ReentrantReadWriteLock中有2個(gè)對(duì)象ReadLock,WriteLock分別都有l(wèi)ock函數(shù)：
readlock.lock()

public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
}

protected final int tryAcquireShared(int unused) {
            Thread current = Thread.currentThread();
            int c = getState();
        
            if (exclusiveCount(c) != 0 && //如果已經(jīng)有寫鎖
                getExclusiveOwnerThread() != current) //且寫鎖線程不是當(dāng)前線程，則獲得鎖失敗返回
                return -1;
            int r = sharedCount(c);
            if (!readerShouldBlock() && //看下面，如果讀不需要被阻塞
                r < MAX_COUNT &&
                compareAndSetState(c, c + SHARED_UNIT)) {//且cas成功，表示獲得到讀鎖
                //下面這一大段都是緩存，提高執(zhí)行效率，可以看上一節(jié)
                if (r == 0) {
                    firstReader = current;
                    firstReaderHoldCount = 1;
                } else if (firstReader == current) {
                    firstReaderHoldCount++;
                } else {
                    HoldCounter rh = cachedHoldCounter;
                    if (rh == null || rh.tid != getThreadId(current))
                        cachedHoldCounter = rh = readHolds.get();
                    else if (rh.count == 0)
                        readHolds.set(rh);
                    rh.count++;
                }
                return 1;
            }
        //如果獲取鎖失敗，則再次去嘗試獲取鎖
            return fullTryAcquireShared(current);
        }

readerShouldBlock分別在公平鎖和非公平鎖中：
FairSync.readerShouldBlock

final boolean readerShouldBlock() {
           //看隊(duì)列中是否有排隊(duì)的線程，公平鎖中只要前面有人在排隊(duì)那么當(dāng)前線程就應(yīng)該被阻塞，不能去競(jìng)爭(zhēng)鎖，必須進(jìn)入排隊(duì)
            return hasQueuedPredecessors();
        }

NonfairSync.readerShouldBlock

final boolean readerShouldBlock() {
         //隊(duì)列中第一個(gè)排隊(duì)的線程(隊(duì)列中第二個(gè)元素)是寫線程
           //非公平鎖中，如果第一個(gè)排隊(duì)的是寫，那么不能去競(jìng)爭(zhēng)；如果是讀，讀讀是共享的，而且是非公平，那么就可以去搶一下
            return apparentlyFirstQueuedIsExclusive();
        }