Java并发之FairSync和NonfairSync

Java并发中的fairSync和NonfairSync主要区别为：

如果当前线程不是锁的占有者,则NonfairSync并不判断是否有等待队列,直接使用compareAndSwap去进行锁的占用;

如果当前线程不是锁的占有者,则FairSync则会判断当前是否有等待队列,如果有则将自己加到等待队列尾;

对应的源码如下：

FairSync （注意FairSync和NonFairSync均继承自AbstractQueuedSynchronizer）：

protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
//判断是否有等待队列,没有队列时，进行占用，如果占用失败,将自己加到等待队列尾
if(!hasQueuedPredecessors() &&
            compareAndSetState(0, acquires)) {
return true;
        }
            setExclusiveOwnerThread(current);
            compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
            compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
            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;
}

　　

NonFairSync：

final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        //不判断是否有等待队列,直接进行占用,如果占用失败也进到等待队列尾
        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;
}

　　

占用失败后，将自己加到等待队列尾的动作在AbstractQueuedSynchronizerder类acquire方法中:

public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}

　　

由于FairSync和NonFairSync均继承AbstractQueuedSynchronizerder(AQS)，
这里使用了到了一个设计模式（模板模式）来设计NonFairSync和FairSync类