CountDownLatch类位于java.util.concurrent包下，利用它可以实现计数器的功能。比如有一堆任务需要多线程去执行，需要在所有任务执行完之后才能进行下一步这个场景，此时就可以利用CountDownLatch来实现这种功能了。

基本介绍

CountDownLatch类只提供了一个构造器，只有一个int类型的参数，表示计数器的初始值。

/**
 * Constructs a {@code CountDownLatch} initialized with the given count.
 *
 * @param count the number of times {@link #countDown} must be invoked
 *        before threads can pass through {@link #await}
 * @throws IllegalArgumentException if {@code count} is negative
 */
public CountDownLatch(int count) {
    if (count < 0) throw new IllegalArgumentException("count < 0");
    this.sync = new Sync(count);
}

重要方法

前两个是等待所有任务结束的方法，第二个方法有一个超时控制，第一个参数是时间，第二个参数是时间的单位，一般是秒或者毫秒。第三个方法就是计数器减一的方法。性能测试中常用的也就这三个。

public void await() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}

public boolean await(long timeout, TimeUnit unit)
    throws InterruptedException {
    return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}

public void countDown() {
    sync.releaseShared(1);
}

实践

下面是我写的性能框架用到的地方，首先是构建任务：

/**
  * 执行多线程任务
  */
 public PerformanceResultBean start() {
     startTime = Time.getTimeStamp();
     for (int i = 0; i < threadNum; i++) {
         ThreadBase thread = getThread(i);
         thread.setCountDownLatch(countDownLatch);
         executorService.execute(thread);
     }
     shutdownService(executorService, countDownLatch);
     endTime = Time.getTimeStamp();
     threads.forEach(x -> {
         if (x.status()) failTotal++;
         errorTotal += x.errorNum;
         excuteTotal += x.excuteNum;
     });
     logger.info("总计{}个线程，共用时：{} s,执行总数:{},错误数:{},失败数:{}", threadNum, Time.getTimeDiffer(startTime, endTime), excuteTotal, errorTotal, failTotal);
     return over();
 }

下面是多线程基类run()用到的地方：

@Override
public void run() {
    try {
        before();
        List<Long> t = new ArrayList<>();
        long ss = Time.getTimeStamp();
        long et = ss;
        while (true) {
            try {
                threadmark = mark == null ? EMPTY : this.mark.mark(this);
                long s = Time.getTimeStamp();
                doing();
                et = Time.getTimeStamp();
                excuteNum++;
                long diff = et - s;
                t.add(diff);
                if (diff > HttpClientConstant.MAX_ACCEPT_TIME) marks.add(diff + CONNECTOR + threadmark);
                if ((et - ss) > time || status() || key) break;
            } catch (Exception e) {
                logger.warn("执行任务失败！", e);
                logger.warn("执行失败对象的标记:{}", threadmark);
                errorNum++;
            }
        }
        long ee = Time.getTimeStamp();
        logger.info("执行次数：{}, 失败次数: {},总耗时: {} s", excuteNum, errorNum, (ee - ss) / 1000 + 1);
        Concurrent.allTimes.addAll(t);
        Concurrent.requestMark.addAll(marks);
    } catch (Exception e) {
        logger.warn("执行任务失败！", e);
    } finally {
        after();
    }

}

下面是after()方法的内容：

@Override
protected void after() {
    super.after();
    marks = new ArrayList<>();
    GCThread.stop();
}

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