Semaphore分析

1.类声明

public class Semaphore implements java.io.Serializable {
}

2.依旧是主要委托Sync内部类实现

• Sync抽象类

// 经典AQS实现版本
abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 1192457210091910933L;

        Sync(int permits) { // 资源初始化
            setState(permits);
        }

        final int getPermits() { // 获得当前资源数
            return getState();
        }

        // 非公平方式获取许可，为什么要写在抽象类中，因为所有的tryAcquire方法都会直接以非公平方式
        // 尝试获取许可，无论初始化时使sync是公平还是非公平的。
        final int nonfairTryAcquireShared(int acquires) {
            for (;;) {  // cas减许可
                int available = getState();
                int remaining = available - acquires;
                // 如果剩余的数量小于0(共享模式下判断tryAcquireShared是看返回的整数是否小于0，小于0代表获取失败)，代表获取许可失败，AQS中tryAcquire失败将会进队列等待。
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }

        // cas释放许可，公平和非公平模式都一样
        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)) // 没有条件判断，只要许可被加上去就返回true
                    return true;
            }
        }
        // 纯粹的cas减掉一部分许可
        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;
            }
        }
        // 许可归零
        final int drainPermits() {
            for (;;) {
                int current = getState();
                if (current == 0 || compareAndSetState(current, 0))
                    return current;
            }
        }
    }

• 非公平模式

static final class NonfairSync extends Sync {
        private static final long serialVersionUID = -2694183684443567898L;

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

        protected int tryAcquireShared(int acquires) {
            // 直接使用抽象类中的方法
            return nonfairTryAcquireShared(acquires);
        }
    }

• 公平模式

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

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

        protected int tryAcquireShared(int acquires) {
            for (;;) {
                // 公平模式下将会先判断是否有线程在等待，在等待先排队
                if (hasQueuedPredecessors())
                    return -1;
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }
    }

3.具体的获取许可方法

// 获取许可直接委托sync
public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }