juc包之CountDownLatch与CyclicBarrier

CountDownLatch与CyclicBarrier各自的实现原理

CountDownLatch类
示例代码: (在CountDownLatch类的注释上方提供了两种使用方式)

• 第一种

  class Driver {
  
      void main() throws InterruptedException {
          CountDownLatch startSignal = new CountDownLatch(1);
          CountDownLatch doneSignal = new CountDownLatch(N);
  
          for (int i = 0; i < N; ++i) // create and start threads
              new Thread(new Worker(startSignal, doneSignal)).start();
  
          doSomethingElse();
          startSignal.countDown();
          doSomethingElse();
          doneSignal.await();
      }
  }
  
  class Worker implements Runnable {
      private final CountDownLatch startSignal;
      private final CountDownLatch doneSignal;
      Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
          this.startSignal = startSignal;
          this.doneSignal = doneSignal;
      }
      public void run() {
          try {
              startSignal.await();
              doWork();
              doneSignal.countDown();
          } catch (InterruptedException ex) {}
      }
  
      void doWork() {
          //something to do
      }
  }
  

先让所有Worker等Driver去唤醒,然后Driver等所有Worker执行完再唤醒

• 第二种

  class Driver2 { // ...
      void main() throws InterruptedException {
          CountDownLatch doneSignal = new CountDownLatch(N);
          Executor e = ...
  
          for (int i = 0; i < N; ++i) // create and start threads
              e.execute(new WorkerRunnable(doneSignal, i));
  
          doneSignal.await();           // wait for all to finish
      }
  }
  
  class WorkerRunnable implements Runnable {
      private final CountDownLatch doneSignal;
      private final int i;
      WorkerRunnable(CountDownLatch doneSignal, int i) {
          this.doneSignal = doneSignal;
          this.i = i;
      }
      public void run() {
          try {
              doWork(i);
              doneSignal.countDown();
          } catch (InterruptedException ex) {} // return;
      }
  
      void doWork() { ... }
  }  
  

让Drive等待所有Worker运行完毕(注释说这种写法可以用CyclicBarrier代替)

看一下几个核心方法

• CountDownLatch构造方法

初始化主要是给state变量设置了值,并且该值不能小于0

• CountDownLatch的countdown方法

countdown执行了Sync内部类的releaseShared方法,该方法先执行了tryReleaseShared方法,返回true才会去执行doReleaseShared方法

该方法首先判断state值是否为0,如果为0,直接返回false,代表没有资源可以释放了,要是不为0,则将state值减1,并将结果更新给state变量,然后判断-1后的值是否为0,如果为0,则返回true.所以返回true的含义就是在此次释放资源后已经没有需要释放的资源了.

该方法是在资源全部被释放后才会执行的.首先获取队列的head节点,并且判断head节点不为null也不为tail(head只有已经唤醒过才有可能与tail节点相同).然后获取head的waitStatus.如果waitStatus是Signal则会去唤醒该节点线程,如果执行期间head节点被替换了,则会继续循环,否则就跳出.

• CountDownLatch的await方法

await执行了Sync内部类的acquireSharedInterruptibly方法,该方法首先判断当前线程是否中断了,如果中断直接抛出错误,否则执行tryAcquireShared方法,当返回负数时才会去执行doAcquireSharedInterruptibly方法

判断当前state变量值是否为0,只有当不为0才会返回负数-1,代表还有资源未释放

该方法内部与ReentranLock类的acquiredQueued很像,都是遍历链表节点,去获取资源,获取失败则会调用LockSupport的park方法将其挂起,且设置节点为SIGNAL.获取成功则会调用setHeadAndPropagate方法将所有未取消的节点都唤醒