System类构造函数由private修饰，不可以被实例化，加载时调用static代码块。
System类提供了标准输入输出流，错误输出流，获取外部属性和系统环境的方法，加载类库和文件的方法，快速copy数组的方法；其中out和err的类型是PrintStream
/**
* The <code>System</code> class contains several useful class fields
* and methods. It cannot be instantiated.
*
* <p>Among the facilities provided by the <code>System</code> class
* are standard input, standard output, and error output streams;
* access to externally defined properties and environment
* variables; a means of loading files and libraries; and a utility
* method for quickly copying a portion of an array.
*
* @author unascribed
* @since JDK1.0
*/
public final class System {
/* register the natives via the static initializer.
*
* VM will invoke the initializeSystemClass method to complete
* the initialization for this class separated from clinit.
* Note that to use properties set by the VM, see the constraints
* described in the initializeSystemClass method.
*/
private static native void registerNatives();
static {
registerNatives();
}
/** Don't let anyone instantiate this class */
private System() {
}
System.exit(int),终止当前虚拟机的运行，什么叫做当前虚拟机呢？
public static void exit(int status) {
Runtime.getRuntime().exit(status);
}
如果在某个线程中调用了exit会怎样？下面的例子输出结果是hello，把子线程和主线程的内容交换也是一样的结果。就是说不管在哪个线程调用了exit，整个应用都退出了。
package com.ysdx.test;
public class JavaTest {
public static void main(String[] args) {
Thread thread = new Thread(() -> {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("hello!");
System.exit(1);
});
thread.start();
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("Hello from main thread!");
}
}
两个虚拟机之间是互不影响的
其中JavaTest1如下
public class JavaTest1 {
public static void main(String[] args) {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("Hello from main thread!");
}
}
---------------------
System中gc相关操作
/**
* Runs the garbage collector.
* <p>
* Calling the <code>gc</code> method suggests that the Java Virtual
* Machine expend effort toward recycling unused objects in order to
* make the memory they currently occupy available for quick reuse.
* When control returns from the method call, the Java Virtual
* Machine has made a best effort to reclaim space from all discarded
* objects.
* <p>
* The call <code>System.gc()</code> is effectively equivalent to the
* call:
* <blockquote><pre>
* Runtime.getRuntime().gc()
* </pre></blockquote>
*
* @see java.lang.Runtime#gc()
*/
public static void gc() {
Runtime.getRuntime().gc();
}
/**
* Runs the finalization methods of any objects pending finalization.
* <p>
* Calling this method suggests that the Java Virtual Machine expend
* effort toward running the <code>finalize</code> methods of objects
* that have been found to be discarded but whose <code>finalize</code>
* methods have not yet been run. When control returns from the
* method call, the Java Virtual Machine has made a best effort to
* complete all outstanding finalizations.
* <p>
* The call <code>System.runFinalization()</code> is effectively
* equivalent to the call:
* <blockquote><pre>
* Runtime.getRuntime().runFinalization()
* </pre></blockquote>
*
* @see java.lang.Runtime#runFinalization()
*/
public static void runFinalization() {
Runtime.getRuntime().runFinalization();
}
顺便看一下Runtime，
即便不显示调用gc，JVM也会自动在一个单独的线程中执行recycling process；
即便不显示调用runFinalization，JVM也会自动在一个单独的线程中执行finalization process；
可以很明显的看出gc和runFinalization是不同的，一个叫做回收过程，一个叫做终结过程。
调用runFinalization后，终结过程会调用已经被废弃但是尚未调用其finalization方法的对象。
函数注释里的描述都使用了suggest和has made its best effort，可能这两个函数还有不足之处。
/**
* Runs the garbage collector.
* Calling this method suggests that the Java virtual machine expend
* effort toward recycling unused objects in order to make the memory
* they currently occupy available for quick reuse. When control
* returns from the method call, the virtual machine has made
* its best effort to recycle all discarded objects.
* <p>
* The name <code>gc</code> stands for "garbage
* collector". The virtual machine performs this recycling
* process automatically as needed, in a separate thread, even if the
* <code>gc</code> method is not invoked explicitly.
* <p>
* The method {@link System#gc()} is the conventional and convenient
* means of invoking this method.
*/
public native void gc();
/* Wormhole for calling java.lang.ref.Finalizer.runFinalization */
private static native void runFinalization0();
/**
* Runs the finalization methods of any objects pending finalization.
* Calling this method suggests that the Java virtual machine expend
* effort toward running the <code>finalize</code> methods of objects
* that have been found to be discarded but whose <code>finalize</code>
* methods have not yet been run. When control returns from the
* method call, the virtual machine has made a best effort to
* complete all outstanding finalizations.
* <p>
* The virtual machine performs the finalization process
* automatically as needed, in a separate thread, if the
* <code>runFinalization</code> method is not invoked explicitly.
* <p>
* The method {@link System#runFinalization()} is the conventional
* and convenient means of invoking this method.
*
* @see java.lang.Object#finalize()
*/
public void runFinalization() {
runFinalization0();
}
System中获取系统变量的方法：
getenv(String name)返回跟name对应的变量值；
getenv()，这个函数返回一个不可修改的、包含系统变量名和对应值的map，这个map在ProcessEnvironment中声明：private static final Map<String,String> theUnmodifiableEnvironment;
public static String getenv(String name) {
SecurityManager sm = getSecurityManager();
if (sm != null) {
sm.checkPermission(new RuntimePermission("getenv."+name));
}
return ProcessEnvironment.getenv(name);
}
public static java.util.Map<String,String> getenv() {
SecurityManager sm = getSecurityManager();
if (sm != null) {
sm.checkPermission(new RuntimePermission("getenv.*"));
}
return ProcessEnvironment.getenv();
}
System类关于系统属性的操作有get、set和clear方法，而Environment只有读取的函数。
environment是操作系统环境变量信息，property感觉更像是jvm相关参数。下面例子的输出结果是：
java.runtime.name = Java(TM) SE Runtime Environment sun.boot.library.path = C:\Program Files\Java\jre1.8.0_121\bin java.vm.version = 25.121-b13 java.vm.vendor = Oracle Corporation java.vendor.url = http://java.oracle.com/ path.separator = ; java.vm.name = Java HotSpot(TM) 64-Bit Server VM file.encoding.pkg = sun.io user.script = user.country = CN sun.java.launcher = SUN_STANDARD sun.os.patch.level = java.vm.specification.name = Java Virtual Machine Specification user.dir = F:\CodingSource\neon-workspace\JavaSystem java.runtime.version = 1.8.0_121-b13 java.awt.graphicsenv = sun.awt.Win32GraphicsEnvironment java.endorsed.dirs = C:\Program Files\Java\jre1.8.0_121\lib\endorsed os.arch = amd64 java.io.tmpdir = C:\Users\ADMINI~1\AppData\Local\Temp\ line.separator = java.vm.specification.vendor = Oracle Corporation user.variant = os.name = Windows 8 sun.jnu.encoding = GBK java.library.path = C:\Program Files\Java\jre1.8.0_121\bin;C:\Windows\Sun\Java\bin;C:\Windows\system32;C:\Windows;C:/Program Files/Java/jre1.8.0_121/bin/server;C:/Program Files/Java/jre1.8.0_121/bin;C:/Program Files/Java/jre1.8.0_121/lib/amd64;E:\CodingIDE\Python36\Scripts\;E:\CodingIDE\Python36\;E:\CodingIDE\DataBase\oraclexe\app\oracle\product\11.2.0\server\bin;;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files\Java\jdk1.8.0_121\bin;C:\Program Files\Java\jdk1.8.0_121\jre\bin;E:\CodingIDE\DataBase\mysql-5.7.17-winx64\bin;E:\CodingIDE\eclipse-jee-neon-3-RC3-win32-x86_64\eclipse;;. java.specification.name = Java Platform API Specification java.class.version = 52.0 sun.management.compiler = HotSpot 64-Bit Tiered Compilers os.version = 6.2 user.home = C:\Users\Administrator user.timezone = java.awt.printerjob = sun.awt.windows.WPrinterJob file.encoding = GBK java.specification.version = 1.8 user.name = Administrator java.class.path = F:\CodingSource\neon-workspace\JavaSystem\bin java.vm.specification.version = 1.8 sun.arch.data.model = 64 java.home = C:\Program Files\Java\jre1.8.0_121 sun.java.command = com.ysdx.jsys.JavaSystem java.specification.vendor = Oracle Corporation user.language = zh awt.toolkit = sun.awt.windows.WToolkit java.vm.info = mixed mode java.version = 1.8.0_121 java.ext.dirs = C:\Program Files\Java\jre1.8.0_121\lib\ext;C:\Windows\Sun\Java\lib\ext sun.boot.class.path = C:\Program Files\Java\jre1.8.0_121\lib\resources.jar;C:\Program Files\Java\jre1.8.0_121\lib\rt.jar;C:\Program Files\Java\jre1.8.0_121\lib\sunrsasign.jar;C:\Program Files\Java\jre1.8.0_121\lib\jsse.jar;C:\Program Files\Java\jre1.8.0_121\lib\jce.jar;C:\Program Files\Java\jre1.8.0_121\lib\charsets.jar;C:\Program Files\Java\jre1.8.0_121\lib\jfr.jar;C:\Program Files\Java\jre1.8.0_121\classes java.vendor = Oracle Corporation file.separator = \ java.vendor.url.bug = http://bugreport.sun.com/bugreport/ sun.cpu.endian = little sun.io.unicode.encoding = UnicodeLittle sun.desktop = windows sun.cpu.isalist = amd64 ----------------------------------- USERDOMAIN_ROAMINGPROFILE=PC-201703120850 LOCALAPPDATA=C:\Users\Administrator\AppData\Local PROCESSOR_LEVEL=6 FP_NO_HOST_CHECK=NO USERDOMAIN=PC-201703120850 LOGONSERVER=\\PC-201703120850 JAVA_HOME=C:\Program Files\Java\jdk1.8.0_121 SESSIONNAME=Console ALLUSERSPROFILE=C:\ProgramData PROCESSOR_ARCHITECTURE=AMD64 PSModulePath=C:\Windows\system32\WindowsPowerShell\v1.0\Modules\ #envTSLOGSHELLEXT3464=510784384 SystemDrive=C: =C:=C:\ APPDATA=C:\Users\Administrator\AppData\Roaming USERNAME=Administrator ProgramFiles(x86)=C:\Program Files (x86) VBOX_MSI_INSTALL_PATH=F:\VirtualBox\ CommonProgramFiles=C:\Program Files\Common Files Path=C:/Program Files/Java/jre1.8.0_121/bin/server;C:/Program Files/Java/jre1.8.0_121/bin;C:/Program Files/Java/jre1.8.0_121/lib/amd64;E:\CodingIDE\Python36\Scripts\;E:\CodingIDE\Python36\;E:\CodingIDE\DataBase\oraclexe\app\oracle\product\11.2.0\server\bin;;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files\Java\jdk1.8.0_121\bin;C:\Program Files\Java\jdk1.8.0_121\jre\bin;E:\CodingIDE\DataBase\mysql-5.7.17-winx64\bin;E:\CodingIDE\eclipse-jee-neon-3-RC3-win32-x86_64\eclipse; PATHEXT=.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC;.PY;.PYW OS=Windows_NT COMPUTERNAME=PC-201703120850 PROCESSOR_REVISION=3a09 CommonProgramW6432=C:\Program Files\Common Files #envTSLOGsss3464=166647136 ComSpec=C:\Windows\system32\cmd.exe #envTSLOGXMediaLibrary3464=166645216 ProgramData=C:\ProgramData =F:=F:\ ProgramW6432=C:\Program Files HOMEPATH=\Users\Administrator SystemRoot=C:\Windows TEMP=C:\Users\ADMINI~1\AppData\Local\Temp HOMEDRIVE=C: PROCESSOR_IDENTIFIER=Intel64 Family 6 Model 58 Stepping 9, GenuineIntel USERPROFILE=C:\Users\Administrator TMP=C:\Users\ADMINI~1\AppData\Local\Temp CommonProgramFiles(x86)=C:\Program Files (x86)\Common Files ProgramFiles=C:\Program Files PUBLIC=C:\Users\Public NUMBER_OF_PROCESSORS=4 windir=C:\Windows #envTSLOGTSLOG3464=166646496 =::=::\
package com.ysdx.jsys;
import java.util.Enumeration;
import java.util.Map;
import java.util.Properties;
public class JavaSystem {
public static void main(String[] args) {
Properties properties = System.getProperties();
Enumeration<?> propertyNames = properties.propertyNames();
for (; propertyNames.hasMoreElements();) {
String propertyName = (String) propertyNames.nextElement();
System.out.println(propertyName + " = " + properties.getProperty(propertyName));
}
System.out.println("-----------------------------------");
Map<String, String> map = System.getenv();
for (String key : map.keySet()) {
System.out.println(key + "=" + map.get(key));
}
}
}
来源：https://www.cnblogs.com/afraidToForget/p/6625357.html