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();
  }
}