6).CountDownLatch、Semaphore和CyclicBarrier - 6.1 CountDownLatch、Semaphore和CyclicBarrier。 - 《Java架构师-2021》

1.CountDownLatch
2.Semaphore
3.CyclicBarrier

JAVA并发包中有三个类用于同步一批线程的行为，分别是CountDownLatch、Semaphore和CyclicBarrier。

1.CountDownLatch

6.1 CountDownLatch、Semaphore和CyclicBarrier。 - 图1
CountDownLatch是一个计数器闭锁，通过它可以完成类似于阻塞当前线程的功能，即：一个线程或多个线程一直等待，直到其他线程执行的操作完成。CountDownLatch用一个给定的计数器来初始化，该计数器的操作是原子操作，即同时只能有一个线程去操作该计数器。调用该类await方法的线程会一直处于阻塞状态，直到其他线程调用countDown方法使当前计数器的值变为零，每次调用countDown计数器的值减1。当计数器值减至零时，所有因调用await()方法而处于等待状态的线程就会继续往下执行。这种现象只会出现一次，因为计数器不能被重置，如果业务上需要一个可以重置计数次数的版本，可以考虑使用CycliBarrier。
在某些业务场景中，程序执行需要等待某个条件完成后才能继续执行后续的操作；典型的应用如并行计算，当某个处理的运算量很大时，可以将该运算任务拆分成多个子任务，等待所有的子任务都完成之后，父任务再拿到所有子任务的运算结果进行汇总。

@Slf4j
class CountDownLatchExample1 {
    private final static int threadCount = 10;
    public static void main(String[] args) throws Exception {
        ExecutorService exec = Executors.newCachedThreadPool();
        final CountDownLatch countDownLatch = new CountDownLatch(threadCount);
        for (int i = 0; i < threadCount; i++) {
            final int threadNum = i;
            exec.execute(() -> {
                try {
                    test(threadNum);
                } catch (Exception e) {
                    log.error("exception", e);
                } finally {
                    countDownLatch.countDown();
                }
            });
        }
        countDownLatch.await();
        log.info("finish");
        exec.shutdown();
    }
    private static void test(int threadNum) throws Exception {
        Thread.sleep(100);
        log.info("{}", threadNum);
        Thread.sleep(100);
    }
}

结果：
20:18:32.917 [pool-1-thread-7] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 6 20:18:32.917 [pool-1-thread-6] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 5 20:18:32.919 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 4 20:18:32.918 [pool-1-thread-1] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 0 20:18:32.918 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 2 20:18:32.916 [pool-1-thread-9] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 8 20:18:32.918 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 3 20:18:32.916 [pool-1-thread-10] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 9 20:18:32.916 [pool-1-thread-8] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 7 20:18:32.917 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 1 20:18:33.032 [main] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - finish

下面演示设置countDownLatch.await时间，看看会出现什么结果

@Slf4j
class CountDownLatchExample2 {
    private final static int threadCount = 10;
    public static void main(String[] args) throws Exception {
        ExecutorService exec = Executors.newCachedThreadPool();
        final CountDownLatch countDownLatch = new CountDownLatch(threadCount);
        for (int i = 0; i < threadCount; i++) {
            final int threadNum = i;
            exec.execute(() -> {
                try {
                    test(threadNum);
                } catch (Exception e) {
                    log.error("exception", e);
                } finally {
                    countDownLatch.countDown();
                }
            });
        }
        countDownLatch.await(10, TimeUnit.MILLISECONDS);
        log.info("finish");
        exec.shutdown();
    }
    private static void test(int threadNum) throws Exception {
        Thread.sleep(100);
        log.info("{}", threadNum);
    }
}

结果：超过指定时间跳过等待
20:19:34.878 [main] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - finish 20:19:34.964 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 2 20:19:34.965 [pool-1-thread-10] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 9 20:19:34.964 [pool-1-thread-1] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 0 20:19:34.965 [pool-1-thread-8] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 7 20:19:34.964 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 1 20:19:34.965 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 4 20:19:34.965 [pool-1-thread-7] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 6 20:19:34.964 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 3 20:19:34.965 [pool-1-thread-9] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 8 20:19:34.965 [pool-1-thread-6] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 5

2.Semaphore

6.1 CountDownLatch、Semaphore和CyclicBarrier。 - 图2
Semaphore与CountDownLatch相似，不同的地方在于Semaphore的值被获取到后是可以释放的，并不像CountDownLatch那样一直减到底。它也被更多地用来限制流量，类似阀门的功能。如果限定某些资源最多有N个线程可以访问，那么超过N个主不允许再有线程来访问，同时当现有线程结束后，就会释放，然后允许新的线程进来。有点类似于锁的lock与 unlock过程。相对来说他也有两个主要的方法：
用于获取权限的acquire(),其底层实现与CountDownLatch.countdown()类似;
用于释放权限的release()，其底层实现与acquire()是一个互逆的过程。

import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
public class SemaphoreExample1 {
    private final static int threadCount = 20;
    public static void main(String[] args) throws Exception {
        ExecutorService exec = Executors.newCachedThreadPool();
        // 每次最多三个线程获取许可
        final Semaphore semaphore = new Semaphore(3);
        for (int i = 0; i < threadCount; i++) {
            final int threadNum = i;
            exec.execute(() -> {
                try {
                    semaphore.acquire(); // 获取一个许可
                    test(threadNum);
                    semaphore.release(); // 释放一个许可
                } catch (Exception e) {
                    log.error("exception", e);
                }
            });
        }
        exec.shutdown();
    }
    private static void test(int threadNum) throws Exception {
        log.info("{}", threadNum);
        Thread.sleep(1000);
    }
}

3.CyclicBarrier

CyclicBarrier也是一个同步辅助类，它允许一组线程相互等待，直到到达某个公共屏障点（common barrier point）。通过它可以完成多个线程之间相互等待，只有当每个线程都准备就绪后，才能各自继续往下执行后面的操作。类似于CountDownLatch，它也是通过计数器来实现的。当某个线程调用await方法时，该线程进入等待状态，且计数器加1，当计数器的值达到设置的初始值时，所有因调用await进入等待状态的线程被唤醒，继续执行后续操作。因为CycliBarrier在释放等待线程后可以重用，所以称为循环barrier。CycliBarrier支持一个可选的Runnable，在计数器的值到达设定值后（但在释放所有线程之前），该Runnable运行一次，注，Runnable在每个屏障点只运行一个。
使用场景类似于CountDownLatch与CountDownLatch的区别

CountDownLatch主要是实现了1个或N个线程需要等待其他线程完成某项操作之后才能继续往下执行操作，描述的是1个线程或N个线程等待其他线程的关系。CyclicBarrier主要是实现了多个线程之间相互等待，直到所有的线程都满足了条件之后各自才能继续执行后续的操作，描述的多个线程内部相互等待的关系。

CountDownLatch是一次性的，而CyclicBarrier则可以被重置而重复使用。

@Slf4j
class CyclicBarrierExample1 {
  private static CyclicBarrier barrier = new CyclicBarrier(5);
  public static void main(String[] args) throws Exception {
      ExecutorService executor = Executors.newCachedThreadPool();
      for (int i = 0; i < 10; i++) {
          final int threadNum = i;
          Thread.sleep(1000);
          executor.execute(() -> {
              try {
                  race(threadNum);
              } catch (Exception e) {
                  log.error("exception", e);
              }
          });
      }
      executor.shutdown();
  }
  private static void race(int threadNum) throws Exception {
      Thread.sleep(1000);
      log.info("{} is ready", threadNum);
      barrier.await();
      log.info("{} continue", threadNum);
  }
}

```shell int statusTest1 = 0; CopyOnWriteArrayList> list = new CopyOnWriteArrayList(); AtomicInteger atomicInteger = new AtomicInteger(0);

@GetMapping(“testId”) public void testId() throws InterruptedException { //同步代码块,防止多次点击 synchronized (this) { if (statusTest1 > 0) { return; } statusTest1 = 1; }

    int threadCount = 100;
    CyclicBarrier cyclicBarrier = new CyclicBarrier(threadCount, new Runnable() {
        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName()+"所有线程完成");
            Map<String, List<Map<String, Object>>> gbList = list.stream().collect(Collectors.groupingBy(e -> e.get("snowId").toString()));
            System.out.println(gbList);
        }
    });
    for(int i=1; i<=threadCount; i++){
        final int finalI = i;
        new Thread(new Runnable() {
            @SneakyThrows
            @Override
            public void run() {
                for(int j= 1;j<=6000;j++){
                    Map<String, Object> map = new HashMap<>();
                    map.put("snowId", IdGenerator.nextId());
                    list.add(map);
                    int now = atomicInteger.incrementAndGet();
                    System.out.println("now"+now);
                }
                cyclicBarrier.await();
            }
        }, String.valueOf(finalI)).start();
    }
}

```