守护线程就像一个普通的线程，除了JVM只会在其他非守护线程不存在时关闭。守护进程线程通常用于为应用程序执行服务。

守护线程是在后台执行一些任务的线程，比如处理请求或应用程序中可能存在的各种chronjob。

当你的程序只剩下守护线程时，它将退出。这是因为这些线程通常与普通线程一起工作，并提供事件的后台处理。

你可以使用setDaemon方法指定一个线程是守护线程，它们通常不会退出，也不会被中断。当应用程序停止时，它们就会停止。

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

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
            }
        }
    }
}

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。

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。