Java中CountDownLatch用法解析
CountDownLatch类是一个同步计数器,构造时传入int参数,该参数就是计数器的初始值，每调用一次countDown()方法，计数器减1,计数器大于0时，await()方法会阻塞程序继续执行
CountDownLatch如其所写，是一个倒计数的锁存器，当计数减至0时触发特定的事件。利用这种特性，可以让主线程等待子线程的结束。下面以一个模拟运动员比赛的例子加以说明。
importjava.util.concurrent.CountDownLatch;
importjava.util.concurrent.Executor;
importjava.util.concurrent.ExecutorService;
importjava.util.concurrent.Executors;
publicclassCountDownLatchDemo{
privatestaticfinalintPLAYER_AMOUNT=5;
publicCountDownLatchDemo(){
//TODOAuto-generatedconstructorstub
}
/**
*@paramargs
*/
publicstaticvoidmain(String[]args){
//TODOAuto-generatedmethodstub
//对于每位运动员，CountDownLatch减1后即结束比赛
CountDownLatchbegin=newCountDownLatch(1);
//对于整个比赛，所有运动员结束后才算结束
CountDownLatchend=newCountDownLatch(PLAYER_AMOUNT);
Player[]plays=newPlayer[PLAYER_AMOUNT];
for(inti=0;i<PLAYER_AMOUNT;i++)
plays[i]=newPlayer(i+1,begin,end);
//设置特定的线程池，大小为5
ExecutorServiceexe=Executors.newFixedThreadPool(PLAYER_AMOUNT);
for(Playerp:plays)
exe.execute(p);//分配线程
System.out.println("Racebegins!");
begin.countDown();
try{
end.await();//等待end状态变为0，即为比赛结束
}catch(InterruptedExceptione){
//TODO:handleexception
e.printStackTrace();
}finally{
System.out.println("Raceends!");
}
exe.shutdown();
}
}
接下来是Player类
importjava.util.concurrent.CountDownLatch;
publicclassPlayerimplementsRunnable{
privateintid;
privateCountDownLatchbegin;
privateCountDownLatchend;
publicPlayer(inti,CountDownLatchbegin,CountDownLatchend){
//TODOAuto-generatedconstructorstub
super();
this.id=i;
this.begin=begin;
this.end=end;
}
@Override
publicvoidrun(){
//TODOAuto-generatedmethodstub
try{
begin.await();//等待begin的状态为0
Thread.sleep((long)(Math.random()*100));//随机分配时间，即运动员完成时间
System.out.println("Play"+id+"arrived.");
}catch(InterruptedExceptione){
//TODO:handleexception
e.printStackTrace();
}finally{
end.countDown();//使end状态减1，最终减至0
}
}
}
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