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