Java并发编程Semaphore计数信号量详解

Semaphore 是一个计数信号量，它的本质是一个共享锁。信号量维护了一个信号量许可集。线程可以通过调用acquire()来获取信号量的许可；当信号量中有可用的许可时，线程能获取该许可；否则线程必须等待，直到有可用的许可为止。 线程可以通过release()来释放它所持有的信号量许可（用完信号量之后必须释放，不然其他线程可能会无法获取信号量）。

简单示例：

package me.socketthread;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
public class SemaphoreLearn {
  //信号量总数
  private static final int SEM_MAX = 12;
  public static void main(String[] args) {
    Semaphore sem = new Semaphore(SEM_MAX);
    //创建线程池
    ExecutorService threadPool = Executors.newFixedThreadPool(3);
    //在线程池中执行任务
    threadPool.execute(new MyThread(sem, 7));
    threadPool.execute(new MyThread(sem, 4));
    threadPool.execute(new MyThread(sem, 2));
    //关闭池
    threadPool.shutdown();
  }
}
  class MyThread extends Thread {
    private volatile Semaphore sem;  // 信号量
    private int count;    // 申请信号量的大小  

    MyThread(Semaphore sem, int count) {
      this.sem = sem;
      this.count = count;
    }
    public void run() {
      try {
       // 从信号量中获取count个许可
        sem.acquire(count);
        Thread.sleep(2000);
        System.out.println(Thread.currentThread().getName() + " acquire count="+count);
      } catch (InterruptedException e) {
        e.printStackTrace();
      } finally {
        // 释放给定数目的许可，将其返回到信号量。
        sem.release(count);
        System.out.println(Thread.currentThread().getName() + " release " + count + "");
      }
    }
  } 

执行结果：

pool-1-thread-2 acquire count=4
pool-1-thread-1 acquire count=7
pool-1-thread-1 release 7
pool-1-thread-2 release 4
pool-1-thread-3 acquire count=2
pool-1-thread-3 release 2

线程1和线程2会并发执行，因为两者的信号量和没有超过总信号量，当前两个线程释放掉信号量之后线程3才能继续执行。

源码分析：

1、构造函数

在构造函数中会初始化信号量值，这值最终是作为锁标志位state的值

Semaphore sem = new Semaphore(12);//简单来说就是给锁标识位state赋值为12 

2、Semaphore.acquire(n);简单理解为获取锁资源，如果获取不到线程阻塞

Semaphore.acquire(n);//从锁标识位state中获取n个信号量，简单来说是state = state-n 此时state大于0表示可以获取信号量，如果小于0则将线程阻塞 

public void acquire(int permits) throws InterruptedException {
    if (permits < 0) throw new IllegalArgumentException();
    //获取锁
    sync.acquireSharedInterruptibly(permits);
  } 

acquireSharedInterruptibly中的操作是获取锁资源，如果可以获取则将state= state-permits，否则将线程阻塞

public final void acquireSharedInterruptibly(int arg)
      throws InterruptedException {
    if (Thread.interrupted())
      throw new InterruptedException();
    if (tryAcquireShared(arg) < 0)//tryAcquireShared中尝试获取锁资源
      doAcquireSharedInterruptibly(arg); //将线程阻塞
  } 

tryAcquireShared中的操作是尝试获取信号量值，简单来说就是state=state-acquires ，如果此时小于0则返回负值，否则返回大于新值，再判断是否将当线程线程阻塞

protected int tryAcquireShared(int acquires) {
      for (;;) {
        if (hasQueuedPredecessors())
          return -1;
      //获取state值
        int available = getState();
      //从state中获取信号量
        int remaining = available - acquires;
        if (remaining < 0 ||
          compareAndSetState(available, remaining))
        //如果信号量小于0则直接返回，表示无法获取信号量，否则将state值修改为新值
          return remaining;
      }
    } 

doAcquireSharedInterruptibly中的操作简单来说是将当前线程添加到FIFO队列中并将当前线程阻塞。

/会将线程添加到FIFO队列中，并阻塞
private void doAcquireSharedInterruptibly(int arg)
    throws InterruptedException {
    //将线程添加到FIFO队列中
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
      for (;;) {
        final Node p = node.predecessor();
        if (p == head) {
          int r = tryAcquireShared(arg);
          if (r >= 0) {
            setHeadAndPropagate(node, r);
            p.next = null; // help GC
            failed = false;
            return;
          }
        }
        //parkAndCheckInterrupt完成线程的阻塞操作
        if (shouldParkAfterFailedAcquire(p, node) &&
          parkAndCheckInterrupt())
          throw new InterruptedException();
      }
    } finally {
      if (failed)
        cancelAcquire(node);
    }
  } 

3、Semaphore.release(int permits)，这个函数的实现操作是将state = state+permits并唤起处于FIFO队列中的阻塞线程。

public void release(int permits) {
    if (permits < 0) throw new IllegalArgumentException();
  //state = state+permits，并将FIFO队列中的阻塞线程唤起
    sync.releaseShared(permits);
  } 

releaseShared中的操作是将state = state+permits，并将FIFO队列中的阻塞线程唤起。

public final boolean releaseShared(int arg) {
    //tryReleaseShared将state设置为state = state+arg
    if (tryReleaseShared(arg)) {
      //唤起FIFO队列中的阻塞线程
      doReleaseShared();
      return true;
    }
    return false;
  } 

tryReleaseShared将state设置为state = state+arg

protected final boolean tryReleaseShared(int releases) {
      for (;;) {
        int current = getState();
        int next = current + releases;
        if (next < current) // overflow
          throw new Error("Maximum permit count exceeded");
        //将state值设置为state=state+releases
        if (compareAndSetState(current, next))
          return true;
      }
    } 

doReleaseShared()唤起FIFO队列中的阻塞线程

private void doReleaseShared() {  

    for (;;) {
      Node h = head;
      if (h != null && h != tail) {
        int ws = h.waitStatus;
        if (ws == Node.SIGNAL) {
          if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
            continue;      // loop to recheck cases
          //完成阻塞线程的唤起操作
          unparkSuccessor(h);
        }
        else if (ws == 0 &&
             !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
          continue;        // loop on failed CAS
      }
      if (h == head)          // loop if head changed
        break;
    }
  }  

总结：Semaphore简单来说设置了一个信号量池state，当线程执行时会从state中获取值，如果可以获取则线程执行，并且在执行后将获取的资源返回到信号量池中，并唤起其他阻塞线程；如果信号量池中的资源无法满足某个线程的需求则将此线程阻塞。

Semaphore源码：

public class Semaphore implements java.io.Serializable {
  private static final long serialVersionUID = -3222578661600680210L;
  private final Sync sync;
  abstract static class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = 1192457210091910933L;
    //设置锁标识位state的初始值
    Sync(int permits) {
      setState(permits);
    }
    //获取锁标识位state的值，如果state值大于其需要的值则表示锁可以获取
    final int getPermits() {
      return getState();
    }
    //获取state值减去acquires后的值，如果大于等于0则表示锁可以获取
    final int nonfairTryAcquireShared(int acquires) {
      for (;;) {
        int available = getState();
        int remaining = available - acquires;
        if (remaining < 0 ||
          compareAndSetState(available, remaining))
          return remaining;
      }
    }
    //释放锁
    protected final boolean tryReleaseShared(int releases) {
      for (;;) {
        int current = getState();
        //将state值加上release值
        int next = current + releases;
        if (next < current) // overflow
          throw new Error("Maximum permit count exceeded");
        if (compareAndSetState(current, next))
          return true;
      }
    }
    //将state的值减去reductions
    final void reducePermits(int reductions) {
      for (;;) {
        int current = getState();
        int next = current - reductions;
        if (next > current) // underflow
          throw new Error("Permit count underflow");
        if (compareAndSetState(current, next))
          return;
      }
    }
    final int drainPermits() {
      for (;;) {
        int current = getState();
        if (current == 0 || compareAndSetState(current, 0))
          return current;
      }
    }
  }
  //非公平锁
  static final class NonfairSync extends Sync {
    private static final long serialVersionUID = -2694183684443567898L;
    NonfairSync(int permits) {
      super(permits);
    }
    protected int tryAcquireShared(int acquires) {
      return nonfairTryAcquireShared(acquires);
    }
  }
  //公平锁
  static final class FairSync extends Sync {
    private static final long serialVersionUID = 2014338818796000944L;
    FairSync(int permits) {
      super(permits);
    }
    protected int tryAcquireShared(int acquires) {
      for (;;) {
        if (hasQueuedPredecessors())
          return -1;
        int available = getState();
        int remaining = available - acquires;
        if (remaining < 0 ||
          compareAndSetState(available, remaining))
          return remaining;
      }
    }
  }
  //设置信号量
  public Semaphore(int permits) {
    sync = new NonfairSync(permits);
  }
  public Semaphore(int permits, boolean fair) {
    sync = fair ? new FairSync(permits) : new NonfairSync(permits);
  }
  //获取锁
  public void acquire() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
  }
  public void acquireUninterruptibly() {
    sync.acquireShared(1);
  }
  public boolean tryAcquire() {
    return sync.nonfairTryAcquireShared(1) >= 0;
  }
  public boolean tryAcquire(long timeout, TimeUnit unit)
    throws InterruptedException {
    return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
  }
  public void release() {
    sync.releaseShared(1);
  }
  //获取permits值锁
  public void acquire(int permits) throws InterruptedException {
    if (permits < 0) throw new IllegalArgumentException();
    sync.acquireSharedInterruptibly(permits);
  }
  public void acquireUninterruptibly(int permits) {
    if (permits < 0) throw new IllegalArgumentException();
    sync.acquireShared(permits);
  }
  public boolean tryAcquire(int permits) {
    if (permits < 0) throw new IllegalArgumentException();
    return sync.nonfairTryAcquireShared(permits) >= 0;
  }
  public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
    throws InterruptedException {
    if (permits < 0) throw new IllegalArgumentException();
    return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
  }
  //释放
  public void release(int permits) {
    if (permits < 0) throw new IllegalArgumentException();
    sync.releaseShared(permits);
  }
  public int availablePermits() {
    return sync.getPermits();
  }
  public int drainPermits() {
    return sync.drainPermits();
  }
  protected void reducePermits(int reduction) {
    if (reduction < 0) throw new IllegalArgumentException();
    sync.reducePermits(reduction);
  }
  public boolean isFair() {
    return sync instanceof FairSync;
  }
  public final boolean hasQueuedThreads() {
    return sync.hasQueuedThreads();
  }
  public final int getQueueLength() {
    return sync.getQueueLength();
  }
  protected Collection<Thread> getQueuedThreads() {
    return sync.getQueuedThreads();
  }
  public String toString() {
    return super.toString() + "[Permits = " + sync.getPermits() + "]";
  }
} 

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