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信号量就是通过AQS的共享锁机制来实现的。这个类总体比较简单，就不做过多描述。

Sync同步器

abstract static class Sync extends AbstractQueuedSynchronizer {private static final long serialVersionUID = 1192457210091910933L;//初始化permits许可数，并用AQS的state来存放许可Sync(int permits) {setState(permits);}final int getPermits() {return getState();}//非公平获取共享锁实现final int nonfairTryAcquireShared(int acquires) {for (;;) {int available = getState();int remaining = available - acquires;if (remaining < 0 ||compareAndSetState(available, remaining))return remaining;}}//释放共享锁实现protected final boolean tryReleaseShared(int releases) {for (;;) {int current = getState();int next = current + releases;if (next < current) // 相加后小于原值，证明溢出了throw new Error("Maximum permit count exceeded");if (compareAndSetState(current, next))return true;}}final void reducePermits(int reductions) {for (;;) {int current = getState();int next = current - reductions;if (next > current) // underflowthrow 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;}}
}//非公平实现
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);}
}
//公平实现
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;}}
}

acquire

public void acquire() throws InterruptedException {//调用AQS的可响应中断获取共享锁的方法sync.acquireSharedInterruptibly(1);
}