什么是后台（守护）线程？

所谓的后台（daemon）线程，也叫守护线程，是指程序在运行的时候，在后台提供一种通用服务的线程（例如：守护线程GC），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，同时杀死所有的后台线程。相反，只要有任何非后台线程（例如：非守护线程main()）还存在，程序就不会终止。后台线程具有最低的优先级。

setDaemon(boolean on)方法可以方便的设置线程的Daemon模式，true为Daemon模式，默认（false）为User模式。setDaemon(boolean on)方法必须在线程启动（start()）之前调用，当线程正在运行时调用会产生异常。

isDaemon方法将测试该线程是否为守护线程。注意：当你在一个守护线程中产生了其他线程，那么这些新产生的线程不用设置Daemon属性，都将是守护线程，用户线程同样。　　

守护线程与普通线程的唯一区别是：守护线程就是main同生共死，当main退出，它将终止，而普通线程是在任务执行结束才停止。

普通线程代码：

package com.mianshi.easy;
public class StartRunTest {
    /**
     * 后台（守护）线程实验
     */
    public static void main(String[] args){
        Thread thread=new ThreadDemo();

        //1、为什么没有打印出100句呢?因为我们将thread线程设置为了daemon(守护)线程,程序中只有守护线程存在的时候,是可以退出的,所以只打印了七句便退出了
        //2、当java虚拟机中有守护线程在运行的时候，java虚拟机会关闭。当所有常规线程运行完毕以后，
        //守护线程不管运行到哪里，虚拟机都会退出运行。所以你的守护线程最好不要写一些会影响程序的业务逻辑。否则无法预料程序到底会出现什么问题
        //thread.setDaemon(true);
        thread.start();
    }
    
    public static class ThreadDemo extends Thread{
        @Override
        public void run() {
            for (int i = 0; i < 100; i++) {
                System.out.println("This is a Thread test"+i);
            }
        }
    }
}

结果：

This is a Thread test0
This is a Thread test1
This is a Thread test2
This is a Thread test3
This is a Thread test4
This is a Thread test5
This is a Thread test6
This is a Thread test7
This is a Thread test8
This is a Thread test9
This is a Thread test10
This is a Thread test11
This is a Thread test12
This is a Thread test13
This is a Thread test14
This is a Thread test15
This is a Thread test16
This is a Thread test17
This is a Thread test18
This is a Thread test19
This is a Thread test20
This is a Thread test21
This is a Thread test22
This is a Thread test23
This is a Thread test24
This is a Thread test25
This is a Thread test26
This is a Thread test27
This is a Thread test28
This is a Thread test29
This is a Thread test30
This is a Thread test31
This is a Thread test32
This is a Thread test33
This is a Thread test34
This is a Thread test35
This is a Thread test36
This is a Thread test37
This is a Thread test38
This is a Thread test39
This is a Thread test40
This is a Thread test41
This is a Thread test42
This is a Thread test43
This is a Thread test44
This is a Thread test45
This is a Thread test46
This is a Thread test47
This is a Thread test48
This is a Thread test49
This is a Thread test50
This is a Thread test51
This is a Thread test52
This is a Thread test53
This is a Thread test54
This is a Thread test55
This is a Thread test56
This is a Thread test57
This is a Thread test58
This is a Thread test59
This is a Thread test60
This is a Thread test61
This is a Thread test62
This is a Thread test63
This is a Thread test64
This is a Thread test65
This is a Thread test66
This is a Thread test67
This is a Thread test68
This is a Thread test69
This is a Thread test70
This is a Thread test71
This is a Thread test72
This is a Thread test73
This is a Thread test74
This is a Thread test75
This is a Thread test76
This is a Thread test77
This is a Thread test78
This is a Thread test79
This is a Thread test80
This is a Thread test81
This is a Thread test82
This is a Thread test83
This is a Thread test84
This is a Thread test85
This is a Thread test86
This is a Thread test87
This is a Thread test88
This is a Thread test89
This is a Thread test90
This is a Thread test91
This is a Thread test92
This is a Thread test93
This is a Thread test94
This is a Thread test95
This is a Thread test96
This is a Thread test97
This is a Thread test98
This is a Thread test99

可见，在主线程中启动了默认的普通线程，此后主线程没有任何的执行语句，开始等待子线程执行，子线程得到CPU执行权后，开始执行run()方法，打印完100句话后，线程执行完成并终止，此时，主线程也会结束。程序中所有的非守护线程执行完成，程序结束。

守护线程代码：

package com.mianshi.easy;
public class StartRunTest {
    /**
     * 后台（守护）线程实验
     */
    public static void main(String[] args){
        Thread thread=new ThreadDemo();

        //1、为什么没有打印出100句呢?因为我们将thread线程设置为了daemon(守护)线程,程序中只有守护线程存在的时候,是可以退出的,所以只打印了七句便退出了
        //2、当java虚拟机中有守护线程在运行的时候，java虚拟机会关闭。当所有常规线程运行完毕以后，
        //守护线程不管运行到哪里，虚拟机都会退出运行。所以你的守护线程最好不要写一些会影响程序的业务逻辑。否则无法预料程序到底会出现什么问题
        thread.setDaemon(true);
        thread.start();
    }
    
    public static class ThreadDemo extends Thread{
        @Override
        public void run() {
            for (int i = 0; i < 100; i++) {
                System.out.println("This is a Thread test"+i);
            }
        }
    }
}

结果：

此时，进入主线程，创建了一个线程，随后将它设置为后台线程，并调用start()方法，使子线程就绪，但此时，程序中没有任何非后台线程，所以，主线程可以结束，程序也会停止。