java命令详解

JDK工具 java命令详解

一、查看用法

 

C:\>java -help

Usage: java [-options] class [args...]

           (to execute a class)

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

           (to execute a jar file)

 

where options include:

    -client       to select the "client" VM

    -server       to select the "server" VM

    -hotspot      is a synonym for the "client" VM  [deprecated]

                  The default VM is client.

 

    -cp <class search path of directories and zip/jar files>

    -classpath <class search path of directories and zip/jar files>

                  A ; separated list of directories, JAR archives,

                  and ZIP archives to search for class files.

    -D<name>=<value>

                  set a system property

    -verbose[:class|gc|jni]

                  enable verbose output

    -version      print product version and exit

    -version:<value>

                  require the specified version to run

    -showversion  print product version and continue

    -jre-restrict-search | -jre-no-restrict-search

                  include/exclude user private JREs in the version search

    -? -help      print this help message

    -X            print help on non-standard options

    -ea[:<packagename>...|:<classname>]

    -enableassertions[:<packagename>...|:<classname>]

                  enable assertions

    -da[:<packagename>...|:<classname>]

    -disableassertions[:<packagename>...|:<classname>]

                  disable assertions

    -esa | -enablesystemassertions

                  enable system assertions

    -dsa | -disablesystemassertions

                  disable system assertions

    -agentlib:<libname>[=<options>]

                  load native agent library <libname>, e.g. -agentlib:hprof

                    see also, -agentlib:jdwp=help and -agentlib:hprof=help

    -agentpath:<pathname>[=<options>]

                  load native agent library by full pathname

    -javaagent:<jarpath>[=<options>]

                  load Java programming language agent, see java.lang.instrument

 

这个命令帮助是英文的，不知道JDK咋搞的，也不妨碍使用。

 

另外，这个命令的非标准选项也是很重要的，常常在JVM优化配置方面很关键，可以参看本人的JVM参数配置文章。

C:\myproject>java -X
    -Xmixed           mixed mode execution (default)
    -Xint             interpreted mode execution only
    -Xbootclasspath:<directories and zip/jar files separated by ;>
                      set search path for bootstrap classes and resources
    -Xbootclasspath/a:<directories and zip/jar files separated by ;>
                      append to end of bootstrap class path
    -Xbootclasspath/p:<directories and zip/jar files separated by ;>
                      prepend in front of bootstrap class path
    -Xnoclassgc       disable class garbage collection
    -Xincgc           enable incremental garbage collection
    -Xloggc:<file>    log GC status to a file with time stamps
    -Xbatch           disable background compilation
    -Xms<size>        set initial Java heap size
    -Xmx<size>        set maximum Java heap size
    -Xss<size>        set java thread stack size
    -Xprof            output cpu profiling data
    -Xfuture          enable strictest checks, anticipating future default
    -Xrs              reduce use of OS signals by Java/VM (see documentation)
    -Xcheck:jni       perform additional checks for JNI functions
    -Xshare:off       do not attempt to use shared class data
    -Xshare:auto      use shared class data if possible (default)
    -Xshare:on        require using shared class data, otherwise fail.

 

 

二、实践

 

老规矩，主要看看里面的参数（也叫开关）的使用。环境接着上篇javac的环境。

 

 

1、无参数情况

 

2、-cp

运行Java要使用类的全名来运行。如果遇到文件夹，则需要-cp设置到顶级包下面，例如

 

3、-D

设置一个系统属性，在运行时候，可以通过System.getProperties()来获取到。例如写一段代码：

package com.lavasoft;

import java.util.Properties;

/**
* 列举系统的属性
* User: leizhimin
* Date: 2008-11-12 21:25:08
*/
public class TestProperty {
        public static void main(String[] args) {
                //获取系统属性
                Properties prop = System.getProperties();
                //输出所有到一个流上，
                prop.list(System.out);
        }
}

 

 

如果在运行的时候加上一个参数-DmyProp=999999999，注意，中间不要添加任何空格。这样就相当于设置了一个系统属性myProp，其值为999999999。

 

一旦通过-DmyProp=999999999设置了这个系统属性，在程序中就可以获取到这个属性。

 

3、程序入参

是在运行的时候，给main方法传递参数。为了测试先写一个测试类。

package com.lavasoft;

/**
* 测试main方法的入参
* User: leizhimin
* Date: 2008-11-12 21:46:21
*/
public class TestMainVar {
        public static void main(String[] args) {
                System.out.println("入参列表如下：");
                for(String arg:args){
                        System.out.println(arg);
                }
        }
}

 

 

4、其他的选项

有关选择 客户端/服务端VM、版本、运行在哪个版本下、是否使用断言等一些不常用的选项。还可以查看

 

 

5、-verbose[:class|gc|jni]

查看虚拟机内部动作。