java - Program Launching Command and Options

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∟java - Program Launching Command and Options

This section describes the Java launching tool 'java' and its commonly used command options.

What Is "java"? - "java" is a command line tool that launches Java applications. It starts a Java Virtual Machine (JVM), loads the specified class, and invokes that class's main method.

"java" has been included in JDK installation since JDK 1.0. And it is represented by the %java_home%\bin\java.exe program file.

"java" command has the following syntax:

java [options] class [arguments]

where "options" is a list of options; "class" is the full name of a Java class to be launched; "arguments" is a list of arguments to be passed to the main method of the class to be launched.

Another way of launching a Java class is to use the "-jar" option:

java [options] -jar file [arguments]

where "file" is a JAR file, which should contain a "Main-Class" attribute in the manifest file. The "Main-Class" attribute defines the Java class to be launched.

Commonly used "java" options are:

"-help" - Displays a short help text.
"-verbose" - Generates verbose output to standard output.
"-version" - Prints the version information of the launcher.
"-classpath classpath" - Specifies a list of path names where the launcher will search for compiled type definitions. If "-classpath" is not specified, the current directory will be used as the class path.
"-Dproperty=value" - Defines a new system property, which can be accessed by the application.

You can run "java" by typing in "java" at the command prompt, if you have %java_home%\bin directory included in "path" the environment variable. If you run the "java" command without any options, you will get the quick usage information as shown below:

herong> java

Usage: java [options] <mainclass> [args...]
           (to execute a class)
   or  java [options] -jar <jarfile> [args...]
           (to execute a jar file)
   or  java [options] -m <module>[/<mainclass>] [args...]
       java [options] --module <module>[/<mainclass>] [args...]
           (to execute the main class in a module)

 Arguments following the main class, -jar <jarfile>, -m or --module
 <module>/<mainclass> are passed as the arguments to main class.

 where options include:

    -cp <class search path of directories and zip/jar files>
    -classpath <class search path of directories and zip/jar files>
    --class-path <class search path of directories and zip/jar files>
                  A ; separated list of directories, JAR archives,
                  and ZIP archives to search for class files.
    -p <module path>
    --module-path <module path>...
                  A ; separated list of directories, each directory
                  is a directory of modules.
    --upgrade-module-path <module path>...
                  A ; separated list of directories, each directory
                  is a directory of modules that replace upgradeable
                  modules in the runtime image
    --add-modules <module name>[,<module name>...]
                  root modules to resolve in addition to the initial
                  module. <module name> can also be ALL-DEFAULT,
                  ALL-SYSTEM, ALL-MODULE-PATH.
    --list-modules
                  list observable modules and exit
    -d <module name>
    --describe-module <module name>
                  describe a module and exit
    --dry-run     create VM and load main class but do not execute
                  main method. The --dry-run option may be useful for
                  validating the command-line options such as the module
                  system configuration.
    --validate-modules
                  validate all modules and exit
                  The --validate-modules option may be useful for
                  finding conflicts and other errors with modules on
                  the module path.
    -D<name>=<value>
                  set a system property
    -verbose:[class|module|gc|jni]
                  enable verbose output
    -version      print product version to the error stream and exit
    --version     print product version to the output stream and exit
    -showversion  print product version to the error stream and continue
    --show-version
                  print product version to the output stream and continue
    --show-module-resolution
                  show module resolution output during startup
    -? -h -help
                  print this help message to the error stream
    --help        print this help message to the output stream
    -X            print help on extra options to the error stream
    --help-extra  print help on extra options to the output stream
    -ea[:<packagename>...|:<classname>]
    -enableassertions[:<packagename>...|:<classname>]
                  enable assertions with specified granularity
    -da[:<packagename>...|:<classname>]
    -disableassertions[:<packagename>...|:<classname>]
                  disable assertions with specified granularity
    -esa | -enablesystemassertions
                  enable system assertions
    -dsa | -disablesystemassertions
                  disable system assertions
    -agentlib:<libname>[=<options>]
                  load native agent library <libname>, e.g.
                  -agentlib:jdwp, see also -agentlib:jdwp=help
    -agentpath:<pathname>[=<options>]
                  load native agent library by full pathname
    -javaagent:<jarpath>[=<options>]
                  load Java programming language agent, see
                  java.lang.instrument

    -splash:<imagepath>
                  show splash screen with specified image
                  HiDPI scaled images are automatically supported and
                  used if available. The unscaled image filename, e.g.
                  image.ext, should always be passed as the argument to
                  the -splash option.

                  The most appropriate scaled image provided will be
                  picked up automatically. See the SplashScreen API
                  documentation for more information
    @argument files
                  one or more argument files containing options
    -disable-@files
                  prevent further argument file expansion

To specify an argument for a long option, you can use --<name>=<value>
or --<name> <value>.