主线程、子线程、守护线程

 import static java.util.concurrent.TimeUnit.*;
 public class DaemonTest {
     public static void main(String[] args) throws InterruptedException {
         Runnable r = new Runnable() {
             public void run() {
                 for (int time = 10; time > 0; --time) {
                     System.out.println("Time #" + time);
                     try {
                         SECONDS.sleep(2);
                     } catch (InterruptedException e) {
                         e.printStackTrace();
                     }
                 }
             }
         };

         Thread t = new Thread(r);
         t.setDaemon(true);  // try to set this to "false" and see what happens
         t.start();

         System.out.println("Main thread waiting...");
         SECONDS.sleep(6);
         System.out.println("Main thread exited.");
     }
 }

当t.setDaemon(true)，即t为Daemon线程时，执行结果如下：

t为Daemon线程的输出： 
Time #10

Time #9

Time #8

Main thread exited.

Time #7

当t.setDaemon(false)，即t为非守护线程时，执行结果如下：

Main thread waiting...

Time #10

Time #9

Time #8

Main thread exited.

Time #7

Time #6

Time #5

Time #4

Time #3

Time #2

Time #1

总结： 

1、当主线程一结束、（非守护线程的）子线程不会立即结束。

2、当主线程一结束，（守护线程的）子线程会立即结束。

3、子线程对象，在调用start()方法之前，通过调用setDaemon(true),将本子线程设置为守护线程。