11.5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();

              }

}