官网API解释

setDaemon

public final void setDaemon(boolean on)

Marks this thread as either a daemon thread or a user thread. The Java Virtual Machine exits when the only threads running are all daemon threads.

This method must be invoked before the thread is started.

Parameters:

on - if true, marks this thread as a daemon thread

Throws:

IllegalThreadStateException - if this thread is alive

SecurityException - if checkAccess() determines that the current thread cannot modify this thread

setDaemon设置线程为守护线程，必须放到start()方法之前，目的是当前所有运行中的线程为守护线程时该线程结束

使用守护线程模拟心跳实验

package com.dwz.concurrency.chapter4;
/**
 *     使用守护线程模拟心跳实验
 */
public class DaemonThread2 {
    public static void main(String[] args) {
        Thread t = new Thread(() -> {
            Thread innerThread = new Thread(() -> {
                try {
                    while(true) {
                        System.out.println("Do some thing for health check.");
                        Thread.sleep(1_000);
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }); 

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

        try {
            Thread.sleep(3_000);
            System.out.println("T Thread finish done.");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

原文地址：https://www.cnblogs.com/zheaven/p/12034273.html