reentrantlock如何实现公平锁和非公平锁

下面我们来看一下公平锁与非公平锁的加锁方法的源码。

公平锁的锁获取源码如下：

protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (!hasQueuedPredecessors() && //这里判断了 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;
}
protected final boolean tryAcquire(int acquires) {

    final Thread current = Thread.currentThread();
    int c = getState();

    if (c == 0) {

        if (!hasQueuedPredecessors() && //这里判断了 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;
}
protected final boolean tryAcquire(int acquires) {

    final Thread current = Thread.currentThread();
    int c = getState();

    if (c == 0) {

        if (!hasQueuedPredecessors() && //这里判断了 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;
}

非公平锁的锁获取源码如下：

final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (compareAndSetState(0, acquires)) { //这里没有判断      hasQueuedPredecessors()
            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;
}
final boolean nonfairTryAcquire(int acquires) {

    final Thread current = Thread.currentThread();
    int c = getState();

    if (c == 0) {

        if (compareAndSetState(0, acquires)) { //这里没有判断      hasQueuedPredecessors()

            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;

}
final boolean nonfairTryAcquire(int acquires) {

    final Thread current = Thread.currentThread();
    int c = getState();

    if (c == 0) {

        if (compareAndSetState(0, acquires)) { //这里没有判断      hasQueuedPredecessors()

            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;

}

通过对比，我们可以明显的看出公平锁与非公平锁的lock()方法唯一的区别就在于公平锁在获取锁时多了一个限制条件：hasQueuedPredecessors() 为false，这个方法就是判断在等待队列中是否已经有线程在排队了。

这也就是公平锁和非公平锁的核心区别，如果是公平锁，那么一旦已经有线程在排队了，当前线程就不再尝试获取锁；对于非公平锁而言，无论是否已经有线程在排队，都会尝试获取一下锁，获取不到的话，再去排队。

这里有一个特例需要我们注意，针对 tryLock() 方法，它不遵守设定的公平原则。

例如，当有线程执行 tryLock() 方法的时候，一旦有线程释放了锁，那么这个正在tryLock的线程就能获取到锁，即使设置的是公平锁模式，即使在它之前已经有其他正在等待队列中等待的线程，简单地说就是tryLock可以插队。

看它的源码就会发现：

public boolean tryLock() {
    return sync.nonfairTryAcquire(1);
}
public boolean tryLock() {

    return sync.nonfairTryAcquire(1);

}
public boolean tryLock() {

    return sync.nonfairTryAcquire(1);

}

这里调用的就是 nonfairTryAcquire()，表明了是不公平的，和锁本身是否是公平锁无关。

综上所述，公平锁就是会按照多个线程申请锁的顺序来获取锁，从而实现公平的特性。

非公平锁加锁时不考虑排队等待情况，直接尝试获取锁，所以存在后申请却先获得锁的情况，但由此也提高了整体的效率。