Java中的守护线程是什么?

有人能告诉我Java中的守护线程是什么吗?

当前回答

以上答案都是好的。下面是一个简单的小代码片段，以说明两者的区别。在setDaemon中分别尝试true和false值。

public class DaemonTest {
    
    public static void main(String[] args) {
        new WorkerThread().start();

        try {
            Thread.sleep(7500);
        } catch (InterruptedException e) {
            // handle here exception
        }

        System.out.println("Main Thread ending") ;
    }

}

class WorkerThread extends Thread {
    
    public WorkerThread() {
        // When false, (i.e. when it's a non daemon thread),
        // the WorkerThread continues to run.
        // When true, (i.e. when it's a daemon thread),
        // the WorkerThread terminates when the main 
        // thread or/and user defined thread(non daemon) terminates.
        setDaemon(true); 
    }
    
    public void run() {
        int count = 0;

        while (true) {
            System.out.println("Hello from Worker "+count++);

            try {
                sleep(5000);
            } catch (InterruptedException e) {
                // handle exception here
            }
        }
    }
}

2012-03-09 00:54:21

其他回答

让我们只讨论工作示例中的代码。我喜欢russ上面的回答，但为了消除我的疑虑，我稍微加强了一下。我运行了两次，一次工作线程设置为deamon true (deamon线程)，另一次设置为false(用户线程)。它确认守护线程在主线程结束时结束。

public class DeamonThreadTest {

public static void main(String[] args) {

    new WorkerThread(false).start();    //set it to true and false and run twice.

    try {
        Thread.sleep(7500);
    } catch (InterruptedException e) {
        // handle here exception
    }

    System.out.println("Main Thread ending");
    }
   }

   class WorkerThread extends Thread {

    boolean isDeamon;

    public WorkerThread(boolean isDeamon) {
        // When false, (i.e. when it's a user thread),
        // the Worker thread continues to run.
        // When true, (i.e. when it's a daemon thread),
        // the Worker thread terminates when the main
        // thread terminates.
        this.isDeamon = isDeamon;
        setDaemon(isDeamon);
    }

    public void run() {
        System.out.println("I am a " + (isDeamon ? "Deamon Thread" : "User Thread (none-deamon)"));

        int counter = 0;

        while (counter < 10) {
            counter++;
            System.out.println("\tworking from Worker thread " + counter++);

            try {
                sleep(5000);
            } catch (InterruptedException e) {
                // handle exception here
            }
        }
        System.out.println("\tWorker thread ends. ");
    }
}



result when setDeamon(true)
=====================================
I am a Deamon Thread
    working from Worker thread 0
    working from Worker thread 1
Main Thread ending

Process finished with exit code 0


result when setDeamon(false)
=====================================
I am a User Thread (none-deamon)
    working from Worker thread 0
    working from Worker thread 1
Main Thread ending
    working from Worker thread 2
    working from Worker thread 3
    working from Worker thread 4
    working from Worker thread 5
    working from Worker thread 6
    working from Worker thread 7
    working from Worker thread 8
    working from Worker thread 9
    Worker thread ends. 

Process finished with exit code 0

2016-10-06 03:57:53

守护线程

在后台运行的线程称为守护线程。

Daemon线程示例:

垃圾收集器。信号分配器。

守护线程的目标:

守护进程线程的主要目标是为非守护进程线程提供支持。

有关守护线程的其他信息:

通常，守护线程在MIN_PRIORITY中运行，但是，也可以使用MAX_PRIORITY运行守护线程。示例:当需要额外的内存时，GC通常以MIN_PRIORITY优先级运行。JVM将GC的优先级从MIN_PRIORITY增加到MAX_PRIORITY。

一旦线程已经启动，就不能将其从守护线程更改为非守护线程，这会导致IllegalThreadStateException异常。例子: public static void main(String[] args) { Thread.currentThread () .setDaemon(真正的); ｝输出: java.lang.IllegalThreadStateException线程异常在java.base / java.lang.Thread.setDaemon (Thread.java: 1403)

If we are branching off a thread, the child thread inherits the nature of the parent thread. If the parent thread is a non-daemon thread automatically the child thread will be non-daemon as well and if the parent thread is a daemon, the child thread will be a daemon as well. class Scratch { public static void main(String[] args) { CustomThread customThread = new CustomThread(); customThread.start(); } } class CustomThread extends Thread{ @Override public void run() { System.out.println(currentThread().isDaemon()); } } Output: false

When the last non-daemon thread terminates, all the daemon threads get terminated automatically. class Scratch { public static void main(String[] args) { System.out.println("Main Thread Started."); CustomThread customThread = new CustomThread(); customThread.setDaemon(true); customThread.start(); System.out.println("Main Thread Finished."); } } class CustomThread extends Thread{ @Override public void run() { System.out.println("Custom Thread Started."); try { sleep(2000); } catch (InterruptedException ignore) {} System.out.println("Custom Thread Finished."); //Won't get executed. } } Output: Main Thread Started. Main Thread Finished. Custom Thread Started.

2022-09-14 12:10:57

守护线程是在程序完成但线程仍在运行时不阻止JVM退出的线程。守护进程线程的一个例子是垃圾收集。

您可以使用setDaemon(boolean)方法在线程启动之前更改线程守护进程属性。

2010-02-06 14:10:57

java线程中的用户线程与守护线程

守护进程的线程

这些线程在Java中是低优先级的线程，在后台运行以执行诸如垃圾收集之类的任务。Java中的守护线程也是一个服务提供者线程，它向用户线程提供服务。

用户线程

这些线程是高优先级线程。JVM将等待任何用户线程在终止它之前完成它的任务

“请记住用户和守护线程都包装在操作系统线程上”

最近OpenJdk在Loom项目中提出了虚拟线程(它们也是基于用户的)，你可以在这里找到更多关于Java虚拟机线程的纤维和延续。

2022-09-27 07:41:31