Java併發-公平鎖與非公平鎖

1.公平鎖和非公平鎖釋放時，最後都要寫一個volatile變量的state。

 protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }

2.公平鎖獲取時，首先會去讀volatile變量。

protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!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;
        }

3.非公平鎖獲取時，首先會用CAS更新volatile變量，這個操作同時具有volatile讀和voaltile寫的內存語義。

final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

鎖釋放-獲取的內存語義的實現至少有2種:
1.利用volatile變量的寫-讀所具有的內存語義。
2.利用CAS所附帶的volatile讀和volatile寫的內存語義。