守护进程，顾名思义就是默默在后台运行的进程，当JVM上没有其他进程运行时运行的进程，最典型的一个列子就是

JVM自带的垃圾清理机，他就是在其他程序不运行的时候占用CPU来进行清理内存垃圾，因此他具有很强的不确定性，

因为你不知道啥时候他就会运行，也不知道啥时候他不会运行，因此适合做一个不太重要的清理工作或者服务器上的监

听工作。

下面我们使用JVM的daemon来写一个小例子，实现的效果如下

In this recipe, we will learn how to create a daemon thread developing an example with two threads; one user thread that writes events on a queue and a daemon one that cleans that queue, removing the events which were generated more than 10 seconds ago。

这里的队列是一个双向列表。代码如下

package com.bird.concursey;

import java.util.Date;
import java.util.Deque;
import java.util.concurrent.TimeUnit;

public class WriterTask implements Runnable {

	public WriterTask(Deque<Event> deque) {
		this.deque = deque;
	}

	// 这个为双向队列
	private Deque<Event> deque;

	public Deque<Event> getDeque() {
		return deque;
	}

	public void setDeque(Deque<Event> deque) {
		this.deque = deque;
	}

	@Override
	public void run() {
		for(int i = 0; i < 100; i++) {
			Event event = new Event();
			event.setDate(new Date());
			event.setEvent("The Thread " + Thread.currentThread().getId() + " has generated a event");
			deque.addFirst(event);
			try {
				TimeUnit.SECONDS.sleep(1);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

}

package com.bird.concursey;

import java.util.Date;
import java.util.Deque;

public class CleanerTask extends Thread {

	private Deque<Event> deque;

	public CleanerTask(Deque<Event> deque) {
		this.deque = deque;
		//设置为守护进程
		setDaemon(true);
	}
	
	@Override
	public void run() {
		while(true) {
			Date date = new Date();
			clean(date);
		}
	}

	private void clean(Date date) {
		long difference = 0;
		boolean delete = false;
		
		if(deque.size() == 0) {
			return;
		}
		
		do {
			Event e = deque.getLast();
			difference = date.getTime() - e.getDate().getTime();
			if(difference > 10000) {
				System.out.println("cleaner " + e.getEvent());
				deque.removeLast();
				delete = true;
			}
		}while(difference > 10000);
		
		if(delete) {
			System.out.println("cleaner : the size of the deque " + deque.size());
		}
	}

	public Deque<Event> getDeque() {
		return deque;
	}

	public void setDeque(Deque<Event> deque) {
		this.deque = deque;
	}
	
	
}

package com.bird.concursey;

import java.util.ArrayDeque;
import java.util.Date;
import java.util.Deque;

public class Event {

	private Date date;
	private String event;

	public Date getDate() {
		return date;
	}

	public void setDate(Date date) {
		this.date = date;
	}

	public String getEvent() {
		return event;
	}

	public void setEvent(String event) {
		this.event = event;
	}
	
	
	public static void main(String[] args) {
		Deque<Event> deque = new ArrayDeque<Event>();
		WriterTask writer = new WriterTask(deque);
		for(int i = 0; i < 3; i++) {
			Thread thread = new Thread(writer);
			thread.start();
		}
		CleanerTask cleaner = new CleanerTask(deque);
		cleaner.start();
	}
}

cleaner The Thread 12 has generated a event
cleaner The Thread 11 has generated a event
cleaner : the size of the deque 27
cleaner The Thread 13 has generated a event
cleaner : the size of the deque 26
cleaner The Thread 13 has generated a event
cleaner The Thread 12 has generated a event
cleaner The Thread 11 has generated a event
cleaner : the size of the deque 26
cleaner The Thread 12 has generated a e

程就会占用CPU time 来运行清理建立好的事件，这样就会让队列一直维护在25-30左右，最关键的一句

//设置为守护进程 setDaemon(true);必须在start之前设置，因为线程一旦运行起来了就不能更改他的状态，这样就开

始了监控的状态，守护进程就OK了

Java 多线程之-----守护进程