Java DaemonThread(守护线程)

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Java中有两类线程:User Thread(用户线程)、Daemon Thread(守护线程) 。

只要当前JVM实例中尚存在任何一个非守护线程没有结束,守护线程就全部工作;当最后一个非守护线程结束时,守护线程就会随着JVM一同结束工作。

GC (垃圾回收器)就是一个最典型的守护线程。它始终在低级别的状态中运行,用于实时监控和管理系统中的可回收资源。

下面看4个例子

(1)主线程结束,JVM会等到User Thread(用户线程)执行完毕才退出。这说明在Java中创建的线程默认是User Thread(用户线程)。

package com.demo.test;public class DeamonThreadDemo {    public static void main(String[] args) {        Thread thread = new Thread(new Runnable() {            @Override            public void run() {                try {                    Thread.sleep(1000);                } catch (InterruptedException e) {                    e.printStackTrace();                }                System.out.println("user thread end, isDaemon = " + Thread.currentThread().isDaemon());            }        });        //thread.setDaemon(true);        thread.start();        Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {            @Override            public void run() {                System.out.println("JVM is close");            }        }));        System.out.println("main end");    }}

输出

main endthread end, isDaemon = falseJVM is close

(2)主线程结束,JVM并没有等待Daemon Thread(守护线程)执行结束就直接退出了。此时Daemon Thread(守护线程)被终止。

同”例(1)”的代码,取消掉thread.setDaemon(true);的注释。

输出

main endJVM is close

(3)在Daemon Thread(守护线程)中新建一个线程并启动,发现JVM在主线程执行结束时就退出了,并没有等待Daemon Thread(守护线程)中创建的新线程执行结束,这说明在Daemon Thread(守护线程)中创建的线程默认是Daemon Thread(守护线程)。

package com.demo.test;public class DeamonThreadDemo {    public static void main(String[] args) {        Thread thread = new Thread(new Runnable() {            @Override            public void run() {                Thread t = new Thread(new Runnable(){                    @Override                    public void run() {                        System.out.println("child daemon thread start, isDaemon = " + Thread.currentThread().isDaemon());                        try {                            Thread.sleep(1000);                        } catch (InterruptedException e) {                            e.printStackTrace();                        }                        System.out.println("child daemon thread end, isDaemon = " + Thread.currentThread().isDaemon());                    }                });                //t.setDaemon(false);                t.start();                System.out.println("daemon thread end");            }        });        thread.setDaemon(true);        thread.start();        try {            //这里休眠是为了保证能正常创建"child daemon thread"            Thread.sleep(100);        } catch (InterruptedException e) {            e.printStackTrace();        }        Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {            @Override            public void run() {                System.out.println("JVM is close");            }        }));        System.out.println("main end");    }}

输出

child daemon thread start, isDaemon = truedaemon thread end, isDaemon = truemain endJVM is close

(4)在Daemon Thread(守护线程)中创建的新线程虽然默认也是Daemon Thread(守护线程),但可以通过设置让其变为User Thread(用户线程)

同”例(3)”的代码,取消掉t.setDaemon(true);的注释。

输出

child daemon thread start, isDaemon = falsedaemon thread end, isDaemon = truemain endchild daemon thread endJVM is close
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