CountDownLatch

CountDownLatch允许一个或多个线程等待其他线程完成操作。

CountDownLatch的构造函数接收一个int类型的参数作为计数器，如果你想等待n个节点完成，就传入n。

注意：CountDownLatch不可能重新初始化或者修改CountDownLatch对象的内部的计数器的值

简单示例：

class CountDownLatchTest{
    static CountDownLatch c = new CountDownLatch(2);
    public static void main(String[] args) throws InterruptedException{
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(1);
                c.countDown();
                System.out.println(2);
                c.countDown();//计数器减一
            }
        }).start();
        c.await();////只要计数器为0，那么线程就可以结束阻塞往下执行,进而保证线程同时开始
        System.out.println("3");
    }
}

复杂示例：

/*
测试，客户端模拟ConnectionRunner获取、使用、释放连接的过程
 */
class ConnectionPoolTest{
    static ConnectionPool pool = new ConnectionPool(10);
    //保证所有的ConnectionRunner能同时开始
    static CountDownLatch start = new CountDownLatch(1);
    //main线程将会等待所有ConnectionRunner结束后才能继续执行
    static CountDownLatch end;
    public static void main(String[] args) throws Exception{
        //线程数量，可以修改线程数量进行观察
        int threadCount = 20;
        end = new CountDownLatch(threadCount);
        int count = 20;
        AtomicInteger got = new AtomicInteger();
        AtomicInteger notGot = new AtomicInteger();
        for (int i = 0; i < threadCount; i++){
            Thread thread = new Thread(new ConnectionRunner(count, got, notGot), "ConnectionRunnerThread");
            thread.start();
        }
        start.countDown();//只要计数器为0，那么线程就可以结束阻塞往下执行,进而保证线程同时开始
        end.await();
        System.out.println("total invoke: " + (threadCount * count));
        System.out.println("got connection: " + got);
        System.out.println("not got connection: " + notGot);
    }

    static class ConnectionRunner implements Runnable{
        int count;
        AtomicInteger got;
        AtomicInteger notGot;
        public ConnectionRunner(int count, AtomicInteger got, AtomicInteger notGot){
            this.count = count;
            this.got = got;
            this.notGot = notGot;
        }
        @Override
        public void run(){
            try{
                start.await();
            }catch (Exception ex){
            }
            while (count > 0){
                try {
                    //从线程池中获取连接，如果1000ms内无法获取到，将会返回null
                    //分别统计连接获取的数量got和未获取到的数量notGot
                    Connection connection = pool.fetchConnection(1000);
                    if (connection != null){
                        try {
                            connection.createStatement();//创建一个 Statement 对象来将 SQL 语句发送到数据库
                            connection.commit();// 使所有上一次提交/回滚后进行的更改成为持久更改，
                                                // 并释放此 Connection 对象当前持有的所有数据库锁。
                        }finally {
                            pool.releaseConnection(connection);
                            got.getAndIncrement();
                        }
                    }else {
                        notGot.getAndIncrement();
                    }
                }catch (Exception ex){
                }finally {
                    count--;
                }
            }
            end.countDown();
        }
    }
}