java多线程总结二：后台线程(守护线程)

所谓的后台线程，是指在程序运行的时候在后台提供一种通用服务的线程，并且这种线程并不属于程序中不可或缺的部分。因此当所有的非后台线程结束时，程序也就终止了，同时会杀死所有后台线程。反过来说，只要有任何非后台线程(用户线程)还在运行，程序就不会终止。后台线程在不执行finally子句的情况下就会终止其run方法。后台线程创建的子线程也是后台线程。

下面是一个后台线程的示例：

package com.muzeet.testThread;

import java.util.concurrent.TimeUnit;

public class DeamonDemo implements Runnable {
    @Override
    public void run() {
        try {
            while (true) {
                Thread.sleep(1000);
                System.out.println("#" + Thread.currentThread().getName());
                
                
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            // 后台线程不执行finally子句
            System.out.println("finally ");
        }
    }

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            Thread daemon = new Thread(new DeamonDemo());
            // 必须在start之前设置为后台线程
            daemon.setDaemon(true);
            daemon.start();
            
        }
        System.out.println("All deamons started");
        try {
            TimeUnit.MILLISECONDS.sleep(10000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

运行结果：

All deamons started
#Thread-2
#Thread-0
#Thread-6
#Thread-4
#Thread-8
#Thread-9
#Thread-5
#Thread-7
#Thread-3
#Thread-1
#Thread-2
#Thread-4
#Thread-6
#Thread-0
#Thread-8
#Thread-9
#Thread-7
#Thread-5
#Thread-3
#Thread-1
#Thread-4
#Thread-6
#Thread-0
#Thread-2
#Thread-8
#Thread-9
#Thread-7
#Thread-5
#Thread-3
#Thread-1
#Thread-6
#Thread-4
#Thread-2
#Thread-0
#Thread-8
#Thread-9
#Thread-5
#Thread-7
#Thread-3
#Thread-1
#Thread-4
#Thread-0
#Thread-2
#Thread-6
#Thread-8
#Thread-9
#Thread-5
#Thread-7
#Thread-3
#Thread-1
#Thread-0
#Thread-2
#Thread-6
#Thread-4
#Thread-8
#Thread-1
#Thread-3
#Thread-7
#Thread-5
#Thread-9
#Thread-0
#Thread-6
#Thread-2
#Thread-4
#Thread-8
#Thread-9
#Thread-7
#Thread-5
#Thread-1
#Thread-3

　　

分析：从结果可以看出，十个子线程并没有无线循环的打印（而是在主线程sleep的10s中，所有子线程各输出10次，每个子线程sleep1s），而是在主线程(main())退出后，JVM强制关闭所有后台线程。而不会有任何希望出现的确认形式，如finally子句不执行。