11.5.2线程 同步装置之CountDownLatch

importjava.util.concurrent.CountDownLatch;
importjava.util.concurrent.ExecutorService;
importjava.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.
*/
publicclassCountDownLatchTest{
/**初始化组件的线程*/
publicstaticclassComponentThreadimplementsRunnable{
CountDownLatchlatch;//计数器
intID;//组件ID

//构造方法
publicComponentThread(CountDownLatchlatch,intID){
this.latch=latch;
this.ID=ID;
}

publicvoidrun(){
//初始化组件
System.out.println("Initializingcomponent"+ID);
try{
Thread.sleep(500*ID);
}
catch(InterruptedExceptione){}
System.out.println("Component"+ID+"initialized!");
latch.countDown();//将计数器减1
}
}

/**启动服务器*/
publicstaticvoidstartServer()throwsException{
System.out.println("Serverisstarting.");
//初始化一个初始值为3的CountDownLatch
CountDownLatchlatch=newCountDownLatch(3);
//启动3个线程分别去3个组件
ExecutorServiceservice=Executors.newCachedThreadPool();
service.submit(newComponentThread(latch,1));
service.submit(newComponentThread(latch,2));
service.submit(newComponentThread(latch,3));
service.shutdown();
latch.await();//等待3个组件的初始化工作都完成
System.out.println("Serverisup!");//当所需的三个组件都完成时,Server就可以继续了
}

publicstaticvoidmain(String...args)throwsException{
CountDownLatchTest.startServer();
}
}