线程同步工具之CountDownLatch示例

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CountDownLatch的一个非常典型的应用场景是：有一个任务想要往下执行，但必须要等到其他的任务执行完毕后才可以继续往下执行。假如我们这个想要继续往下执行的任务调用一个CountDownLatch对象的await()方法，其他的任务执行完自己的任务后调用同一个CountDownLatch对象上的countDown()方法，这个调用await()方法的任务将一直阻塞等待，直到这个CountDownLatch对象的计数值减到0为止。

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package chapter3;
import java.util.concurrent.CountDownLatch;
public class Videoconference implements Runnable{
private final CountDownLatch controller;
public Videoconference(int number){
controller = new CountDownLatch(number);
}
public void arrive(String name){
System.out.println(name+” has arrived.”);
controller.countDown();
System.out.println(”VideoConference:Waiting for ”+controller.getCount());
}
@Override
public void run() {
System.out.println(”VideoConference:Initialization:”+controller.getCount());
try {
controller.await();
System.out.printf(”VideoConference: All the participants have come\n”);
System.out.printf(”VideoConference: Let’s start…\n”);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

package chapter3;import java.util.concurrent.CountDownLatch;public class Videoconference implements Runnable{    private final CountDownLatch controller;    public Videoconference(int number){        controller = new CountDownLatch(number);    }    public void arrive(String name){        System.out.println(name+" has arrived.");        controller.countDown();        System.out.println("VideoConference:Waiting for "+controller.getCount());    }    @Override    public void run() {        System.out.println("VideoConference:Initialization:"+controller.getCount());        try {            controller.await();            System.out.printf("VideoConference: All the participants have come\n");            System.out.printf("VideoConference: Let's start...\n");        } catch (InterruptedException e) {            e.printStackTrace();        }    }}

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package chapter3;
import java.util.concurrent.TimeUnit;
public class Participant implements Runnable{
private Videoconference conference;
private String name;
public Participant(Videoconference conference,String name){
this.conference = conference;
this.name = name;
}
@Override
public void run() {
long duration = (long)(Math.random()*10);
try {
TimeUnit.SECONDS.sleep(duration);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
conference.arrive(name);
}
}

package chapter3;import java.util.concurrent.TimeUnit;public class Participant implements Runnable{    private Videoconference conference;    private String name;    public Participant(Videoconference conference,String name){        this.conference = conference;        this.name = name;    }    @Override    public void run() {        long duration = (long)(Math.random()*10);        try {            TimeUnit.SECONDS.sleep(duration);        } catch (InterruptedException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }        conference.arrive(name);    }}

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package chapter3;
/**
*
* <p>
* Description: CountDownLatch的学习
* </p>
* @author zhangjunshuai
* @version 1.0
* Create Date: 2014-9-25 下午8:11:55
* Project Name: Java7Thread
*
* <pre>
* Modification History:
* Date Author Version Description
* ———————————————————————————————————–
* LastChange: Date::              Author:  Rev: 
* </pre>
*
*/
public class Main2 {
/**
* <p>
* </p>
* @author zhangjunshuai
* @date 2014-9-25 下午8:11:50
* @param args
*/
public static void main(String[] args) {
Videoconference conference = new Videoconference(9);
Thread threadConference = new Thread(conference);
threadConference.start();
for(int i=0;i<10;i++){
Participant p = new Participant(conference, “Participant”+i);
Thread t = new Thread(p);
t.start();
}
}
}

package chapter3;/** *  * <p> * Description: CountDownLatch的学习 * </p> * @author zhangjunshuai * @version 1.0 * Create Date: 2014-9-25 下午8:11:55 * Project Name: Java7Thread * * <pre> * Modification History:   *             Date                                Author                   Version          Description  * -----------------------------------------------------------------------------------------------------------   * LastChange: $Date::             $      $Author: $          $Rev: $          * </pre> * */public class Main2 {    /**     * <p>     * </p>     * @author zhangjunshuai     * @date 2014-9-25 下午8:11:50     * @param args     */    public static void main(String[] args) {        Videoconference conference = new Videoconference(9);        Thread threadConference = new Thread(conference);        threadConference.start();        for(int i=0;i<10;i++){            Participant p = new Participant(conference, "Participant"+i);            Thread t = new Thread(p);            t.start();        }    }}

CountDownLatch类有3个基本元素：

初始值决定CountDownLatch类需要等待的事件的数量。
await() 方法, 被等待全部事件终结的线程调用。
countDown() 方法，事件在结束执行后调用。

当创建 CountDownLatch 对象时，对象使用构造函数的参数来初始化内部计数器。每次调用 countDown() 方法, CountDownLatch 对象内部计数器减一。当内部计数器达到0时， CountDownLatch 对象唤醒全部使用 await() 方法睡眠的线程们。

不可能重新初始化或者修改CountDownLatch对象的内部计数器的值。一旦计数器的值初始后，唯一可以修改它的方法就是之前用的 countDown() 方法。当计数器到达0时，全部调用 await() 方法会立刻返回，接下来任何countDown() 方法的调用都将不会造成任何影响。

此方法与其他同步方法有这些不同：

CountDownLatch 机制不是用来保护共享资源或者临界区。它是用来同步一个或者多个执行多个任务的线程。它只能使用一次。像之前解说的，一旦CountDownLatch的计数器到达0，任何对它的方法的调用都是无效的。如果你想再次同步，你必须创建新的对象。

CountDownLatch 类有另一种版本的 await() 方法，它是：

await(long time, TimeUnit unit): 此方法会休眠直到被中断； CountDownLatch 内部计数器到达0或者特定的时间过去了。TimeUnit 类包含了:DAYS, HOURS, MICROSECONDS, MILLISECONDS, MINUTES, NANOSECONDS, 和 SECONDS.

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