//默認(rèn)實現(xiàn)為非公平鎖微猖，這個構(gòu)造方法相當(dāng)于new  ReentrantLock(false) false 使用非公平鎖
public ReentrantLock() {
        sync = new NonfairSync();
    }


  public final boolean tryAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
            //判斷當(dāng)前線程是否被中斷
        if (Thread.interrupted())
            throw new InterruptedException();
            //2種方式獲取鎖方法
        return tryAcquire(arg) ||
            doAcquireNanos(arg, nanosTimeout);
    }

//try Acquire(arg) 最終會調(diào)用這個方法
final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
                    //獲取當(dāng)前鎖狀態(tài)
            int c = getState();
                    //如果鎖是空閑時
            if (c == 0) {
              //cas 更改state值
                if (compareAndSetState(0, acquires)) {
                    //成功的話設(shè)置當(dāng)前鎖的獨占線程為當(dāng)前線程
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
                    //鎖已經(jīng)被持有且持有者為當(dāng)前線程
            else if (current == getExclusiveOwnerThread()) {
                //state增加
                int nextc = c + acquires;
                //如果增加后小于0.則報錯
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                //設(shè)置新state值
                setState(nextc);
                return true;
            }
            return false;
        }

        //以獨占定時模式獲取鎖 轿曙，在指定時間nanosTimeout內(nèi)不斷獲取鎖犀勒，并且始終都是當(dāng)前節(jié)點的上一個節(jié)點先獲取到鎖
        //應(yīng)該是公平鎖的獲取方式
    private boolean doAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
      //獲取到超時的時間點
        final long deadline = System.nanoTime() + nanosTimeout;
        //加入隊列末尾
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
              //獲取當(dāng)前節(jié)點的上一個節(jié)點
                final Node p = node.predecessor();
              //如果是頭節(jié)點且獲取鎖成功 tryAcquire（非公平鎖方法）
                if (p == head && tryAcquire(arg)) {
                  //將頭節(jié)點變?yōu)楫?dāng)前節(jié)點
                    setHead(node);
                  //頭節(jié)點認(rèn)為設(shè)置null副女，方便垃圾回收
                    p.next = null; // help GC
                    failed = false;
                    return true;
                }
                nanosTimeout = deadline - System.nanoTime();
                //如果超時，則直接返回
                if (nanosTimeout <= 0L)
                    return false;
                
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                  //虛假中斷
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
          //失敗的話垦写，取消獲取
            if (failed)
                cancelAcquire(node);
        }
    }

//加入等待隊列
private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
            //最新的對尾元素
        Node pred = tail;
            //如果隊列存在
        if (pred != null) {
            node.prev = pred;
            //設(shè)置當(dāng)前隊尾元素為當(dāng)前任務(wù)
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }


//初始化隊列
 private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
              //設(shè)置當(dāng)前頭元素
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
              //加入隊為 
                node.prev = t;
              //設(shè)置當(dāng)前隊尾元素
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }


    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            //說明需要通知后續(xù)節(jié)點
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
          //大于0說明該搶鎖任務(wù)被取消了
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
          //waitStatus 為0 或者-3時 說明需要重試 設(shè)置waitStatus的值
          //設(shè)置一個信號量吕世，為后續(xù)喚醒
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

//公平鎖的加鎖方式
   protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
              //如果當(dāng)前線程前面有一個排隊線程，則等待梯投，如果當(dāng)前線程位于隊列的頭部或隊列為空命辖，則直接獲取
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
                    //如果當(dāng)前線程是持有鎖線程，則直接獲取
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }