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 deque) {
this.deque = deque;
}

// 这个为双向队列
private Deque deque;

public Deque getDeque() {
return deque;
}

public void setDeque(Deque 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 deque;

public CleanerTask(Deque 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 getDeque() {
return deque;
}

public void setDeque(Deque 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 deque = new ArrayDeque();
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();
}
}

运行结果就是

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

这个队列总是维护在25-30左右，因为上来三个线程创建了30个事件，然后等创建事件的线程休息的时候，daemon线程就会占用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.