java併發編程學習之再談公平鎖和非公平鎖

在java併發編程學習之顯示鎖Lock裏有提過公平鎖和非公平鎖，咱們知道他的使用方式，以及非公平鎖的性能較高，在AQS源碼分析的基礎上，咱們看看NonfairSync和FairSync的區別在什麼地方。

lock方法

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

從源碼能夠看出，獲取鎖的時候，非公平鎖會先嚐試獲取，獲取不到再調用acquire方法，而公平鎖直接調用acquire方法。

tryAcquire方法

////非公平鎖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;
}
////公平鎖FairSync
protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();//無線程佔用
    if (c == 0) {
        if (!hasQueuedPredecessors() &&//看是否有前面節點在等待
            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;
}

從源碼能夠看出，公平鎖再嘗試獲取鎖的時候，先判斷隊列是否有其餘節點在等待，沒有再獲取。而非公平鎖直接嘗試獲取鎖。