详解java CountDownLatch和CyclicBarrier在内部实现和场景上的区别

前言

CountDownLatch和CyclicBarrier两个同为java并发编程的重要工具类，它们在诸多多线程并发或并行场景中得到了广泛的应用。但两者就其内部实现和使用场景而言是各有所侧重的。

内部实现差异

前者更多依赖经典的AQS机制和CAS机制来控制器内部状态的更迭和计数器本身的变化，而后者更多依靠可重入Lock等机制来控制其内部并发安全性和一致性。

 public class {
   //Synchronization control For CountDownLatch.
   //Uses AQS state to represent count.
  private static final class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = 4982264981922014374L;

    Sync(int count) {
      setState(count);
    }

    int getCount() {
      return getState();
    }

    protected int tryAcquireShared(int acquires) {
      return (getState() == 0) ? 1 : -1;
    }

    protected boolean tryReleaseShared(int releases) {
      // Decrement count; signal when transition to zero
      for (;;) {
        int c = getState();
        if (c == 0)
          return false;
        int nextc = c-1;
        if (compareAndSetState(c, nextc))
          return nextc == 0;
      }
    }
  }

  private final Sync sync;
  ... ...//
 }

 public class CyclicBarrier {
  /**
   * Each use of the barrier is represented as a generation instance.
   * The generation changes whenever the barrier is tripped, or
   * is reset. There can be many generations associated with threads
   * using the barrier - due to the non-deterministic way the lock
   * may be allocated to waiting threads - but only one of these
   * can be active at a time (the one to which {@code count} applies)
   * and all the rest are either broken or tripped.
   * There need not be an active generation if there has been a break
   * but no subsequent reset.
   */
  private static class Generation {
    boolean broken = false;
  }

  /** The lock for guarding barrier entry */
  private final ReentrantLock lock = new ReentrantLock();
  /** Condition to wait on until tripped */
  private final Condition trip = lock.newCondition();
  /** The number of parties */
  private final int parties;
  /* The command to run when tripped */
  private final Runnable barrierCommand;
  /** The current generation */
  private Generation generation = new Generation();

  /**
   * Number of parties still waiting. Counts down from parties to 0
   * on each generation. It is reset to parties on each new
   * generation or when broken.
   */
  private int count;

  /**
   * Updates state on barrier trip and wakes up everyone.
   * Called only while holding lock.
   */
  private void nextGeneration() {
    // signal completion of last generation
    trip.signalAll();
    // set up next generation
    count = parties;
    generation = new Generation();
  }

  /**
   * Sets current barrier generation as broken and wakes up everyone.
   * Called only while holding lock.
   */
  private void breakBarrier() {
    generation.broken = true;
    count = parties;
    trip.signalAll();
  }

  /**
   * Main barrier code, covering the various policies.
   */
  private int dowait(boolean timed, long nanos)
    throws InterruptedException, BrokenBarrierException,
        TimeoutException {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      final Generation g = generation;

      if (g.broken)
        throw new BrokenBarrierException();

      if (Thread.interrupted()) {
        breakBarrier();
        throw new InterruptedException();
      }

      int index = --count;
      if (index == 0) { // tripped
        boolean ranAction = false;
        try {
          final Runnable command = barrierCommand;
          if (command != null)
            command.run();
          ranAction = true;
          nextGeneration();
          return 0;
        } finally {
          if (!ranAction)
            breakBarrier();
        }
      }

      // loop until tripped, broken, interrupted, or timed out
      for (;;) {
        try {
          if (!timed)
            trip.await();
          else if (nanos > 0L)
            nanos = trip.awaitNanos(nanos);
        } catch (InterruptedException ie) {
          if (g == generation && ! g.broken) {
            breakBarrier();
            throw ie;
          } else {
            // We're about to finish waiting even if we had not
            // been interrupted, so this interrupt is deemed to
            // "belong" to subsequent execution.
            Thread.currentThread().interrupt();
          }
        }

        if (g.broken)
          throw new BrokenBarrierException();

        if (g != generation)
          return index;

        if (timed && nanos <= 0L) {
          breakBarrier();
          throw new TimeoutException();
        }
      }
    } finally {
      lock.unlock();
    }
  }
  ... ... //
 }

实战 - 展示各自的使用场景

/**
 *类说明：共5个初始化子线程，6个闭锁扣除点，扣除完毕后，主线程和业务线程才能继续执行
 */
public class UseCountDownLatch {

  static CountDownLatch latch = new CountDownLatch(6);

  /*初始化线程*/
  private static class InitThread implements Runnable{

    public void run() {
      System.out.println("Thread_"+Thread.currentThread().getId()
         +" ready init work......");
      latch.countDown();
      for(int i =0;i<2;i++) {
        System.out.println("Thread_"+Thread.currentThread().getId()
           +" ........continue do its work");
      }
    }
  }

  /*业务线程等待latch的计数器为0完成*/
  private static class BusiThread implements Runnable{

    public void run() {
      try {
        latch.await();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      for(int i =0;i<3;i++) {
        System.out.println("BusiThread_"+Thread.currentThread().getId()
           +" do business-----");
      }
    }
  }

  public static void main(String[] args) throws InterruptedException {
    new Thread(new Runnable() {
      public void run() {
        SleepTools.ms(1);
        System.out.println("Thread_"+Thread.currentThread().getId()
           +" ready init work step 1st......");
        latch.countDown();
        System.out.println("begin step 2nd.......");
        SleepTools.ms(1);
        System.out.println("Thread_"+Thread.currentThread().getId()
           +" ready init work step 2nd......");
        latch.countDown();
      }
    }).start();
    new Thread(new BusiThread()).start();
    for(int i=0;i<=3;i++){
      Thread thread = new Thread(new InitThread());
      thread.start();
    }
    latch.await();
    System.out.println("Main do ites work........");
  }
}

/**
 *类说明：共4个子线程，他们全部完成工作后，交出自己结果，
 *再被统一释放去做自己的事情，而交出的结果被另外的线程拿来拼接字符串
 */
class UseCyclicBarrier {
  private static CyclicBarrier barrier
      = new CyclicBarrier(4,new CollectThread());

  //存放子线程工作结果的容器
  private static ConcurrentHashMap<String,Long> resultMap
      = new ConcurrentHashMap<String,Long>();

  public static void main(String[] args) {
    for(int i=0;i<4;i++){
      Thread thread = new Thread(new SubThread());
      thread.start();
    }

  }

  /*汇总的任务*/
  private static class CollectThread implements Runnable{

    @Override
    public void run() {
      StringBuilder result = new StringBuilder();
      for(Map.Entry<String,Long> workResult:resultMap.entrySet()){
        result.append("["+workResult.getValue()+"]");
      }
      System.out.println(" the result = "+ result);
      System.out.println("do other business........");
    }
  }

  /*相互等待的子线程*/
  private static class SubThread implements Runnable{
    @Override
    public void run() {
      long id = Thread.currentThread().getId();
      resultMap.put(Thread.currentThread().getId()+"",id);
      try {
          Thread.sleep(1000+id);
          System.out.println("Thread_"+id+" ....do something ");
        barrier.await();
        Thread.sleep(1000+id);
        System.out.println("Thread_"+id+" ....do its business ");
        barrier.await();
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
  }
}

 两者总结

1. Cyclicbarrier结果汇总的Runable线程可以重复被执行，通过多次触发await()方法，countdownlatch可以调用await()方法多次；cyclicbarrier若没有结果汇总，则调用一次await()就够了；

2. New cyclicbarrier(threadCount)的线程数必须与实际的用户线程数一致；

3. 协调线程同时运行：countDownLatch协调工作线程执行，是由外面线程协调;cyclicbarrier是由工作线程之间相互协调运行；

4. 从构造函数上看出：countDownlatch控制运行的计数器数量和线程数没有关系；cyclicbarrier构造中传入的线程数等于实际执行线程数;

5. countDownLatch在不能基于执行子线程的运行结果做处理，而cyclicbarrier可以;

6. 就使用场景而言，countdownlatch 更适用于框架加载前的一系列初始化工作等场景; cyclicbarrier更适用于需要多个用户线程执行后，将运行结果汇总再计算等典型场景;

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