新Java并发线程控制：CountDownLatch等待所有线程运行完毕才执行

川长思鸟来 2023-10-18 10:31 75阅读 0赞

## 新Java并发线程控制：CountDownLatch等待所有线程运行完毕才执行 ##

创建一批Java线程，然后这批Java线程几乎同时全部跑起来，但是有些开发场景下，开发者需要等到这些Java线程全部执行完毕后，才去执行接下去的业务流程。这个时候就可以CountDownLatch就可以派上用场，CountDownLatch是Java语言从1.5引入的线程控制类，

* A synchronization aid that allows one or more threads to wait until
    * a set of operations being performed in other threads completes.

CountDownLatch在创建之初的构造函数时候需要传递一个count值，这个值作为计数器，在每一个Java线程运行完毕后，通过CountDownLatch的countDown()函数，给CountDownLatch维护的计数（count值）减1，当CountDownLatch的countDown()把计数器减至0时候，CountDownLatch的await()函数阻塞状态解除，就可以接着继续执行后续逻辑代码。  
如代码：

private void test() {
            int count = 5;
    
            CountDownLatch countDownLatch = new CountDownLatch(count);
            for (int i = 0; i < count; i++) {
                new WorkThread(i, countDownLatch).start();
            }
    
            try {
                System.out.println("等待" + count + "个线程运行完毕...");
                countDownLatch.await();
    
                System.out.println(count + "个线程已运行完毕");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    
        private class WorkThread extends Thread {
            private int seq;
            private CountDownLatch countDownLatch;
    
            public WorkThread(int seq, CountDownLatch countDownLatch) {
                this.seq = seq;
                this.countDownLatch = countDownLatch;
            }
    
            @Override
            public void run() {
                try {
                    System.out.println("线程" + this.seq + "开始运行...");
                    Thread.sleep(2000);
                    System.out.println("线程" + this.seq + "运行结束");
    
                    countDownLatch.countDown();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }

输出：

2018-10-27 10:20:00.727 26486-26486/zhangphil.test I/System.out: 等待5个线程运行完毕...
    2018-10-27 10:20:00.728 26486-26513/zhangphil.test I/System.out: 线程4开始运行...
    2018-10-27 10:20:00.728 26486-26512/zhangphil.test I/System.out: 线程3开始运行...
    2018-10-27 10:20:00.728 26486-26510/zhangphil.test I/System.out: 线程1开始运行...
    2018-10-27 10:20:00.728 26486-26508/zhangphil.test I/System.out: 线程0开始运行...
    2018-10-27 10:20:00.729 26486-26511/zhangphil.test I/System.out: 线程2开始运行...
    2018-10-27 10:20:02.728 26486-26513/zhangphil.test I/System.out: 线程4运行结束
    2018-10-27 10:20:02.729 26486-26512/zhangphil.test I/System.out: 线程3运行结束
    2018-10-27 10:20:02.729 26486-26510/zhangphil.test I/System.out: 线程1运行结束
    2018-10-27 10:20:02.729 26486-26508/zhangphil.test I/System.out: 线程0运行结束
    2018-10-27 10:20:02.729 26486-26511/zhangphil.test I/System.out: 线程2运行结束
    2018-10-27 10:20:02.730 26486-26486/zhangphil.test I/System.out: 5个线程已运行完毕

附：  
1，《新Java线程Semaphore：并行环境下访问竞争资源控制》链接：https://blog.csdn.net/zhangphil/article/details/83410270