AQS源码浅析(7)——独占模式ReentrantLock

ReentrantLock的基本语义与synchronized相同，但相对synchronized更加灵活。
ReentrantLock有两种模式：公平模式和非公平模式。

ReentrantLock加锁的实现

公平模式下，如果有线程正在等待，则不参与竞争，对AQS的tryAcquire实现如下：

protected final boolean tryAcquire(int acquires) { //尝试获取锁
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) { //锁没有被占用
        if (!hasQueuedPredecessors() && // 队列前面是否有正在等待的线程
            compareAndSetState(0, acquires)) { //没有则尝试加锁
            setExclusiveOwnerThread(current); //加锁成功，设置独占线程
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) { //锁正被当前线程占用
        int nextc = c + acquires;
        if (nextc < 0) //溢出了，最大可重入是int的最大值
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

非公平模式下，直接参与竞争，对AQS的tryAcquire实现如下：

final void lock() {//非公平，直接竞争一次
    if (compareAndSetState(0, 1))
        setExclusiveOwnerThread(Thread.currentThread());
    else
        acquire(1);
}
protected final boolean tryAcquire(int acquires) {
    return nonfairTryAcquire(acquires);
}
final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (compareAndSetState(0, acquires)) {//不管队列有没有排队的，被唤醒就去竞争锁
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

ReentrantLock对解锁实现

先看一下ReentrantLock的基本实现，我们仅看解锁相关方法，实现很简单易懂

protected final boolean tryRelease(int releases) {
    int c = getState() - releases;
    if (Thread.currentThread() != getExclusiveOwnerThread())
        throw new IllegalMonitorStateException();
    boolean free = false;
    if (c == 0) {
        free = true;
        setExclusiveOwnerThread(null);
    }
    setState(c);
    return free;
}