Lock的实现之ReentrantLock详解

摘要

Lock在硬件层面依赖CPU指令，完全由Java代码完成，底层利用LockSupport类和Unsafe类进行操作；

虽然锁有很多实现，但是都依赖AbstractQueuedSynchronizer类，我们用ReentrantLock进行讲解；

ReentrantLock调用过程

ReentrantLock类的API调用都委托给一个内部类 Sync ,而该类继承了 AbstractQueuedSynchronizer类；

public class ReentrantLock implements Lock, java.io.Serializable {
    ......
    abstract static class Sync extends AbstractQueuedSynchronizer {
......

而Sync又分为两个子类：公平锁和非公平锁，默认为非公平锁

 /** * Sync object for non-fair locks */ static final class NonfairSync extends Sync { 

 /** * Sync object for fair locks */ static final class FairSync extends Sync { 

Lock的调用过程如下图(其中涉及到 ReentrantLock类、Sync(抽象类)、AbstractQueuedSynchronizer类，NofairSync类，这些类将 Template方法用的淋漓尽致，相当赞)：

先来一张类依赖图：

再来一张lock调用图：

Lock API详解

自底而上来看，由被调用一步步向上分析

nofairTryAcquire

/**
 * Performs non-fair tryLock.  tryAcquire is implemented in
 * subclasses, but both need nonfair try for trylock method.
 */
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;
}

来看这段代码，首先获取当前状态（初始化为0），当它等于0的时候，代表还没有任何线程获得该锁，然后通过CAS（底层是通过CompareAndSwapInt实现）改变state，并且设置当前线程为持有锁的线程；其他线程会直接返回false;当该线程重入的时候，state已经不等于0，这个时候并不需要CAS，因为该线程已经持有锁，然后会重新通过setState设置state的值，这里就实现了一个偏向锁的功能，即锁偏向该线程；

addWaiter

只有当锁被一个线程持有，另外一个线程请求获得该锁的时候才会进入这个方法

/**
 * Creates and enqueues node for current thread and given mode.
 *
 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
 * @return the new node
 */
private Node addWaiter(Node mode) {
    Node node = new Node(Thread.currentThread(), mode);
    // Try the fast path of enq; backup to full enq on failure
    Node pred = tail;
    if (pred != null) {
        node.prev = pred;
        if (compareAndSetTail(pred, node)) {
            pred.next = node;
            return node;
        }
    }
    enq(node);
    return node;
}

首先持有该锁之外的线程进入到该方法，这里涉及到一个CLH（三个人的名字首字母：Craig, Landin, and Hagersten）队列，其实就是一个链表，

简单说下CLH队列：

CLH队列由node节点组成，mode代表每个Node有两种模式：共享模式和排他模式，并且维护了一个状态：waitStatus，可取值如下：

1. CANCELLED = 1    由于超时或者被打断，该线程被取消，将不会被block；
2. SIGNAL = -1    当前线程的后继节点线程通过park正处于或即将处于block状态；
3. CONDITION = -2    当前线程正处于条件队列，正式因为调用了condition.await造成阻塞；
4. PROPAGATE = -3    共享锁应该被传播出去

首先，new一个节点，这个时候模式为：mode为 Node.EXCLUSIVE，默认为null即排它锁；

然后：

如果该队列已经有node即tail!=null，则将新节点的前驱节点置为tail，再通过CAS将tail指向当前节点，前驱节点的后继节点指向当前节点，然后返回当前节点；

如果队列为空或者CAS失败，则通过enq入队：

/**
 * Inserts node into queue, initializing if necessary. See picture above.
 * @param node the node to insert
 * @return node's predecessor
 */
private Node enq(final Node node) {
    for (;;) {
        Node t = tail;
        if (t == null) { // Must initialize
            if (compareAndSetHead(new Node()))
                tail = head;
        } else {
            node.prev = t;
            if (compareAndSetTail(t, node)) {
                t.next = node;
                return t;
            }
        }
    }
}

进队的时候，要么是第一个入队并且设置head节点并且循环设置tail，要么是add tail，如果CAS不成功，则会无限循环，直到设置成功，即使高并发的场景，也最终能够保证设置成功，然后返回包装好的node节点；

acquireQueued

/**
 * Acquires in exclusive uninterruptible mode for thread already in
 * queue. Used by condition wait methods as well as acquire.
 *
 * @param node the node
 * @param arg the acquire argument
 * @return {@code true} if interrupted while waiting
 */
final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

该方法的主要作用就是将已经进入虚拟队列的节点进行阻塞，我们看到，如果当前节点的前驱节点是head并且尝试获取锁的时候成功了，则直接返回，不需要阻塞；

如果前驱节点不是头节点或者获取锁的时候失败了，则进行判定是否需要阻塞：

/**
 * Checks and updates status for a node that failed to acquire.
 * Returns true if thread should block. This is the main signal
 * control in all acquire loops.  Requires that pred == node.prev.
 *
 * @param pred node's predecessor holding status
 * @param node the node
 * @return {@code true} if thread should block
 */
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.
         */
        return true;
    if (ws > 0) {
        /*
         * Predecessor was cancelled. Skip over predecessors and
         * indicate retry.
         */
        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.
         */
        compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
    }
    return false;
}

这段代码对该节点的前驱节点的状态进行判断，如果前驱节点已经处于signal状态，则返回true，表明当前节点可以进入阻塞状态；

否则，将前驱节点状态CAS置为signal状态，然后通过上层的for循环进入parkAndCheckInterrupt代码块park：

/**
 * Convenience method to park and then check if interrupted
 *
 * @return {@code true} if interrupted
 */
private final boolean parkAndCheckInterrupt() {
    LockSupport.park(this);
    return Thread.interrupted();
}

这个时候将该线程交给操作系统内核进行阻塞；

总体来讲，acquireQueued就是依靠前驱节点的状态来决定当前线程是否应该处于阻塞状态，如果前驱节点处于cancel状态，则丢弃这些节点，重新构建队列；

Unlock API详解

流程类似lock api相关类的流程，这里讲主要的代码，unlock相对的比较简单

首先 ReentrantLock 调用 Sync的release接口也就是AbstractQueuedSynchronizer的release接口

/**
 * Releases in exclusive mode.  Implemented by unblocking one or
 * more threads if {@link #tryRelease} returns true.
 * This method can be used to implement method {@link Lock#unlock}.
 *
 * @param arg the release argument.  This value is conveyed to
 *        {@link #tryRelease} but is otherwise uninterpreted and
 *        can represent anything you like.
 * @return the value returned from {@link #tryRelease}
 */
public final boolean release(int arg) {
    if (tryRelease(arg)) {
        Node h = head;
        if (h != null && h.waitStatus != 0)
            unparkSuccessor(h);
        return true;
    }
    return false;
}

这个时候会先调用Sync的tryRelease，如果返回true，则释放锁成功

protected final boolean tryRelease(int releases) {
    int c = getState() - releases;
    if (Thread.currentThread() != getExclusiveOwnerThread())
        throw new IllegalMonitorStateException();
    boolean free = false;
    if (c == 0) {
        free = true;
        setExclusiveOwnerThread(null);
    }
    setState(c);
    return free;
}

这个接口的作用很简单，如果不是获得锁的线程调用直接抛出异常，否则，如果当前state-releases==0也就是lock已经完全释放，返回true，清除资源；

这个返回free之后，release拿到head节点，进入以下代码：

/**
 * Wakes up node's successor, if one exists.
 *
 * @param node the node
 */
private void unparkSuccessor(Node node) {
    /*
     * If status is negative (i.e., possibly needing signal) try
     * to clear in anticipation of signalling.  It is OK if this
     * fails or if status is changed by waiting thread.
     */
    int ws = node.waitStatus;
    if (ws < 0)
        compareAndSetWaitStatus(node, ws, 0);
 
    /*
     * Thread to unpark is held in successor, which is normally
     * just the next node.  But if cancelled or apparently null,
     * traverse backwards from tail to find the actual
     * non-cancelled successor.
     */
    Node s = node.next;
    if (s == null || s.waitStatus > 0) {
        s = null;
        for (Node t = tail; t != null && t != node; t = t.prev)
            if (t.waitStatus <= 0)
                s = t;
    }
    if (s != null)
        LockSupport.unpark(s.thread);
}

这个作用即：当头结点的状态小于0，则将头结点的状态CAS为0，然后通过链表获取下一个节点，如果下一个节点为null或者不符合要求的状态，则从队尾遍历整个链表，直到遍历到离head节点最近的一个节点并且

等待状态符合预期，则将头结点的后继节点置为该节点；

对刚刚筛出来的符合要求的节点唤醒，也就是该节点获得 争夺 锁的权利；

这就是非公平锁的特点：在队列一直等待的线程不一定比后来的线程先获得锁，至此，unlock 已经解释完成；