Java CountDownLatch计数器与CyclicBarrier循环屏障
作者：Alphathur
定义
CountDownLatch: A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
CyclicBarrier: A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.
上述是Oracle官方定义。简单来说
CountDownLatch：计数器，允许一个或多个线程等待，直到在其他线程中执行的一组操作完成。
CyclicBarrier：循环屏障，它允许一组线程相互等待以达到一个共同的屏障点。
区别
- CountDownLatch 是 AQS (AbstractQueuedSynchronizer) 的一员，但 CyclicBarrier 不是。
- CountDownLatch 的使用场景中，有两类线程，一类是调用await()方法的等待线程，另一类是调用countDownl() 方法的操作线程。CyclicBarrier 的场景中，只有一类线程，都是相互等待的等待线程。
- CountDownLatch 是减计数，递减完后不能复位，CyclicBarrier 是加计数，递增完后自动复位
CountDownLatch 示例
创建两组线程，一组等待另一组执行完才继续进行
CountDownLatch countDownLatch = new CountDownLatch(5);
ExecutorService executorService = Executors.newCachedThreadPool();
for (int i = 0; i < 5; i++) {
executorService.execute(() -> {
countDownLatch.countDown();
System.out.println("run..");
});
}
for (int i = 0; i < 3; i++) { //我们要等上面执行完成才继续
executorService.execute(() -> {
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("await..");
});
}
executorService.shutdown();
打印：
run..
run..
run..
run..
run..
await..
await..
await..
等待累加线程执行完，主线程再输出累加结果
public class ThreadUnsafeExample {
private int cnt = 0;
public void add() {
cnt++;
}
public int get() {
return cnt;
}
public static void main(String[] args) throws InterruptedException {
final int threadSize = 1000;
ThreadUnsafeExample example = new ThreadUnsafeExample();
final CountDownLatch countDownLatch = new CountDownLatch(threadSize);
ExecutorService executorService = Executors.newCachedThreadPool();
for (int i = 0; i < threadSize; i++) {
executorService.execute(() -> {
example.add();
countDownLatch.countDown();
});
}
countDownLatch.await();
executorService.shutdown();
System.out.println(example.get());
}
}
打印：
997
3 模拟并发
ExecutorService executorService = Executors.newCachedThreadPool();
CountDownLatch countDownLatch = new CountDownLatch(1);
for (int i = 0; i < 5; i++) {
executorService.submit( () -> {
try {
countDownLatch.await();
System.out.println("【" + Thread.currentThread().getName() + "】开始执行……");
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
Thread.sleep(2000);
countDownLatch.countDown();//开始并发
executorService.shutdown();
打印：
【pool-2-thread-2】开始执行……
【pool-2-thread-5】开始执行……
【pool-2-thread-3】开始执行……
【pool-2-thread-1】开始执行……
【pool-2-thread-4】开始执行……
CyclicBarrier 示例
所有线程相互等待，直到某一步完成后再继续执行
final int totalThread = 3;
CyclicBarrier cyclicBarrier = new CyclicBarrier(3);
ExecutorService executorService = Executors.newCachedThreadPool();
for (int i = 0; i < totalThread; i++) {
executorService.execute(() -> {
System.out.println("before..");
try {
cyclicBarrier.await();
} catch (InterruptedException | BrokenBarrierException e) {
e.printStackTrace();
}
System.out.println("after..");
});
}
executorService.shutdown();
打印：
before..
before..
before..
after..
after..
after..
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