守护线程的创建和运行

　　Java中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，当同一个应用程序里没有其他的线程运作的时候，守护线程才运行。当守护线程是程序中唯一的线程时，守护线程执行结束后，JVM也就结束了这个程序。一个典型的守护线程是Java的垃圾回收器（Garbage Collector）。

示例程序如下：

1.创建Event类

import java.util.Date;

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;
    }

}

2. 创建WriterTask类

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

public class WriterTask implements Runnable {
    private Deque<Event> deque;
    public WriterTask(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(String.format(
                    "The thread %s has been generated an event", Thread
                            .currentThread().getId()));
            deque.addFirst(event);
            // System.out.println("event: "+event.getDate());
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

    }

}

3. 创建CleanerTask类

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

public class CleanerTask extends Thread {
    private Deque<Event> deque;
    public CleanerTask(Deque<Event> deque){
        this.deque = deque;
        this.setDaemon(true);
    }
    @Override
    public void run() {
        while(true){
            Date date = new Date();
            clean(date);
        }
    }

    private void clean(Date date){
        long diff;
        boolean delete;
        if(deque.isEmpty())
            return;
        delete = false;
        do{
            Event e = deque.getLast();
            diff = date.getTime()-e.getDate().getTime();
            if(diff>10000){
                System.out.printf("Cleaner: %s\n", e.getEvent());
                deque.removeLast();
                delete = true;
            }
        }while(diff>10000&&!deque.isEmpty());
        if(delete){
            System.out.printf("Cleaner: Size of the queue: %d\n", deque.size());
        }
    }
}

4. 实现主类Main

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

public class Main {

    public static void main(String[] args) {
        Deque<Event> deque = new ArrayDeque<Event>();
        WriterTask wt = new WriterTask(deque);
        //创建三个客户端进程
        for(int i=0;i<3;i++){
            Thread t = new Thread(wt);
            t.start();
        }
        //创建一个守护进程
        CleanerTask ct = new CleanerTask(deque);
        ct.start();
    }
}

注：setDaemon()方法只能在start()方法被调用之前设置。一旦程序开始运行，将不能在修改守护状态。