在同步器中的節(jié)點，始終是有順序的。非公平指的是沒有進入同步隊列的線程與首節(jié)點的后繼節(jié)點中的線程非公平，也有可能是首節(jié)點中的線程和首節(jié)點后繼節(jié)點中的線程非公平競爭，比如首節(jié)點中的線程釋放鎖后又立即去獲取鎖。
體現(xiàn)在ReentrantLock代碼中話，就是NonfairSync的tryAcquire實現(xiàn)和FairSync的tryAcquire實現(xiàn)是不一樣的。

static final class NonfairSync extends Sync {
        。。。。。。
        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;
}
static final class FairSync extends Sync {
        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;
        }
}
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());
}

FairSync的tryAcquire方法，多了一個!hasQueuedPredecessors()判斷。如果同步隊列為空，或者只有首節(jié)點，或者首節(jié)點的后繼節(jié)點中的線程是當(dāng)前線程，那么hasQueuedPredecessors()為false,!hasQueuedPredecessors()為true，調(diào)用tryAcquire的當(dāng)前線程就可以去獲取同步狀態(tài)。