Section: User Commands (1)
Updated: 18 Jul 2013
java - the Java application launcher
''' java' [ options ] class [ argument ...'' ] java [ options ] -jar file.jar [ argument ... ]
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.
For default Java VM selection, see Server-Class Machine Detection at http://docs.oracle.com/javase/7/docs/technotes/guides/vm/server-class.html
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
@] See JVMTI Agent Command-Line Options at http://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.html#starting
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").:
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.:
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.:
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.:
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
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.:
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.:
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.:
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.:
''' -Xms6291456 -Xms6144k -Xms6m '''
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.:
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: :
''' java -XX:+DisableAttachMechanism '''
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.:
''' java -XX:+UnlockExperimentalVMOptions -XX:PredictedClassLoadCount=60013 '''
This option is true by default.:
Large pages are enabled by default on Solaris.:
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:
''' "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.:
''' "1.6.0_10+&1.6*" '''
''' "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.:
The following examples show how to use experimental tuning flags to optimize either throughput or faster response time.
''' java -d64 -server -XX:+AggressiveOpts -XX:+UseLargePages -Xmn10g -Xms26g -Xmx26g '''
''' 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).
Tutoriais de Tecnologia Web