Semaphore 源码解析.md

#### Semaphore

限制可以访问某些资源（物理或逻辑的）的线程数目

* 实际上是就是普通共享锁，内部静态类 Sync 继承了 AQS，实现了共享锁的 tryAcquireShared、tryReleaseShared。

* 逻辑就是正常共享锁逻辑，同时还可以实现公平和非公平
* 可以说是 ReentrantLock 的共享版，所以注意**要在 finally 里面调用 semaphore.release() 释放锁**

##### Sync 抽象类

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

        Sync(int permits) {
            setState(permits);
        }

        final int getPermits() {
            return getState();
        }

    	// 父类提供了非公平的 tryAcquireShared 方法
        final int nonfairTryAcquireShared(int acquires) {
            for (;;) {
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }

        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");
                if (compareAndSetState(current, next))
                    return true;
            }
        }

    	// 和 tryReleaseShared 一个效果，但是不会唤醒 CLH 的线程，不过下一次 tryReleaseShared 唤醒后就可以用这个释放的。
        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;
            }
        }

    	// 重置为 0，但是就算重置为 0，后面通过 aquirceShared 也就能进 permits 个
        final int drainPermits() {
            for (;;) {
                int current = getState();
                if (current == 0 || compareAndSetState(current, 0))
                    return current;
            }
        }
}
```



##### Sync 实现类

* NonfairSync

```java
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);
    }
}
```

* FairSync

```java
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()) // 多了个判断 CLH 队列还有没有等待的节点
                return -1;
            int available = getState();
            int remaining = available - acquires;
            if (remaining < 0 ||
                compareAndSetState(available, remaining))
                return remaining;
        }
    }
}
```



##### 方法

###### 构造方法

```java
public Semaphore(int permits) {
    // 默认选择非公平锁
    sync = new NonfairSync(permits);
}

public Semaphore(int permits, boolean fair) {
    sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}
```



###### acquire

```java
public void acquire() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}
```



###### release

```java
public void release() {
    sync.releaseShared(1);
}
```



###### 应用场景

就是限制访问同步块的线程数