并发编程 --Semaphore 计数信号量

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：

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：

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() + "]";
    }
}