定義

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官方定義。簡單來說

CountDownLatch：計數器，允許一個或多個執(zhí)行緒等待，直到在其他執(zhí)行緒中執(zhí)行的一組操作完成。

CyclicBarrier：循環(huán)屏障，它允許一組執(zhí)行緒相互等待以達到一個共同的屏障點。

區(qū)別

• CountDownLatch 是 AQS (AbstractQueuedSynchronizer) 的一員，但 CyclicBarrier 不是。

• CountDownLatch 的使用場景中，有兩類線程，一類是呼叫await()方法的等待線程，另一類是呼叫countDownl() 方法的操作線程。在 CyclicBarrier 的情境中，所有線程都是等待彼此的線程，屬於同一類型。

• CountDownLatch 是減計數，遞減完後無法重位，CyclicBarrier 是加計數，遞增完後自動重設

CountDownLatch 範例

建立兩組線程，一組等待另一組執(zhí)行完才繼續(xù)進行

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++) {  //我們要等上面執(zhí)行完成才繼續(xù)
    executorService.execute(() -> {
        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("await..");
    });
}
executorService.shutdown();

列?。?/p>

#run..
run..
run ..
run..
run..
await..
await..
await..

等待累加執(zhí)行緒執(zhí)行完，主執(zhí)行緒再輸出累加結果

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());
    }
}

列印：

#997

3 模擬並發(fā)

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() + "】開始執(zhí)行……");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        }
        Thread.sleep(2000);
        countDownLatch.countDown();//開始并發(fā)
        executorService.shutdown();

列?。?/p>

#【pool-2-thread-2】開始執(zhí)行……
【pool-2-thread-5】開始執(zhí)行……
【pool-2-thread-3】開始執(zhí)行&hellip ;…
【pool-2-thread-1】開始執(zhí)行……
【pool-2-thread-4】開始執(zhí)行……

#CyclicBarrier 範例

所有執(zhí)行緒互相等待，直到某一步完成後再繼續(xù)執(zhí)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();

列?。?/p>

before..
before..
before ..
after..
after..
after..

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