Latch的字面意思：弹簧锁
CountDownLatch是java.util.concurrent包里的一个同步工具类。
CountDownLatch的构造函数，接收一个类型为整型的参数，代表CountDownLatch所在的线程，在执行await方法后能够返回，所需要在其他线程内调用其countDown方法的次数。
timer.schedule新启动了一个线程，在新线程里调用countDown，而主线程执行await进入阻塞状态，待新线程调用一次countDown之后，主线程立即从await方法的阻塞状态中返回。
package jerry; import java.util.concurrent.CountDownLatch import java.util.concurrent.TimeUnit CountDownLatch called = new CountDownLatch(1) println "main thread id: " + Thread.currentThread().getId(); Timer timer = new Timer() timer.schedule(new TimerTask() { void run() { println "call countDown in another thread: " + Thread.currentThread().getId(); called.countDown() } }, 220) println "before calling called.await in main thread: " + Thread.currentThread().getId(); called.await(10, TimeUnit.SECONDS) println "after calling called.await in main thread: " + Thread.currentThread().getId();
