ReentrantLock的公平与非公平

/**
 * Performs lock.  Try immediate barge, backing up to normal
 * acquire on failure.
 * 在加锁的时候直接进行了cas操作去获取锁，不加入等待队列，当然如果失败，还是会和公平锁一样
 */
final void lock() {
    if (compareAndSetState(0, 1))
        setExclusiveOwnerThread(Thread.currentThread());
    else
        acquire(1);
}

protected final boolean tryAcquire(int acquires) {
    return nonfairTryAcquire(acquires);
}


/**
 * Performs non-fair tryLock.  tryAcquire is implemented in
 * subclasses, but both need nonfair try for trylock method.
 */
final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (compareAndSetState(0, acquires)) {
            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;
}

/**
 * 没有进行任何操作，直接获取锁
 */
final void lock() {
    acquire(1);
}

/**
 * Fair version of tryAcquire.  Don't grant access unless
 * recursive call or no waiters or is first.
 * 这里与非公平锁只有一个区别：判断等待队列是否有线程处于等待状态
 */
protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (!hasQueuedPredecessors() &&
            compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0)
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}