守护程序线程是低优先级背景线程，它为用户线程提供服务。
它的生命取决于用户线程。
如果没有用户线程正在运行，则即使守护程序线程正在运行，JVM也可以退出。
JVM不要等待守护程序线程完成。

守护程序线程执行后台任务，如垃圾收集，终结器等。

守护程序线程的唯一目的是为用户线程服务，因此如果没有用户线程，则没有JVM运行这些线程，这就是为什么JVM退出后没有用户线程。

两个与守护程序线程相关的方法

public void setdaemon(布尔状态)：此方法可用于将线程标记为用户或者守护程序线程。
如果我们将SetDaemon(true)放置，则将线程作为守护程序。

Public boolean isDaemon()此方法可用于检查线程是否是守护程序。

守护程序线程示例：

package org.arpit.theitroad;
 
 
class SimpleThread implements Runnable{
 
 public void run()
 {
  if(Thread.currentThread().isDaemon())
   System.out.println(Thread.currentThread().getName()+"  is daemon thread");
  else
   System.out.println(Thread.currentThread().getName()+"  is user thread");
 }
 
}
 
public class DaemonThreadMain {
 public static void main(String[] args){  
  SimpleThread st=new SimpleThread();
    Thread th1=new Thread(st,"Thread 1");//creating threads
    Thread th2=new Thread(st,"Thread 2");  
    Thread th3=new Thread(st,"Thread 3");  
    
    th2.setDaemon(true);//now th2 is daemon thread  
      
    th1.start();//starting all threads  
    th2.start();  
    th3.start();  
   }  
}

运行上面的程序时，我们将得到以下输出：

Thread 1  is user thread
Thread 3  is user thread
Thread 2  is daemon thread

Please note that you can not convert user thread to daemon thread once it is started otherwise it will throw IllegalThreadStateException.

package org.arpit.theitroad;
 
 
class SimpleThread implements Runnable{
 
 public void run()
 {
  if(Thread.currentThread().isDaemon())
   System.out.println(Thread.currentThread().getName()+"  is daemon thread");
  else
   System.out.println(Thread.currentThread().getName()+"  is user thread");
 }
 
}
 
public class DaemonThreadMain {
 public static void main(String[] args){  
  SimpleThread st=new SimpleThread();
    Thread th1=new Thread(st,"Thread 1");//creating threads
    Thread th2=new Thread(st,"Thread 2");  
    Thread th3=new Thread(st,"Thread 3");  
    
    
      
    th1.start();//starting all threads  
    th2.start();  
    th3.start();  
                  th2.setDaemon(true);//now converting user thread to daemon thread after starting the thread. 
   }  
}

运行上面的程序时，我们将得到以下输出：

Thread 1  is user threadException in thread “main" Thread 2  is user thread Thread 3  is user thread java.lang.IllegalThreadStateException at java.lang.Thread.setDaemon(Thread.java:1388) at org.arpit.theitroad.DaemonThreadMain.main(DaemonThreadMain.java:28)