Java countDownLatch 线程辅助类

countDownLatch 线程辅助类，是一个线程计数类，用来线程同步，每当一个线程执行完成后，就是加1，当达到计数，就算执行完成。初始值必须小于等于要执行的线程数，为0 是，不其作用，大于0，小于等于线程数时，为同步线程数，当大于线程数时，为死循环（可能不对）反正就是一直等待。

import java.text.SimpleDateFormat;  import java.util.Date;  import java.util.concurrent.CountDownLatch;      public class T1 {      final static SimpleDateFormat sdf = new SimpleDateFormat(              "yyyy-MM-dd HH:mm:ss");          public static void main(String[] args) throws Exception {          CountDownLatch countDownLatch = new CountDownLatch(3);          Worker w1 = new Worker("w1", 2000, countDownLatch);          Worker w2 = new Worker("w2", 5000, countDownLatch);          w1.start();//          w2.start();//          countDownLatch.await();// 等待所有工人完成工作          System.out.println("all work done at " + sdf.format(new Date()));      }          static class Worker extends Thread {          String workerName;          int workTime;          CountDownLatch latch;              public Worker(String workerName, int workTime, CountDownLatch latch) {              this.workerName = workerName;              this.workTime = workTime;              this.latch = latch;          }              @Override          public void run() {              System.out.println("Worker " + workerName + " do work begin at "                      + sdf.format(new Date()));              try {                  Thread.sleep(workTime);              } catch (InterruptedException e) {                  e.printStackTrace();              }              System.out.println("Worker " + workerName + " do work complete at "                      + sdf.format(new Date()));              latch.countDown();// 工人完成工作，计数器减一          }      }  }