用户线程和守护线程

Stackoverflow中关于守护线程有如下比较好的解释：

Daemon threads are like assistants. Non-Daemon i.e. user threads are like front performers. Assistants help performers to complete a job. When the job is completed, no help is needed by performers to perform anymore. As no help is needed the assistants leave the place. So when the jobs of Non-Daemon threads is over, Daemon threads march away.

使用守护线程的目的

• 守护线程（Daemon Thread）是一个后台（background）线程，它用来辅助用户线程。
• 守护线程通常用来执行一些后台任务，比如说垃圾回收。

关于守护线程的一些重要点

• 守护线程通常都是低优先级执行
• 守护线程的生命周期依赖于用户线程
• 守护进程线程是在同一进程中运行的用户线程的服务提供者
• 如果非守护进程线程正在执行，Java VM就不会退出
• 如果只执行守护进程线程，Java VM将退出

使用守护线程

案例一

public class DaemonThread {

    public static void main(String[] args) throws InterruptedException {
        
        Thread t = new Thread() {

            @Override
            public void run() {
                try {
                    System.out.println(Thread.currentThread().getName() + " running");
                    Thread.sleep(10000);
                    System.out.println(Thread.currentThread().getName() + " done");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

        };

        t.setDaemon(true);
        // runnable -> running|dead|blocked
        t.start();

        Thread.sleep(5000);
        System.out.println(Thread.currentThread().getName());

    }
    
}

执行结果：main线程执行结束后，t线程就会停止。

案例二

public class DaemonThread1 {

    public static void main(String[] args)  {
        
        Thread t = new Thread() {

            @Override
            public void run() {

                Thread innerThread = new Thread() {

                    @Override
                    public void run() {
                        try {
                            while(true) {
                                System.out.println("Do something for health check.");
                                Thread.sleep(10000);
                            }
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }

                };

                innerThread.setDaemon(true);
                innerThread.start();

                try {
                    Thread.sleep(1000);
                    System.out.println("T thread finish done.");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

            }

        };

        // runnable -> running|dead|blocked
        t.start();
    }
    
}

执行结果：t线程停止后，innerThread就会停止。