2.1.1 初步使用

在本节将实现一个当CountDownLatch的计数不为0时，线程呈阻塞状态的效果。通过本实验可以看到线程执行任务具有“组团”的特性了。

创建测试用的项目CountDownLatch_test1，类MyService.java代码如下：

        package service;
        import java.util.concurrent.CountDownLatch;
        public class MyService {
            private CountDownLatch down = new CountDownLatch(1);
            public void testMethod() {
                try {
                    System.out.println("A");
                    down.await();
                    System.out.println("B");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            public void downMethod() {
                System.out.println("X");
                down.countDown();
            }
        }

代码new CountDownLatch(1)的作用是创建1个计数的CountDownLatch类的对象，当线程执行down.await()代码时呈等待状态，程序不向下继续运行。程序执行down. countDown()代码时计数由1变成0，以前呈等待状态的线程继续向下运行。

线程类MyThread.java代码如下：

        package extthread;
              import service.MyService;
              public class MyThread extends Thread {
                  private MyService myService;
                  public MyThread(MyService myService) {
                      super();
                      this.myService = myService;
                  }
                  @Override
                  public void run() {
                      myService.testMethod();
                  }
              }

运行类Run.java代码如下：

              package test.run;
              import service.MyService;
              import extthread.MyThread;
              public class Run {
                  public static void main(String[] args) throws InterruptedException {
                      MyService service = new MyService();
                      MyThread t = new MyThread(service);
                      t.start();
                      Thread.sleep(2000);
                      service.downMethod();
                  }
              }

程序运行结果如图2-2所示。

图2-2 运行结果

await()方法还有重载的写法await (long timeout, TimeUnit unit)，此方法在2.1.5节有测试的代码。