### 概述 在API中System类介绍的比较简单，我们给出定义，System中代表程序所在系统，提供了对应的一些系统属性信息，和系统操作。System类不能手动创建对象，因为构造方法被private修饰，阻止外界创建对象。System类中的都是static方法，类名访问即可。在JDK中，有许多这样的类。 ### 常用方法 #### currentTimeMillis() 获取1970年至今的毫秒值 , long型. ~~~ System.out.println(System.currentTimeMillis()); ~~~ ~~~ 1523431212444 ~~~ #### exit() ~~~ System.exit(int) ~~~ 终止当前正在运行的Java虚拟机 . 数传入一个数字即可。通常传入0记为正常状态，其他为异常状态 ### gc() 用来运行JVM中的垃圾回收器，完成内存中垃圾的清除。 完成类 ~~~ public class Test { public void finalize() { System.out.println("垃圾回收了"); } } ~~~ 测试 ~~~ new Test(); System.gc(); ~~~ 结果 ~~~ 垃圾回收了 ~~~ #### arraycopy() * object src 要复制的源数组 * int srcPos 数组源的起始索引 * object dest 复制后的目标数组 * int destPos 目标数组起始索引 * int length 复制长度 ~~~ int[] arr1 = {0,2,3}; int[] arr2 = {4,5,6}; System.arraycopy(arr1,1,arr2,0,2); for(int i=0;i< arr2.length;i++){ System.out.println(arr2[i]); } ~~~ 结果 ~~~ 2 3 6 ~~~ 实际上是对目标数组的内容进行了替换 . #### getProperties() 用来获取指定键(字符串名称)中所记录的系统属性信息 . ~~~ System.out.println(System.getProperties()); ~~~ ~~~ {java.runtime.name=Java(TM) SE Runtime Environment, sun.boot.library.path=C:\Java\jdk1.7.0_72\jre\bin, java.vm.version=24.72-b04, java.vm.vendor=Oracle Corporation, java.vendor.url=http://java.oracle.com/, path.separator=;, java.vm.name=Java HotSpot(TM) Client VM, file.encoding.pkg=sun.io, user.country=CN, user.script=, sun.java.launcher=SUN_STANDARD, sun.os.patch.level=, java.vm.specification.name=Java Virtual Machine Specification, user.dir=C:\Users\Artisan\Desktop\Java, java.runtime.version=1.7.0_72-b14, java.awt.graphicsenv=sun.awt.Win32GraphicsEnvironment, java.endorsed.dirs=C:\Java\jdk1.7.0_72\jre\lib\endorsed, os.arch=x86, java.io.tmpdir=C:\Users\Artisan\AppData\Local\Temp\, line.separator= , java.vm.specification.vendor=Oracle Corporation, user.variant=, os.name=Windows 8.1, sun.jnu.encoding=GBK, java.library.path=C:\Java\jdk1.7.0_72\bin;C:\WINDOWS\Sun\Java\bin;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;C:\WINDOWS\System32\WindowsPowerShell\v1.0\;D:\wamp\Apache24\bin;D:\wamp\PHP7;C:\Program Files\MySQL\MySQL Utilities 1.6\;C:\ProgramData\ComposerSetup\bin;C:\Program Files\Git\cmd;C:\Program Files\nodejs\;C:\Program Files (x86)\NVIDIA Corporation\PhysX\Common;C:\Program Files\Redis\;C:\Java\jdk1.7.0_72\bin;C:\Program Files\MySQL\MySQL Server 5.7\bin;C:\Users\Artisan\AppData\Local\Programs\Python\Python36\Scripts\;C:\Users\Artisan\AppData\Local\Programs\Python\Python36\;C:\Users\Artisan\AppData\Local\Microsoft\WindowsApps;C:\Users\Artisan\AppData\Roaming\Composer\vendor\bin;C:\Users\Artisan\AppData\Roaming\npm;., java.specification.name=Java Platform API Specification, java.class.version=51.0, sun.management.compiler=HotSpot Client Compiler, os.version=6.3, user.home=C:\Users\Artisan, user.timezone=, java.awt.printerjob=sun.awt.windows.WPrinterJob, file.encoding=UTF-8, java.specification.version=1.7, java.class.path=C:\Java\jdk1.7.0_72\jre\lib\charsets.jar;C:\Java\jdk1.7.0_72\jre\lib\deploy.jar;C:\Java\jdk1.7.0_72\jre\lib\ext\access-bridge-32.jar;C:\Java\jdk1.7.0_72\jre\lib\ext\dnsns.jar;C:\Java\jdk1.7.0_72\jre\lib\ext\jaccess.jar;C:\Java\jdk1.7.0_72\jre\lib\ext\localedata.jar;C:\Java\jdk1.7.0_72\jre\lib\ext\sunec.jar;C:\Java\jdk1.7.0_72\jre\lib\ext\sunjce_provider.jar;C:\Java\jdk1.7.0_72\jre\lib\ext\sunmscapi.jar;C:\Java\jdk1.7.0_72\jre\lib\ext\sunpkcs11.jar;C:\Java\jdk1.7.0_72\jre\lib\ext\zipfs.jar;C:\Java\jdk1.7.0_72\jre\lib\javaws.jar;C:\Java\jdk1.7.0_72\jre\lib\jce.jar;C:\Java\jdk1.7.0_72\jre\lib\jfr.jar;C:\Java\jdk1.7.0_72\jre\lib\jfxrt.jar;C:\Java\jdk1.7.0_72\jre\lib\jsse.jar;C:\Java\jdk1.7.0_72\jre\lib\management-agent.jar;C:\Java\jdk1.7.0_72\jre\lib\plugin.jar;C:\Java\jdk1.7.0_72\jre\lib\resources.jar;C:\Java\jdk1.7.0_72\jre\lib\rt.jar;C:\Users\Artisan\Desktop\Java\out\production\Java;C:\Program Files\JetBrains\IntelliJ IDEA 2018.1\lib\idea_rt.jar, user.name=Artisan, java.vm.specification.version=1.7, sun.java.command=com.dome.Main, java.home=C:\Java\jdk1.7.0_72\jre, sun.arch.data.model=32, user.language=zh, java.specification.vendor=Oracle Corporation, awt.toolkit=sun.awt.windows.WToolkit, java.vm.info=mixed mode, sharing, java.version=1.7.0_72, java.ext.dirs=C:\Java\jdk1.7.0_72\jre\lib\ext;C:\WINDOWS\Sun\Java\lib\ext, sun.boot.class.path=C:\Java\jdk1.7.0_72\jre\lib\resources.jar;C:\Java\jdk1.7.0_72\jre\lib\rt.jar;C:\Java\jdk1.7.0_72\jre\lib\sunrsasign.jar;C:\Java\jdk1.7.0_72\jre\lib\jsse.jar;C:\Java\jdk1.7.0_72\jre\lib\jce.jar;C:\Java\jdk1.7.0_72\jre\lib\charsets.jar;C:\Java\jdk1.7.0_72\jre\lib\jfr.jar;C:\Java\jdk1.7.0_72\jre\classes, java.vendor=Oracle Corporation, file.separator=\, java.vendor.url.bug=http://bugreport.sun.com/bugreport/, sun.io.unicode.encoding=UnicodeLittle, sun.cpu.endian=little, sun.desktop=windows, sun.cpu.isalist=pentium_pro+mmx pentium_pro pentium+mmx pentium i486 i386 i86} ~~~