守护进程： d（isk） a（nd） e（xecution） mon（itor） or or from de（vice） mon（itor）

Daemon(计算)的定义:

处理打印假脱机和文件传输等服务请求的后台进程，在不需要时处于休眠状态。

——来源:牛津词典英文版

Java中的守护线程是什么?

Daemon threads can shut down any time in between their flow, Non-Daemon i.e. user thread executes completely. Daemon threads are threads that run intermittently in the background as long as other non-daemon threads are running. When all of the non-daemon threads complete, daemon threads terminates automatically. Daemon threads are service providers for user threads running in the same process. The JVM does not care about daemon threads to complete when in Running state, not even finally block also let execute. JVM do give preference to non-daemon threads that is created by us. Daemon threads acts as services in Windows. The JVM stops the daemon threads when all user threads (in contrast to the daemon threads) are terminated. Hence daemon threads can be used to implement, for example, a monitoring functionality as the thread is stopped by the JVM as soon as all user threads have stopped.

我想澄清一个误解:

Assume that if daemon thread (say B) is created within user thread (say A); then ending of this user thread/parent thread (A) will not end the daemon thread/child thread (B) it has created; provided user thread is the only one currently running. So there is no parent-child relationship on thread ending. All daemon threads (irrespective of where it is created) will end once there is no single live user thread and that causes JVM to terminate. Even this is true for both (parent/child) are daemon threads. If a child thread created from a daemon thread then that is also a daemon thread. This won't need any explicit daemon thread flag setting. Similarly if a child thread created from a user thread then that is also a user thread, if you want to change it, then explicit daemon flag setting is needed before start of that child thread.

守护进程线程通常被称为“服务提供者”线程。这些线程不应该用来执行程序代码，而应该用来执行系统代码。这些线程与您的代码并行运行，但JVM可以随时终止它们。当JVM没有发现用户线程时，它将停止用户线程，所有守护进程线程立即终止。我们可以使用以下方法将非守护线程设置为守护线程:

setDaemon(true)

守护线程是在进程的其他非守护线程仍在运行时在后台运行的线程。因此，当所有非守护进程线程完成时，守护进程线程将终止。非守护进程线程的一个例子是运行Main的线程。 通过在线程启动之前调用setDaemon()方法，将线程设置为守护进程

更多参考:Java中的守护进程线程

对我来说，守护线程就像用户线程的管家。 如果所有用户线程都已完成，守护进程线程就没有任务 JVM杀死。 我在YouTube视频里解释过了。

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