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：
1 package com.mianshi.easy;
2 public class StartRunTest {
3 /**
4 * 后台（守护）线程实验
5 */
6 public static void main(String[] args){
7 Thread thread=new ThreadDemo();
8
9 //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
10 //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，
11 //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
12 //thread.setDaemon(true);
13 thread.start();
14 }
15
16 public static class ThreadDemo extends Thread{
17 @Override
18 public void run() {
19 for (int i = 0; i < 100; i++) {
20 System.out.println("This is a Thread test"+i);
21 }
22 }
23 }
24 }
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：
1 package com.mianshi.easy;
2 public class StartRunTest {
3 /**
4 * 后台（守护）线程实验
5 */
6 public static void main(String[] args){
7 Thread thread=new ThreadDemo();
8
9 //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
10 //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，
11 //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
12 thread.setDaemon(true);
13 thread.start();
14 }
15
16 public static class ThreadDemo extends Thread{
17 @Override
18 public void run() {
19 for (int i = 0; i < 100; i++) {
20 System.out.println("This is a Thread test"+i);
21 }
22 }
23 }
24 }
结果：
1
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。
时间： 2024-10-29 09:16:02