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java并发编程学习之再谈公平锁和非公平锁

在 java并发编程学习之显示锁Lock 里有提过公平锁和非公平锁，我们知道他的使用方式，以及非公平锁的性能较高，在AQS源码分析的基础上，我们看看NonfairSync和FairSync的区别在什么地方。

lock方法

//非公平锁NonfairSync
final void lock() {
    if (compareAndSetState(0, 1))
        setExclusiveOwnerThread(Thread.currentThread());
    else
        acquire(1);
}
//公平锁FairSync
final void lock() {
    acquire(1);
}

从源码可以看出，获取锁的时候，非公平锁会先尝试获取，获取不到再调用acquire方法，而公平锁直接调用acquire方法。

tryAcquire方法

////非公平锁NonfairSync
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)) {
            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;
}
////公平锁FairSync
protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();//无线程占用
    if (c == 0) {
        if (!hasQueuedPredecessors() &&//看是否有前面节点在等待
            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;
}

从源码可以看出，公平锁再尝试获取锁的时候，先判断队列是否有其他节点在等待，没有再获取。而非公平锁直接尝试获取锁。

原文 https://segmentfault.com/a/1190000019856037

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