Java并发编程Semaphore计数信号量详解

阴焱

2023-03-14

本文向大家介绍Java并发编程Semaphore计数信号量详解，包括了Java并发编程Semaphore计数信号量详解的使用技巧和注意事项，需要的朋友参考一下

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() + "]"; 
  } 
}

总结

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Java并发编程Semaphore计数信号量详解

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