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.

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

守护线程和用户线程。通常程序员创建的所有线程都是用户线程(除非你指定它为守护线程或者你的父线程是守护线程)。用户线程通常用于运行我们的程序代码。除非所有用户线程都终止，否则JVM不会终止。

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。

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.