Java并发编程：CountDownLatch

在jdk5之后，提供了一些辅助类来帮助我们进行并发编程，CountDownLatch就是其中之一。看alibaba开源项目中用到CountDownLatch，决定总结一下

CountDownLatch类位于java.util.concurrent包下，利用它可以实现类似计数器的功能。比如一个任务要等待其他任务执行完毕后才能执行，此时就可以通过CountDownLatch来实现
CountDownLatch提供了一个构造器：

public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
    }

有以下几个重要的方法

public void await() throws InterruptedException { };   //调用await()方法的线程会被挂起，它会等待直到count值为0才继续执行
public boolean await(long timeout, TimeUnit unit) throws InterruptedException { };  //和await()类似，只不过等待一定的时间后count值还没变为0的话就会继续执行
public void countDown() { };  //将count值减1

看下面代码示例

public class CountDownLatchDemo {

public static void main(String[] args) throws InterruptedException {
        CountDownLatchDemo demo = new CountDownLatchDemo();
        demo.testCountDownLatch();
    }

public void testCountDownLatch() throws InterruptedException {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(10,50,10, TimeUnit.SECONDS,new LinkedBlockingQueue());
int threadCount = 5;
        CountDownLatch start = new CountDownLatch(1);
        CountDownLatch end = new CountDownLatch(threadCount);
for (int i=0;i<threadCount;i++){
            threadPoolExecutor.execute(new Runnable() {
                @Override
public void run() {
try {
                        start.await();
                        Thread.sleep(3000);
                        System.out.println("执行子线程" + Thread.currentThread().getName()+"");
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    end.countDown();        //将count值减1
                }
            });
        }
        threadPoolExecutor.shutdown();
        System.out.println("执行主线程-1...");
long begin = System.currentTimeMillis();
        start.countDown();      //执行此语句之前 所有的子线程挂起等待 之后子线程开始执行
        System.out.println("执行主线程-2...");
        end.await();            //所有的子线程执行完之前 挂起主线程
        System.out.println("执行主线程-3...");
        System.out.println("执行时间："+ (System.currentTimeMillis()-begin)+"ms");
    }
}

执行结果如下

执行主线程-1...
执行主线程-2...
执行子线程pool-1-thread-3
执行子线程pool-1-thread-5
执行子线程pool-1-thread-1
执行子线程pool-1-thread-2
执行子线程pool-1-thread-4
执行主线程-3...
执行时间：3000ms