第五章 java中的守护线程与示例

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守护线程是为用户线程提供服务的低优先级后台线程。它的生命取决于用户线程。如果没有用户线程正在运行，那么即使守护线程正在运行，JVM 也可以退出。JVM 不等待守护线程完成。

守护线程执行后台任务，例如垃圾收集、终结器等。

守护线程的唯一目的是为用户线程服务，所以如果没有用户线程，JVM 就没有必要运行这些线程，这就是为什么一旦没有用户线程，JVM 就会退出。

与守护线程相关的两种方法

Public void setDaemon(boolean status) ： 此方法可用于将线程标记为用户或守护线程。如果你把 setDaemon(true)，它使线程作为守护进程。

public boolean isDaemon() 这个方法可以用来检查线程是否是守护进程。

守护线程示例：

package org.arpit.java2blog;

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();  
   }  
}

当你运行上面的程序时，你会得到下面的输出：

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

请注意，一旦启动，就不能将用户线程转换为守护线程，否则会抛出IllegalThreadStateException。

package org.arpit.java2blog;

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.
   }  
}

当你运行上面的程序时，你会得到下面的输出：

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.java2blog.DaemonThreadMain.main(DaemonThreadMain.java:28)

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原文链接：https://codingdict.com/

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