基于ReentrantLock的实现原理讲解

目录

• ReentrantLock实现核心–AQS(AbstractQueuedSynchronizer)

• Node结构

• 二者关联
• NonfairSync分析
• FairSync分析

        ReentrantLock lock = new ReentrantLock();
		//尝试获取锁
        lock.lock();
        //获得锁后执行逻辑......
        //......
		//解锁
        lock.unlock();

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

final void lock() {
    if (compareAndSetState(0, 1))//【step1】
        setExclusiveOwnerThread(Thread.currentThread());//【step2】
    else
        acquire(1);//【step3】
}

protected final boolean compareAndSetState(int expect, int update) {
    // See below for intrinsics setup to support this
    return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}

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();
    int c = getState();
    if (c == 0) {
        //如果此时state已经变为1，再次尝试一次获取锁
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {
        //否则判断当前持有AQS的锁的线程是不是当前请求获取锁的线程，是的话就进行锁重入。
        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) {
    //新建一个Node节点，mode传入表示当前线程之间竞争是互斥模式
    Node node = new Node(Thread.currentThread(), mode);
    // Try the fast path of enq; backup to full enq on failure
    //尝试添加当前新建Node到链表队列末尾
    Node pred = tail;
    if (pred != null) {
        node.prev = pred;
        //利用cas设置尾指针指向的节点为当前线新建节点
        if (compareAndSetTail(pred, node)) {
            pred.next = node;
            return node;
        }
    }
    //当前是空的没有任何正在等待的线程Node的时候，执行enq(node)，初始化等待队列
    enq(node);
    return node;
}

private Node enq(final Node node) {
    for (;;) {
        Node t = tail;
        if (t == null) { // Must initialize
            //新建一个空的头节点
            if (compareAndSetHead(new Node()))
                tail = head;
        } else {
            //跟之前一样，利用cas这只当前新建节点为尾节点
            node.prev = t;
            if (compareAndSetTail(t, node)) {
                t.next = node;
                return t;
            }
        }
    }
}

final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
            //获取当前竞争锁的线程Node的前置节点
            final Node p = node.predecessor();
            //【step1】
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            //【step2】
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
    int ws = pred.waitStatus;
    if (ws == Node.SIGNAL)
        //前置节点是-1，返回true表示线程可挂起
        return true;
    if (ws > 0) {
        //前置节点大于0表示前置节点已经取消，那么进行跳过前置节点的操作，做链表的基本删除节点操作
        do {
            node.prev = pred = pred.prev;
        } while (pred.waitStatus > 0);
        pred.next = node;
    } else {
        //如果前置节点还是0,表示前置节点Node的waitStatus是初始值，需要设置为-1，然后外层循环重新执行shouldParkAfterFailedAcquire方法，即可挂起当前线程。
        compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
    }
    return false;
}

private final boolean parkAndCheckInterrupt() {
    //阻塞挂起线程，等待唤醒
    LockSupport.park(this);
    //唤醒后，重置中断标记，线程中断标记位为不中断
    return Thread.interrupted();
}

final void lock() {
    acquire(1);
}

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

protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        //hasQueuedPredecessors判断队列是否还有别的线程在等待锁，没有的话就尝试获取lock
        //如果有别的线程在等待锁，就不会尝试获取lock；下面如果也不是重入的情况的话就直接进入等待队列
        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 release(int arg) {
    if (tryRelease(arg)) {
        Node h = head;
        if (h != null && h.waitStatus != 0)
            unparkSuccessor(h);
        return true;
    }
    return false;
}

protected final boolean tryRelease(int releases) {
    //获取state减掉realease，对于ReentrantLock就是默认减一
    int c = getState() - releases;
    if (Thread.currentThread() != getExclusiveOwnerThread())
        throw new IllegalMonitorStateException();
    boolean free = false;
    if (c == 0) {
        //如果减到0了，那么久释放锁
        free = true;
        //设置持有线程为null
        setExclusiveOwnerThread(null);
    }
    //设置state为新的
    setState(c);
    return free;
}

    if (Thread.currentThread() != getExclusiveOwnerThread())
        throw new IllegalMonitorStateException();