线程的后台线程

后台线程是在后台运行的，他的任务是为其他的线程提供服务，也成为“守护线程”。JVM的垃圾回收线程就是典型的后台线程。
特征：如果所有的前台线程都死亡，那么后台线程会自动死亡。
调用THREAD对象的setDaemon(true)方法就可将制定线程设置为后台线程。

程序：
package com.test;

public class DaemonThread extends Thread {

public void run()
{
for(int i=0;i<1000;i++)
{
System.out.println(Thread.currentThread().getName()+" "+i);
}
}

public static void main(String[] args)
{
DaemonThread dt = new DaemonThread();
dt.setDaemon(true);
dt.start();
for(int i=0;i<10;i++)
{

System.out.println(Thread.currentThread().getName()+" "+i);

}
}
}

打印结果：
main 0
Thread-0 0
main 1
Thread-0 1
main 2
Thread-0 2
Thread-0 3
Thread-0 4
Thread-0 5
main 3
Thread-0 6
main 4
Thread-0 7
main 5
Thread-0 8
main 6
main 7
main 8
main 9
Thread-0 9
Thread-0 10
Thread-0 11
Thread-0 12
Thread-0 13
Thread-0 14
Thread-0 15
Thread-0 16
Thread-0 17
Thread-0 18
Thread-0 19
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Thread-0 23
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Thread-0 28
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Thread-0 37
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我们设置DaemonThread为后台线程和主线程一起运行，主线程只打印10次，而后台线程打印1000次，当主线程打印完成以后，后台线程就不运行了，所以没有打印完1000次就退出。