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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