [ argument ...'' ] java [ options ] -jar file.jar [ argument ... ]
- Command-line options. See Options.:
- The name of the class to be called.:
- The name of the JAR file to be called. Used only with the -jar command.:
- The arguments passed to the main function.:
The java command starts a Java application. It does this by starting a Java runtime environment, loading a 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) '''
By default, the first argument without an option is the name of the class to be called. A fully qualified class name should be used. If the -jar option is specified, then the first non-option argument is the name of a JAR file containing class and resource files for the application, with the startup class indicated by the Main-Class manifest header.
The Java runtime searches for the startup class, and other classes used, in three sets of locations: the bootstrap class path, the installed extensions, and the user class path.
Non-option arguments after the class name or JAR file name are passed to the main function.
The launcher has a set of standard options that are supported in the current runtime environment.
In addition, the current implementations of the virtual machines support a set of nonstandard options that are subject to change in future releases. See Nonstandard Options.
- Selects the Java HotSpot Client VM. A 64-bit capable JDK currently ignores this option and instead uses the Java Hotspot Server VM.
For default Java VM selection, see Server-Class Machine Detection at http://docs.oracle.com/javase/7/docs/technotes/guides/vm/server-class.html
- Selects the Java HotSpot Server VM. On a 64-bit capable JDK, only the Java Hotspot Server VM is supported so the -server option is implicit.
For default a Java VM selection, see Server-Class Machine Detection at http://docs.oracle.com/javase/7/docs/technotes/guides/vm/server-class.html
- libname[=options]:Loads native agent library libname, for example:
@] See JVMTI Agent Command-Line Options at http://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.html#starting
- pathname[=options]:Loads a native agent library by full pathname. See JVMTI Command-Line Options at http://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.html#starting
- -classpath classpath, -cp classpath
- Specifies a list of directories, JAR files, and ZIP archives to search for class files. Separate class path entries with colons (:). Specifying -classpath or -cp overrides any setting of the CLASSPATH environment variable.
If -classpath and -cp are not used and CLASSPATH is not 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 * is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR. A Java program cannot tell the difference between the two invocations.
For example, if directory mydir contains a.jar and b.JAR, then the class path element mydir/* is expanded to a 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 simply of * expands to a list of all the jar files in the current directory. The CLASSPATH environment variable, where defined, will be similarly expanded. Any class path wildcard expansion occurs before the Java VM is started. No Java program will ever see wild cards that are not expanded except by querying the environment. For example, by calling System.getenv("CLASSPATH").:
- Sets a system property value.:
- Run the application in a 32-bit environment. If a 32-bit environment is not installed or is not supported, an error will be reported. By default, the application is run in a 32-bit environment unless a 64-bit only system is used.:
- Run the application in a 64-bit environment. If a 64-bit environment is not installed or is not supported, an error will be reported. By default, the application is run in a 32-bit environment unless a 64-bit only system is used.
Currently only the Java HotSpot Server VM supports 64-bit operation, and the -server option is implicit with the use of -d64. The -client option is ignored with the use of -d64. This is subject to change in a future release.:
- package name"..." | :class name ], -da[:package name"..." | :class name ]:Disable assertions. This is the default.
With no arguments, -disableassertions or -da disables assertions. With one argument ending in "...", the switch disables assertions in the specified package and any subpackages. If the argument is "...", then the switch disables assertions in the unnamed package in the current working directory. With one argument not ending in "...", the switch disables assertions in the specified class.
To run a program with assertions enabled in package com.wombat.fruitbat but disabled in class com.wombat.fruitbat.Brickbat, the following command could be used:
''' java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat <Main Class> '''
@] The -disableassertions and -da switches apply to all class loaders and to system classes (which do not have a class loader). There is one exception to this rule: in their no-argument form, the switches do not apply to system. This makes it easy to turn on asserts in all classes except for system classes. The -disablesystemassertions option provides a separate swith to enable assertions in all system classes.:
- package name"..." | :class name ], -ea[:package name"..." | :class name ]:Enable assertions. Assertions are disabled by default.
With no arguments, -enableassertions or -ea enables assertions. With one argument ending in "...", the switch enables assertions in the specified package and any subpackages. If the argument is "...", then the switch enables assertions in the unnamed package in the current working directory. With one argument not ending in "...", the switch enables assertions in the specified class.
If a single command contains multiple instances of these switches, then they are processed in order before loading any classes. So, for example, to run a program with assertions enabled only in package com.wombat.fruitbat (and any subpackages), the following command could be used:
''' java -ea:com.wombat.fruitbat... <Main Class> '''
@] The -enableassertions and -ea switches apply to all class loaders and to system classes (which do not have a class loader). There is one exception to this rule: in their no-argument form, the switches do not apply to system. This makes it easy to turn on asserts in all classes except for system classes. The -enablesystemassertions option provides a separate switch to enable assertions in all system classes.:
- -enablesystemassertions, -esa
- Enable assertions in all system classes (sets the default assertion status for system classes to true).:
- -disablesystemassertions, -dsa
- Disables assertions in all system classes.:
- -help or -?
- Displays usage information and exit.:
- Executes a program encapsulated in a JAR file. The first argument is the name of a JAR file instead of a startup class name. For this option to work, the manifest of the JAR file must contain a line in the form Main-Class: classname. Here, classname identifies the class with the public static void main(String args) method that serves as your application's starting point.
When you use this option, the JAR file is the source of all user classes, and other user class path settings are ignored.
JAR files that can be run with the java -jar option can have their execute permissions set so they can be run without using java -jar. See JAR File Overview at http://docs.oracle.com/javase/7/docs/technotes/guides/jar/jarGuide.html
- jarpath[=options]:Loads a Java programming language agent. For more information about instrumenting Java applications, see the java.lang.instrument package description in the Java API documentation at
- Includes user-private JREs in the version search.:
- Excludes user-private JREs in the version search.:
- Displays version information and continues.:
- imagepath:Shows splash screen with image specified by imagepath.:
- -verbose, -verbose
- class:Displays information about each class loaded.:
- gc:Reports on each garbage collection event.:
- jni:Reports information about use of native methods and other Java Native Interface activity.:
- Displays version information and exits. See also the -showversion option.:
- release:Specifies that the version specified by the release is required by the class or JAR file specified on the command line. If the version of the java command called does not meet this specification and an appropriate implementation is found on the system, then the appropriate implementation will be used.
The release option specifies an exact version and a list of versions called a version string. A version string is an ordered list of version ranges separated by spaces. A version range is either a version-id, a version-id followed by an asterisk (*), a version-id followed by a plus sign (+), or a version range that consists of two version-ids combined using an ampersand (&). The asterisk means prefix match, the plus sign means this version or greater, and the ampersand means the logical and of the two version-ranges, for example:
''' -version:"1.6.0_13 1.6*&1.6.0_10+" '''
@] The meaning of the previous example is that the class or JAR file requires either version 1.6.0_13, or a version with 1.6 as a version-id prefix and that is not less than 1.6.0_10. The exact syntax and definition of version strings can be found in Appendix A of the Java Network Launching Protocol & API Specification (JSR-56).
For JAR files, the preference is to specify version requirements in the JAR file manifest rather than on the command line.
See Notes for important policy information on the use of this option.:
- Displays information about nonstandard options and exits.:
- Operates in interpreted-only mode. Compilation to native code is disabled, and all bytecode is executed by the interpreter. The performance benefits offered by the Java HotSpot Client VM adaptive compiler is not present in this mode.:
- Disables background compilation. Typically, the Java VM compiles the method as a background task, running the method in interpreter mode until the background compilation is finished. The -Xbatch flag disables background compilation so that compilation of all methods proceeds as a foreground task until completed.:
- bootclasspath:Specifies a colon-separated list of directories, JAR files, and ZIP archives to search for boot class files. These are used in place of the boot class files included in the Java platform JDK.
Applications that use this option for the purpose of overriding a class in rt.jar should not be deployed because doing so would contravene the Java Runtime Environment binary code license.:
- path:Specifies a colon-separated path of directories, JAR files, and ZIP archives to append to the default bootstrap class path.:
- path:Specifies a colon-separated path of directories, JAR files, and ZIP archives to add in front of the default bootstrap class path.
Do not deploy applications that use this option to override a class in rt.jar because this violates the Java Runtime Environment binary code license.:
- jni:Performs additional checks for Java Native Interface (JNI) functions. Specifically, the Java Virtual Machine validates the parameters passed to the JNI function and the runtime environment data before processing the JNI request. Any invalid data encountered indicates a problem in the native code, and the Java Virtual Machine will terminate with a fatal error in such cases. Expect a performance degradation when this option is used.:
- Performs strict class-file format checks. For backward compatibility, the default format checks performed by the Java virtual machine are no stricter than the checks performed by 1.1.x versions of the JDK software. The -Xfuture option turns on stricter class-file format checks that enforce closer conformance to the class-file format specification. Developers are encouraged to use this flag when developing new code because the stricter checks will become the default in future releases of the Java application launcher.:
- Disables class garbage collection. Use of this option preven memory recovery from loaded classes thus increasing overall memory usage. This could cause OutOfMemoryError to be thrown in some applications.:
- Enables the incremental garbage collector. The incremental garbage collector, which is turned off by default, will reduce the occasional long garbage-collection pauses during program execution. The incremental garbage collector will at times execute concurrently with the program and during such times will reduce the processor capacity available to the program.:
- file:Reports on each garbage collection event, as with -verbose:gc, but logs this data to a file. In addition to the information -verbose:gc gives, each reported event will be preceded by the time (in seconds) since the first garbage-collection event.
Always use a local file system for storage of this file to avoid stalling the Java VM due to network latency. The file may be truncated in the case of a full file system and logging will continue on the truncated file. This option overrides -verbose:gc when both are specified on the command line.:
- -Xmnsize or -XX
- NewSize:Sets the size of the young generation (nursery).:
- Specifies the initial size, in bytes, of the memory allocation pool. This value must be a multiple of 1024 greater than 1 MB. Append the letter k or K to indicate kilobytes, or m or M to indicate megabytes. The default value is chosen at runtime based on system configuration. See Garbage Collector Ergonomics at http://docs.oracle.com/javase/7/docs/technotes/guides/vm/gc-ergonomics.html
''' -Xms6291456 -Xms6144k -Xms6m '''
- Specifies the maximum size, in bytes, of the memory allocation pool. This value must a multiple of 1024 greater than 2 MB. Append the letter k or K to indicate kilobytes, or m or M to indicate megabytes. The default value is chosen at runtime based on system configuration.
For server deployments, -Xms and -Xmx are often set to the same value. See Garbage Collector Ergonomics at http://docs.oracle.com/javase/7/docs/technotes/guides/vm/gc-ergonomics.html
''' -Xmx83886080 -Xmx81920k -Xmx80m '''
@] On Solaris 7 and Solaris 8 SPARC platforms, the upper limit for this value is approximately 4000 m minus overhead amounts. On Solaris 2.6 and x86 platforms, the upper limit is approximately 2000 m minus overhead amounts. On Linux platforms, the upper limit is approximately 2000 m minus overhead amounts.:
- Profiles the running program, and sends profiling data to standard output. This option is provided as a utility that is useful in program development and is not intended to be used in production systems.:
- Reduces use of operating-system signals by the Java VM.
In an earlier release, the Shutdown Hooks facility was added to enable orderly shutdown of a Java application. The intent was to enable user cleanup code (such as closing database connections) to run at shutdown, even if the Java VM terminates abruptly.
The Java VM catches signals to implement shutdown hooks for unexpected Java VM termination. The Java VM uses SIGHUP, SIGINT, and SIGTERM to initiate the running of shutdown hooks.
The JVM uses a similar mechanism to implement the feature of dumping thread stacks for debugging purposes. The JVM uses SIGQUIT to perform thread dumps.
Applications embedding the Java VM frequently need to trap signals such as SIGINT or SIGTERM, which can lead to interference with the Java VM signal handlers. The -Xrs command-line option is available to address this issue. When -Xrs is used on the Java VM, the signal masks for SIGINT, SIGTERM, SIGHUP, and SIGQUIT are not changed by the Java VM, and signal handlers for these signals are not installed.
There are two consequences of specifying -Xrs: :
- SIGQUIT thread dumps are not available.:
- User code is responsible for causing shutdown hooks to run, for example by calling System.exit() when the Java VM is to be terminated.:
- Sets the thread stack size.:
- AllocationPrefetchStyle=n:Sets the style of prefetch used during allocation. default=2.:
- +AggressiveOpts:Enables aggressive optimization.:
- +|-DisableAttachMechanism:Specifies whether commands (such as jmap and jconsole) can attach to the Java VM. By default, this feature is disabled. That is, attaching is enabled, for example:
''' java -XX:+DisableAttachMechanism '''
- Specifies the maximum size for large pages.:
- MaxGCPauseMillis=n:Sets a target for the maximum GC pause time.
This is a soft goal, and the Java VM will make its best effort to achieve it. There is no maximum value set by default.:
- NewSize:Sets the size of the young generation (nursery). Sames as -Xmnsize.:
- ParallelGCThreads=n:Sets the number of GC threads in the parallel collectors.:
- PredictedClassLoadCount=n:This option requires that the UnlockExperimentalVMOptions flag be set first. Use the PredictedClassLoadCount flag if your application loads a lot of classes and especially if class.forName() is used heavily. The recommended value is the number of classes loaded as shown in the output from -verbose:class.
''' java -XX:+UnlockExperimentalVMOptions -XX:PredictedClassLoadCount=60013 '''
- +PrintCompilation:Prints verbose output from the Java HotSpot VM dynamic runtime compiler.:
- +PrintGCDetails -XX:+PrintGCTimeStamps:Prints garbage collection output along with time stamps.:
- SoftRefLRUPolicyMSPerMB=0:This flag enables aggressive processing of software references. Use this flag if the software reference count has an impact on the Java HotSpot VM garbage collector.:
- TLABSize=n:Thread local allocation buffers (TLAB) are enabled by default in the Java HotSpot VM. The Java HotSpot VM sizes TLABs based on allocation patterns. The -XX:TLABSize option enables fine-tuning the size of TLABs.:
- +UseAltSigs:The Java VM uses SIGUSR1 and SIGUSR2 by default, which can sometimes conflict with applications that signal-chain SIGUSR1 and SIGUSR2. The -XX:+UseAltSigs option causes the Java VM to use signals other than SIGUSR1 and SIGUSR2 as the default.:
- +|-UseCompressedOops:Enables compressed references in 64-bit Java VMs.
This option is true by default.:
- +UseConcMarkSweepGC or -XX:+UseG1GC:Enables either the Concurrent Mark Sweep (CMS) or the G1 garbage collectors.:
- +|-UseLargePages:Enables large page support.
Large pages are enabled by default on Solaris.:
- +UseParallelOldGC:Enables the parallel garbage collectors, which are optimized for throughput and average response time.:
The -version:release option places no restrictions on the complexity of the release specification. However, only a restricted subset of the possible release specifications represent sound policy and only these are fully supported. These policies are:
- Any version, represented by not using this option.:
- Any version greater than an arbitrarily precise version-id value, for example:
''' "1.6.0_10+" '''
@] This would utilize any version greater than 1.6.0_10. This is useful for a case where an interface was introduced (or a bug fixed) in the release specified.:
- A version greater than an arbitrarily precise version-id, bounded by the upper bound of that release family, for example:
''' "1.6.0_10+&1.6*" '''
- An or expressions of items 2 or 3, for example:
''' "1.6.0_10+&1.6* 1.7+" '''
@] Similar to item 2. This is useful when a change was introduced in a release (1.7) but also made available in updates to earlier releases.:
Performance Tuning Examples
The following examples show how to use experimental tuning flags to optimize either throughput or faster response time.
Example 1, Tuning for Higher Throughput
''' java -d64 -server -XX:+AggressiveOpts -XX:+UseLargePages -Xmn10g -Xms26g -Xmx26g '''
Example 2, Tuning for Lower Response Time
''' java -d64 -XX:+UseG1GC -Xms26g Xmx26g -XX:MaxGCPauseMillis=500 -XX:+PrintGCTimeStamps '''
The following exit values are typically returned by the launcher, typically when the launcher is called with the wrong arguments, serious errors, or exceptions thrown from the Java Virtual Machine. However, a Java application may choose to return any value using the API call System.exit(exitValue).
- 0: Successful completion:
- >0: An error occurred: