Java中的守护线程是指运行在后台的线程，主要由JVM创建，用于执行后台任务，如垃圾收集和其他家政任务。

注意事项:

Any thread created by main thread, which runs main method in Java is by default non daemon because Thread inherits its daemon nature from the Thread which creates it i.e. parent Thread and since main thread is a non daemon thread, any other thread created from it will remain non-daemon until explicitly made daemon by calling setDaemon(true). Thread.setDaemon(true) makes a Thread daemon but it can only be called before starting Thread in Java. It will throw IllegalThreadStateException if corresponding Thread is already started and running.

Java中守护线程与非守护线程的区别:

1) JVM在存在之前不会等待任何守护线程完成。

2)当JVM终止时，守护线程与用户线程被区别对待，最终块不被调用，堆栈不受伤，JVM只是退出。

Java守护线程

(守护进程)

Java使用用户线程和守护进程线程概念。

JVM流

1. If there are no `user treads` JVM starts terminating the program
2. JVM terminates all `daemon threads` automatically without waiting when they are done
3. JVM is shutdown

正如你所看到的，守护线程是用户线程的服务线程。

守护线程是低优先级线程。 线程从父线程继承它的属性。要从外部设置它，你可以在启动它之前使用setDaemon()方法或通过isDaemon()检查它

Java中的守护线程是指运行在后台的线程，主要由JVM创建，用于执行后台任务，如垃圾收集和其他家政任务。

注意事项:

Any thread created by main thread, which runs main method in Java is by default non daemon because Thread inherits its daemon nature from the Thread which creates it i.e. parent Thread and since main thread is a non daemon thread, any other thread created from it will remain non-daemon until explicitly made daemon by calling setDaemon(true). Thread.setDaemon(true) makes a Thread daemon but it can only be called before starting Thread in Java. It will throw IllegalThreadStateException if corresponding Thread is already started and running.

Java中守护线程与非守护线程的区别:

1) JVM在存在之前不会等待任何守护线程完成。

2)当JVM终止时，守护线程与用户线程被区别对待，最终块不被调用，堆栈不受伤，JVM只是退出。

Java有一种特殊的线程，称为守护线程。

非常低的优先级。 仅在同一程序没有其他线程正在运行时执行。 当守护线程结束时，JVM结束程序，完成这些线程 程序中运行的唯一线程。

守护线程用于什么?

通常用作普通线程的服务提供者。 通常有一个等待服务请求或执行线程任务的无限循环。 他们不能做重要的工作。(因为我们不知道它们什么时候会有CPU时间，如果没有任何其他线程在运行，它们可以在任何时间完成。)

这类线程的一个典型示例是Java垃圾收集器。

有更多的…

在调用start()方法之前，只能调用setDaemon()方法。线程运行后，就不能修改它的守护进程状态。 使用isDaemon()方法检查线程是守护线程还是用户线程。

守护线程类似于助手线程。非daemon线程就像前面的表演者。助手帮助表演者完成一项工作。工作完成后，执行者不再需要帮助来执行。由于不需要帮助，助手们离开了这个地方。因此，当非守护线程的任务结束时，守护线程就会离开。

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