jdk源码解析四之Semaphore

野性酷女 2023-02-20 12:08 11阅读 0赞

### 文章目录 ###

*  Semaphore
 *   *  acquire
     *  release
     *  总结

# Semaphore #

计数信号量:控制同时访问某个特定资源的操作数量,或者同时执行某个指定操作的数量,实现某种资源池,对容器施加边界

## acquire ##

步骤同CountDownLatch.wait

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

protected int tryAcquireShared(int acquires) {
        
                return nonfairTryAcquireShared(acquires);
            }

final int nonfairTryAcquireShared(int acquires) {
        
            //<0才阻塞
                for (;;) {
        
                    //获取同步状态的值。
                    int available = getState();
                    int remaining = available - acquires;
                    //available为0或者更新成功,直接返回
                    if (remaining < 0 ||
                        compareAndSetState(available, remaining))
                        return remaining;
                }
            }

## release ##

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

protected final boolean tryReleaseShared(int releases) {
        
                for (;;) {
        
                    //获取同步状态的值。
                    int current = getState();
                    //累加
                    int next = current + releases;
                    //内存溢出,也就是超过了Integer范围上限
                    if (next < current) // overflow
                        throw new Error("Maximum permit count exceeded");
                    //修改值返回
                    if (compareAndSetState(current, next))
                        return true;
                }
            }

## 总结 ##

与闭锁和其类似,闭锁是为0,才可以访问,信号量恰恰相反,  
信号量和闭锁的区别是不是一次性的,可以重复限定多少线程可以访问资源.  
如果不限定释放的条件,可以不停的释放从而超过构造设置的许可个数,  
建议这样使用

public boolean add(T o) throws InterruptedException {
        
            //阻塞获得许可
            sem.acquire();
            boolean wasAdded = false;
            try {
        
                wasAdded = set.add(o);
                return wasAdded;
            } finally {
        
                //返回许可给信号量
                if (!wasAdded)
                    sem.release();
            }
        }
    
        public boolean remove(Object o) {
        
            boolean wasRemoved = set.remove(o);
            if (wasRemoved)
                sem.release();
            return wasRemoved;
        }