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：

 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 }

View Code

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

View Code

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 }

View Code

 

结果：

1       

View Code

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。