一 点睛

针对守护线程，只要有一个用户线程在执行，这个进程就不会结束。

当线程中只剩下守护线程时，JVM会自动退出，反之，如果还有其他任何用户线程存在，JVM都不会退出。

二 代码

public class ThreadDaemon{  public static void main( String args[] )  {   ThreadTest t = new ThreadTest();   Thread tt = new Thread( t );   tt.setDaemon( true ); // 设置后台运行   tt.start( );   try   { // 睡眠1毫秒，避免可能出现的没有输出的现象     Thread.sleep( 1 );   }   catch( InterruptedException e )   {     e.printStackTrace();   }  }}class ThreadTest implements Runnable{  public void run()  {   for( int i = 0; true; ++i )   {     System.out.println( i + " " + Thread.currentThread().getName()        + " is running." );   }  }}

三 运行

0 Thread-0 is running.1 Thread-0 is running.2 Thread-0 is running.3 Thread-0 is running.4 Thread-0 is running.5 Thread-0 is running.6 Thread-0 is running.7 Thread-0 is running.8 Thread-0 is running.9 Thread-0 is running.10 Thread-0 is running.11 Thread-0 is running.12 Thread-0 is running.13 Thread-0 is running.14 Thread-0 is running.15 Thread-0 is running.16 Thread-0 is running.17 Thread-0 is running.18 Thread-0 is running.19 Thread-0 is running.20 Thread-0 is running.21 Thread-0 is running.22 Thread-0 is running.23 Thread-0 is running.24 Thread-0 is running.25 Thread-0 is running.26 Thread-0 is running.27 Thread-0 is running.28 Thread-0 is running.