JDK5.0新特性系列---11.5.2线程 同步装置之CountDownLatch

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
* CountDownLatch维护一个计数器,等待这个CountDownLatch的线程必须等到计数器为0时才可以继续.
* 以下实例模拟服务器的启动,假设启动一个服务器需要初始化3个组件,当3个组件初始化完毕后,服务器才算成功启动.
*/
/**
* 使用CountDownLatch的关键技术点如下:
* 1.构造CountDownLatch对象时,需要指定计数器的初始值,该值必须大于等于0,一旦对象被创建,其初始值将不能被改变.
* 2.CountDownLatch的await方法使当前线程进入等待状态,直到计数器为0
* 3.CountDownLatch的 和countDown方法使计数器减1.
*/
public class CountDownLatchTest {
/** 初始化组件的线程 */
public static class ComponentThread implements Runnable {
CountDownLatch latch; //计数器
int ID; //组件ID

//构造方法
public ComponentThread(CountDownLatch latch, int ID) {
this.latch = latch;
this.ID = ID;
}

public void run() {
//初始化组件
System.out.println("Initializing component " + ID);
try {
Thread.sleep(500 * ID);
}
catch (InterruptedException e) {}
System.out.println("Component " + ID + " initialized!");
latch.countDown(); //将计数器减1
}
}

/** 启动服务器 */
public static void startServer() throws Exception {
System.out.println("Server is starting.");
//初始化一个初始值为3的CountDownLatch
CountDownLatch latch = new CountDownLatch(3);
//启动3个线程分别去3个组件
ExecutorService service = Executors.newCachedThreadPool();
service.submit(new ComponentThread(latch, 1));
service.submit(new ComponentThread(latch, 2));
service.submit(new ComponentThread(latch, 3));
service.shutdown();
latch.await();//等待3个组件的初始化工作都完成
System.out.println("Server is up!");//当所需的三个组件都完成时,Server就可以继续了
}

public static void main(String... args) throws Exception {
CountDownLatchTest.startServer();
}
}