多线程条件通行工具——Semaphore

Semaphore的作用是，限制线程通行的数量，如果线程进入时达到通行数量，便等待其它正在通行的线程释放。

• acquire()
  获取通行
• release()
  释放通行
• availablePermits()
  查看通行剩余次数
• int drainPermits()
  缩减剩余通行次数为0，并返回缩减量
• reducePermits(int)
  缩减通行次数为指定次数
• boolean isFair()
  查看是否公平分配通行
• boolean hasQueuedThreads()
  查看是否有线程在排队
• int getQueueLength()
  查看排队的长度
• Collection<Thread> getQueuedThreads()
  查看排队中的所有线程

源码：

/**
 * @since 1.5
 * @author Doug Lea
 */
public class Semaphore implements java.io.Serializable {
    private static final long serialVersionUID = -3222578661600680210L;
    
    private final Sync sync;

    public Semaphore(int permits) {
        // 默认使用非公平锁，设置初始的剩余通行量
        sync = new NonfairSync(permits);
    }

    public Semaphore(int permits, boolean fair) {
        // 参数决定使用公平还是非公平锁，设置初始的剩余通行量
        sync = fair ? new FairSync(permits) : new NonfairSync(permits);
    }

    abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 1192457210091910933L;

        Sync(int permits) {
            // 设置初始的剩余通行量
            setState(permits);
        }

        final int getPermits() {
            // 获取剩余通行量
            return getState();
        }

        final int nonfairTryAcquireShared(int acquires) {
            for (;;) {
                int available = getState();
                int remaining = available - acquires;
                // 设置获取后的剩余通行量
                // 当剩余量小于0时，则阻塞，否则通行
                if (remaining < 0 || compareAndSetState(available, remaining))
                    return remaining;
            }
        }

        protected final boolean tryReleaseShared(int releases) {
            for (;;) {
                int current = getState();
                int next = current + releases;
                // 不允许释放量是负数
                if (next < current) // overflow
                    throw new Error("Maximum permit count exceeded");
                // 设置释放后的剩余通行量，返回释放成功，表示需要通知下一个线程通行
                if (compareAndSetState(current, next))
                    return true;
            }
        }

        final void reducePermits(int reductions) {
            for (;;) {
                int current = getState();
                int next = current - reductions;
                // 不允许缩减值是负数
                if (next > current) // underflow
                    throw new Error("Permit count underflow");
                // 设置缩减后的剩余通行量
                if (compareAndSetState(current, next))
                    return;
            }
        }

        final int drainPermits() {
            for (;;) {
                int current = getState();
                // 直接把剩余通行量设置为0
                if (current == 0 || compareAndSetState(current, 0))
                    return current;
            }
        }
    }

    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);
        }
    }

    static final class FairSync extends Sync {
        private static final long serialVersionUID = 2014338818796000944L;

        FairSync(int permits) {
            super(permits);
        }

        protected int tryAcquireShared(int acquires) {
            for (;;) {
                // 如果当前线程不位于队头，则阻塞
                if (hasQueuedPredecessors())
                    return -1;
                int available = getState();
                int remaining = available - acquires;
                // 设置获取后的剩余通行量
                // 当剩余量小0时，则阻塞，否则通行
                if (remaining < 0 || compareAndSetState(available, remaining))
                    return remaining;
            }
        }
    }

    public void acquire() throws InterruptedException {
        // 获取一次通行，可中断
        sync.acquireSharedInterruptibly(1);
    }

    public void acquireUninterruptibly() {
        // 获取一次通行
        sync.acquireShared(1);
    }

    public boolean tryAcquire() {
        // 尝试获取一次通行
        return sync.nonfairTryAcquireShared(1) >= 0;
    }

    public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException {
        // 尝试在指定时间内获取一次通行
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    public void acquire(int permits) throws InterruptedException {
        if (permits < 0)
            throw new IllegalArgumentException();
        // 获取指定通行次数，可中断
        sync.acquireSharedInterruptibly(permits);
    }

    public void acquireUninterruptibly(int permits) {
        if (permits < 0)
            throw new IllegalArgumentException();
        // 获取指定通行次数
        sync.acquireShared(permits);
    }

    public boolean tryAcquire(int permits) {
        if (permits < 0)
            throw new IllegalArgumentException();
        // 尝试获取指定通行次数
        return sync.nonfairTryAcquireShared(permits) >= 0;
    }

    public boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException {
        if (permits < 0)
            throw new IllegalArgumentException();
        // 尝试在指定时间内获取通行
        return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
    }

    public void release() {
        // 释放一次通行
        sync.releaseShared(1);
    }

    public void release(int permits) {
        if (permits < 0)
            throw new IllegalArgumentException();
        // 释放指定的通行次数
        sync.releaseShared(permits);
    }

    public int availablePermits() {
        // 返回剩余的通行量
        return sync.getPermits();
    }

    public int drainPermits() {
        // 直接把通行量设置为0
        return sync.drainPermits();
    }

    protected void reducePermits(int reduction) {
        if (reduction < 0)
            throw new IllegalArgumentException();
        // 缩减通行量
        sync.reducePermits(reduction);
    }

    public boolean isFair() {
        // 返回是否公平锁（即是否公平分配通行）
        return sync instanceof FairSync;
    }

    public final boolean hasQueuedThreads() {
        // 返回队列中是否有线程
        return sync.hasQueuedThreads();
    }

    public final int getQueueLength() {
        // 返回队列的长度
        return sync.getQueueLength();
    }

    protected Collection<Thread> getQueuedThreads() {
        // 返回队列中的所有线程
        return sync.getQueuedThreads();
    }

    public String toString() {
        return super.toString() + "[Permits = " + sync.getPermits() + "]";
    }
}