Java7并发编程实战（一） 守护线程的创建和运行

Java里有一种特殊的线程叫做守护（Daemon）线程，这种线程的优先级很低，通常来说，当一个应用程序里面没有其他线程运行的时候，守护线程才运行，当线程是程序中唯一运行的线程时，守护线程执行结束后，JVM也就结束了这个程序。因此，守护线程通常被用来作为同一程序中普通线程的服务提供者，通常是无线循环的，以等待服务请求或者线程任务。

　　代码实现

　　1：创建Event类，声明两个私有属性

　　

package com.packtpub.java7.concurrency.chapter1.recipe7.task;

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

import com.packtpub.java7.concurrency.chapter1.recipe7.event.Event;

/**
* Class that review the Event data structure and delete
* the events older than ten seconds
*
*/
public class CleanerTask extends Thread {

/**
* Data structure that stores events
*/
private Deque<Event> deque;

/**
* Constructor of the class
* @param deque data structure that stores events
*/
public CleanerTask(Deque<Event> deque) {
this.deque = deque;
// Establish that this is a Daemon Thread
setDaemon(true);
}

/**
* Main method of the class
*/
@Override
public void run() {
while (true) {
Date date = new Date();
clean(date);
}
}

/**
* Method that review the Events data structure and delete
* the events older than ten seconds
* @param date
*/
private void clean(Date date) {
long difference;
boolean delete;

if (deque.size()==0) {
return;
}

delete=false;
do {
Event e = deque.getLast();
difference = date.getTime() - e.getDate().getTime();
if (difference > 10000) {
System.out.printf("Cleaner: %s\n",e.getEvent());
deque.removeLast();
delete=true;
}
} while (difference > 10000);
if (delete){
System.out.printf("Cleaner: Size of the queue: %d\n",deque.size());
}
}
}

View Code
　　

　　4：实现主类

　　

public class Main {

/**
* Main method of the example. Creates three WriterTasks and a CleanerTask
* @param args
*/
public static void main(String[] args) {

// Creates the Event data structure
Deque<Event> deque=new ArrayDeque<Event>();

// Creates the three WriterTask and starts them
WriterTask writer=new WriterTask(deque);
for (int i=0; i<3; i++){
Thread thread=new Thread(writer);
thread.start();
}

// Creates a cleaner task and starts them
CleanerTask cleaner=new CleanerTask(deque);
cleaner.start();

}

}

　　打印结果

Cleaner: Size of the queue: 28
Cleaner: The thread 9 has generated an event
Cleaner: Size of the queue: 28
Cleaner: The thread 11 has generated an event
Cleaner: Size of the queue: 29
Cleaner: The thread 10 has generated an event
Cleaner: Size of the queue: 28
Cleaner: The thread 9 has generated an event
Cleaner: Size of the queue: 28
Cleaner: The thread 11 has generated an event
Cleaner: Size of the queue: 29
Cleaner: The thread 10 has generated an event
Cleaner: Size of the queue: 28
Cleaner: The thread 9 has generated an event
Cleaner: Size of the queue: 28
Cleaner: The thread 11 has generated an event
Cleaner: Size of the queue: 29
Cleaner: The thread 10 has generated an event
Cleaner: Size of the queue: 29
Cleaner: The thread 9 has generated an event
Cleaner: Size of the queue: 28

我们会发现，队列中的对象会不断增长至30个，然后程序结束，队列的长度维持在27-30之间，这个程序有3个WriteTask线程，每个线程向队列写入一个事件，然后休眠1秒钟，在第一个10s中，队列中有30个事件，直到3个WriterTask都结束后，CleanTask才开始执行，但是他没有删除任何事件，因为所有的事件都小于10秒钟，在接下来运行中，CleanTask每秒钟删除3个事件，同时WriteTask会写入3个对象，所以队列一直在27-30之间。