Daemon thread is like daemon process which is responsible for managing resources,a daemon thread is created by the Java VM to serve the user threads. example updating system for unix,unix is daemon process. child of daemon thread is always daemon thread,so by default daemon is false.you can check thread as daemon or user by using "isDaemon()" method. so daemon thread or daemon process are basically responsible for managing resources. for example when you starting jvm there is garbage collector running that is daemon thread whose priority is 1 that is lowest,which is managing memory. jvm is alive as long as user thread is alive,u can not kill daemon thread.jvm is responsible to kill daemon threads.

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。

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)

答案已经有很多了;然而，也许我可以更清楚地解释这一点，因为当我阅读Daemon Threads时，最初，我有一种感觉，我很好地理解了它;然而，在玩了它并调试了一下之后，我发现了一个奇怪的行为。

我被教导:

如果我想让线程在主线程有序完成执行后立即死亡，我应该将其设置为Diamond。

我尝试了什么:

我从主线程中创建了两个线程，我只将其中一个设置为菱形; 在主线程有序完成执行后，那些新创建的线程都没有退出，但我预计，守护线程应该已经退出; 我浏览了许多博客和文章，到目前为止，我找到的最好、最清晰的定义来自《Java并发实践》一书，它非常清楚地指出:

7.4.2守护线程

Sometimes you want to create a thread that performs some helper function but you don’t want the existence of this thread to prevent the JVM from shutting down. This is what daemon threads are for. Threads are divided into two types: normal threads and daemon threads. When the JVM starts up, all the threads it creates (such as garbage collector and other housekeeping threads) are daemon threads, except the main thread. When a new thread is created, it inherits the daemon status of the thread that created it, so by default any threads created by the main thread are also normal threads. Normal threads and daemon threads differ only in what happens when they exit. When a thread exits, the JVM performs an inventory of running threads, and if the only threads that are left are daemon threads, it initiates an orderly shutdown. When the JVM halts, any remaining daemon threads are abandoned— finally blocks are not executed, stacks are not unwound—the JVM just exits. Daemon threads should be used sparingly—few processing activities can be safely abandoned at any time with no cleanup. In particular, it is dangerous to use daemon threads for tasks that might perform any sort of I/O. Daemon threads are best saved for “housekeeping” tasks, such as a background thread that periodically removes expired entries from an in-memory cache.

Daemon threads are those threads which provide general services for user threads (Example : clean up services - garbage collector) Daemon threads are running all the time until kill by the JVM Daemon Threads are treated differently than User Thread when JVM terminates , finally blocks are not called JVM just exits JVM doesn't terminates unless all the user threads terminate. JVM terminates if all user threads are dies JVM doesn't wait for any daemon thread to finish before existing and finally blocks are not called If all user threads dies JVM kills all the daemon threads before stops When all user threads have terminated, daemon threads can also be terminated and the main program terminates setDaemon() method must be called before the thread's start() method is invoked Once a thread has started executing its daemon status cannot be changed To determine if a thread is a daemon thread, use the accessor method isDaemon()

下面是一个示例，用于测试在jvm由于不存在用户线程而退出时守护线程的行为。

请注意下面输出的倒数第二行，当主线程退出时，守护线程也死了，并且没有在finally块中打印finally executed9语句。这意味着如果由于不存在用户线程而导致JVM退出，守护线程finally块内关闭的任何i/o资源都不会被关闭。

public class DeamonTreadExample {

public static void main(String[] args) throws InterruptedException {

    Thread t = new Thread(() -> {
        int count = 0;
        while (true) {
            count++;
            try {
                System.out.println("inside try"+ count);
                Thread.currentThread().sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } finally {
                System.out.println("finally executed"+ count);
            }
        }
    });
    t.setDaemon(true);
    t.start();

    Thread.currentThread().sleep(10000);
    System.out.println("main thread exited");
  }
}

输出

inside try1
finally executed1
inside try2
finally executed2
inside try3
finally executed3
inside try4
finally executed4
inside try5
finally executed5
inside try6
finally executed6
inside try7
finally executed7
inside try8
finally executed8
inside try9
finally executed9
inside try10
main thread exited