Java并发学习之七——守护线程

1、Java有两种Thread：“守护线程Daemon”与“用户线程User”。用户线程：Java虚拟机在它所有非守护线程已经离开后自动离开；守护线程：则是用来服务用户线程的，如果没有其他用户线程在运行，那么就没有可服务对象，也就没有理由继续下去。

2、setDaemon（boolean on）方法可以方便的设置线程的Daemon模式，true为Daemon模式，此方法必须在线程启动之前调用，当线程正在运行时调用会产生异常。

3、当一个守护线程中产生了其他线程，那么这些新产生的线程不用设置Daemon属性，都将是守护线程。

4、Java垃圾回收线程就是一个典型的守护线程，当我们的程序中不在有任何运行中的Thread，程序就不会再产生垃圾，垃圾回收器也就无事可做，所以当垃圾回收线程是Java虚拟机上仅剩的线程时，Java虚拟机会自动离开。

package chapter;  
  
import java.util.ArrayDeque;  
import java.util.Deque;  
  
  
  
public class Main7 {  
  
    /** 
     * <p> 
     * </p> 
     * @author zhangjunshuai 
     * @date 2014-8-18 上午11:21:30 
     * @param args 
     */  
    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();  
    }  
  
}  

package chapter;  
  
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 generated an event ", Thread.currentThread().getId()));  
            deque.addFirst(event);  
              
            try {  
                TimeUnit.SECONDS.sleep(1);  
            } catch (InterruptedException e) {  
                e.printStackTrace();  
            }  
        }  
          
    }  
  
} 

package chapter;  
  
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;  
        setDaemon(true);  
    }  
      
    @Override  
    public void run() {  
        while(true){  
            Date date = new Date();  
            clean(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());  
        }  
    }  
}