守護線程的創建和運行

　　Java中有一種特殊的線程叫做守護（Daemon)線程。這種線程的優先級很低，通常來說，當同一個應用程序里沒有其他的線程運作的時候，守護線程才運行。當守護線程是程序中唯一的線程時，守護線程執行結束后，JVM也就結束了這個程序。一個典型的守護線程是Java的垃圾回收器（Garbage Collector）。

示例程序如下：

1.創建Event類

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. 創建WriterTask類

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. 創建CleanerTask類

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. 實現主類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);
        //創建三個客戶端進程
        for(int i=0;i<3;i++){
            Thread t = new Thread(wt);
            t.start();
        }
        //創建一個守護進程
        CleanerTask ct = new CleanerTask(deque);
        ct.start();
    }
}

 注：setDaemon()方法只能在start()方法被調用之前設置。一旦程序開始運行，將不能在修改守護狀態。