公平鎖和非公平鎖

Lock鎖分為公平鎖和非公平鎖兩種

公平鎖：線程獲取鎖的順序是按照線程加鎖的順序來分配的，即先來先得的FIFO先進先出順序

非公平鎖：一種獲取鎖的搶占機制，是隨機獲取鎖的，和公平鎖的區別就是先來的不一定先得到鎖，導致某些線程可能一直拿不到鎖，所以是不公平的

　　公平鎖，就是很公平，在並發環境中，每個線程在獲取鎖時會先查看此鎖維護的等待隊列，如果為空，或者當前線程是等待隊列的第一個，就占有鎖，否則就會加入到等待隊列中，以后會按照FIFO的規則從隊列中取到自己；非公平鎖比較粗魯，上來就直接嘗試占有鎖，如果嘗試失敗，就再采用類似公平鎖那種方式

--------------------------------------公平鎖--------------------------------------

    protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {//狀態是0，說明當前線程沒有占有鎖
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);//當前線程占有鎖
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {//如果當前線程本身持有鎖，疊加狀態值，繼續持有
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
------------------------------------------------------------------
    public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }

--------------------------------------非公平鎖--------------------------------------

　　　　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;
        }