ReentrantLock从源码解析Java多线程同步学习

public ReentrantLock() {
    sync = new NonfairSync();
}

public ReentrantLock(boolean fair) {
    sync = fair ? new FairSync() : new NonfairSync();
}

static final class NonfairSync extends Sync {
    private static final long serialVersionUID = 7316153563782823691L;

    
    final void lock() {
        //CAS修改state状态
        if (compareAndSetState(0, 1))
           //修改成功设置exclusiveOwnerThread
            setExclusiveOwnerThread(Thread.currentThread());
        else
           //尝试获取资源
            acquire(1);
    }

    protected final boolean tryAcquire(int acquires) {
        return nonfairTryAcquire(acquires);
    }
}

public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}

final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    //获取当前资源标识
    int c = getState();
    if (c == 0) {
        //如果资源没被占有CAS尝试加锁
        if (compareAndSetState(0, acquires)) {
            //修改成功设置exclusiveOwnerThread
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    //资源被占有要判断占有资源的线程是不是当前线程，加锁成功设置的exclusiveOwnerThread值在这里就派上了用处
    else if (current == getExclusiveOwnerThread()) {
        //这下面就是将重入次数设置到资源标识里
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

private Node addWaiter(Node mode) {
    //构建一个当前线程的node节点 这里prev 和 next 都为null
    Node node = new Node(Thread.currentThread(), mode);
    // 指向双向链表的尾节点的引用
    Node pred = tail;
    //B线程进来目前还未构建任何队列这里肯定是空的
    if (pred != null) {
        //如果已经构建过队列会把当前线程的node节点的上一个node节点指向tail尾节点
        node.prev = pred;
        //CAS操作把当前线程的node节点设置为新的tail尾节点
        if (compareAndSetTail(pred, node)) {
            //把旧的tail尾节点的下一个node节点指向当前线程的node节点
            pred.next = node;
            return node;
        }
    }
    //尾节点为空执行
    enq(node);
    return node;
}

private Node enq(final Node node) {
    //死循环当tail 尾节点不为空才会跳出
    for (;;) {
        Node t = tail;
        if (t == null) { // Must initialize
            //用CAS构建出一个head空的node节点
            if (compareAndSetHead(new Node()))
            //将当前空的node节点交给尾节点（下一次循环就会走else分支）
                tail = head;
        } else {
            //把我们addWaiter中创建的node节点的prev指向了当前线程node节点
            node.prev = t;
            //将tail尾节点更改为当前线程的node节点
            if (compareAndSetTail(t, node)) {
                //将t的下一个节点指向当前线程创建的node节点
                t.next = node;
                return t;
            }
        }
    }
}

final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        //中断标志位
        boolean interrupted = false;
        for (;;) {
            //获取当前线程node节点的上一个节点
            final Node p = node.predecessor();
            //如果上一个节点就是head节点说明当前线程其实是处在队列第一位然后就会再次尝试加锁
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            //这里是重点 这个方法来判断当前线程是否应该进入等待状态
            if (shouldParkAfterFailedAcquire(p, node) &&
                //调用LockSupport.park(this)阻塞了当前线程等待被唤醒
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
    //获取上一个节点的等待状态
    int ws = pred.waitStatus;
    if (ws == Node.SIGNAL)//表示当前节点的后继节点包含的线程需要运行
        
        return true;
    if (ws > 0) {//当前线程被取消
        
        do {
            node.prev = pred = pred.prev;
        } while (pred.waitStatus > 0);
        pred.next = node;
    } else {
        
        compareAndSetWaitStatus(pred, ws, Node.SIGNAL);//设置等待状态为-1
    }
    return false;
}

public void unlock() {
    sync.release(1);
}

public final boolean release(int arg) {
   //尝试释放锁
    if (tryRelease(arg)) {
        Node h = head;
        if (h != null && h.waitStatus != 0)
            //head节点不为空并且等待状态不是0就去进行unpark操作
            unparkSuccessor(h);
        return true;
    }
    return false;
}

private void unparkSuccessor(Node node) {
    
    //head节点等待状态
    int ws = node.waitStatus;
    //
    if (ws < 0)
       //将头节点的等待状态修改为0
        compareAndSetWaitStatus(node, ws, 0);

    
    //获取头节点的下一个节点(也就是B节点)
    Node s = node.next;
    if (s == null || s.waitStatus > 0) {//节点为空或者线程处于取消状态
        s = null;
        //从尾节点往上找符合条件的节点
        for (Node t = tail; t != null && t != node; t = t.prev)
            if (t.waitStatus <= 0)
                s = t;
    }
    if (s != null)
        //对该线程进行unpark唤醒(B节点)
        LockSupport.unpark(s.thread);
}

final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();
            //这里尝试获取锁由于A线程释放了锁这里是肯定获取成功的
            if (p == head && tryAcquire(arg)) {
                //把head设置为当前节点（也就是往前移一位,并且把上一个节点指向指为null）
                setHead(node);
                //把刚刚的上一个节点也指向为null (这里他就没引用了会被GC回收掉)
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                //刚刚在这里阻塞现在被唤醒
                parkAndCheckInterrupt())
                //设置标志中断位为true 然后开始下一次循环
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}