7.7 CyclicBarrier

CyclicBarrier 循环栅栏，用来进行线程协作，等待线程满足某个计数。构造时设置【计数个数】。每个线程执行到某个需要“同步”的时刻调用 await() 方法进行等待，当等待的线程数满足【计数个数】时，继续执行。

@Slf4j(topic = "c.TestCyclicBarrier")
public class TestCyclicBarrier {public static void main(String[] args) {ExecutorService executorService = Executors.newFixedThreadPool(2);CyclicBarrier barrier = new CyclicBarrier(2, () -> {log.debug("task1 task2 finish...");});for (int i = 0; i < 3; i++) {executorService.submit(() -> {log.debug("task1 begin...");sleep(1000);try {barrier.await();log.debug("task1 end...");} catch (InterruptedException | BrokenBarrierException e) {throw new RuntimeException(e);}});executorService.submit(() -> {log.debug("task2 begin...");sleep(2000);try {barrier.await();log.debug("task2 end...");} catch (InterruptedException | BrokenBarrierException e) {throw new RuntimeException(e);}});}}public static void sleep(long time) {try {Thread.sleep(time);} catch (InterruptedException e) {throw new RuntimeException(e);}}
}

输出

19:31:31.163 [pool-1-thread-1] - task1 begin...
19:31:31.163 [pool-1-thread-2] - task2 begin...
19:31:33.170 [pool-1-thread-2] - task1 task2 finish...
19:31:33.170 [pool-1-thread-2] - task2 end...
19:31:33.170 [pool-1-thread-1] - task1 end...
19:31:33.170 [pool-1-thread-2] - task1 begin...
19:31:33.170 [pool-1-thread-1] - task2 begin...
19:31:35.175 [pool-1-thread-1] - task1 task2 finish...
19:31:35.176 [pool-1-thread-1] - task2 end...
19:31:35.176 [pool-1-thread-1] - task1 begin...
19:31:35.176 [pool-1-thread-2] - task1 end...
19:31:35.176 [pool-1-thread-2] - task2 begin...
19:31:37.176 [pool-1-thread-2] - task1 task2 finish...
19:31:37.177 [pool-1-thread-2] - task2 end...
19:31:37.177 [pool-1-thread-1] - task1 end...

task1 需等待 task2 执行结束，task1 才能结束
CyclicBarrier 与 CountDownLatch 的主要区别在于 CyclicBarrier 是可以重用的 CyclicBarrier 可以被比喻为『人满发车』
线程池中的线程数与 CyclicBarrier 中的 parties（第一个参数）的数量最好保持一致。

@Slf4j(topic = "c.TestCyclicBarrier")
public class TestCyclicBarrier {public static void main(String[] args) {// 线程数与 parties 数量未保持一致ExecutorService executorService = Executors.newFixedThreadPool(3);CyclicBarrier barrier = new CyclicBarrier(2, () -> {log.debug("task1 task2 finish...");});// task1 执行时间为 1s，task2 执行时间为 2s// 可能会出现线程1，线程2与线程3同时执行，其中线程1和线程3执行 task1，// 线程2执行 task2。那么线程 1 和线程 3 在执行完 task1 后 CyclicBarrier // 的计数就减为了0，然后就去调用 CyclicBarrier 中的后续任务（参数2）for (int i = 0; i < 3; i++) {executorService.submit(() -> {log.debug("task1 begin...");sleep(1000);try {barrier.await();log.debug("task1 end...");} catch (InterruptedException | BrokenBarrierException e) {throw new RuntimeException(e);}});executorService.submit(() -> {log.debug("task2 begin...");sleep(2000);try {barrier.await();log.debug("task2 end...");} catch (InterruptedException | BrokenBarrierException e) {throw new RuntimeException(e);}});}}public static void sleep(long time) {try {Thread.sleep(time);} catch (InterruptedException e) {throw new RuntimeException(e);}}
}

输出

19:40:04.736 [pool-1-thread-2] - task2 begin...
19:40:04.736 [pool-1-thread-3] - task1 begin...
19:40:04.736 [pool-1-thread-1] - task1 begin...
19:40:05.742 [pool-1-thread-1] - task1 task2 finish...
19:40:05.742 [pool-1-thread-1] - task1 end...
19:40:05.743 [pool-1-thread-3] - task1 end...
19:40:05.743 [pool-1-thread-1] - task2 begin...
19:40:05.743 [pool-1-thread-3] - task1 begin...
19:40:06.748 [pool-1-thread-3] - task1 task2 finish...
19:40:06.748 [pool-1-thread-3] - task1 end...
19:40:06.748 [pool-1-thread-3] - task2 begin...
19:40:06.748 [pool-1-thread-2] - task2 end...
19:40:08.752 [pool-1-thread-3] - task1 task2 finish...
19:40:08.753 [pool-1-thread-3] - task2 end...
19:40:08.753 [pool-1-thread-1] - task2 end...