JDK源码分析之ReentrantLock

ReentrantLock可重入锁基于AQS类实现。AQS提供了一种实现阻塞锁和一系列依赖FIFO等待队列的同步器的框架。ReentrantLock基于AbstractQueuedSynchronizer实现了Sync类

 /**
     * Base of synchronization control for this lock. Subclassed
     * into fair and nonfair versions below. Uses AQS state to
     * represent the number of holds on the lock.
     */
    abstract static class Sync extends AbstractQueuedSynchronizer {}

ReentrantLock分为公平锁和非公平锁，都继承Sync类进行实现，NonfairSync是非公平锁。FairSync 是公平锁。

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

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

ReentrantLock根据构造方法确定使用公平或者非公平锁

    /**
     * Creates an instance of {@code ReentrantLock}.
     * This is equivalent to using {@code ReentrantLock(false)}.
     */
    public ReentrantLock() {
        sync = new NonfairSync();
    }

    /**
     * Creates an instance of {@code ReentrantLock} with the
     * given fairness policy.
     *
     * @param fair {@code true} if this lock should use a fair ordering policy
     */
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }

非公平锁NonfairSync的加锁和释放

    /**
     * Sync object for non-fair locks
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        //尝试加锁
        final void lock() {
        //通过CAS将变量state从0更新为1
            if (compareAndSetState(0, 1))
        //变量更新成功将独占锁的主线程设置为当前线程
                setExclusiveOwnerThread(Thread.currentThread());
            else
        //变量更新失败，调用acquire再次尝试更新,此处回调用到nonfairTryAcquire()
                acquire(1);
        }
       
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

state变量更新失败，调用nonfairTryAcquire再次尝试更新

        /**
         * Performs non-fair tryLock.  tryAcquire is implemented in
         * subclasses, but both need nonfair try for trylock method.
         */
        final boolean nonfairTryAcquire(int acquires) {

            //获取当前线程
            final Thread current = Thread.currentThread();
            //取得当前state
            int c = getState();
            //判断是不是未加锁状态
            if (c == 0) {
                //尝试CAS获取修改state并将当前线程设置为独占主线程
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            //没有设置成功，就判断当前线程是不是独占锁的主线程，如果是则在state的基础上+1
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }

            //如果也不是当前线程就返会false
            return false;
        }

Acquire失败后先进行addWaiter方法，然后进行acquireQueued

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

我们先看addWaiter(Node.EXCLUSIVE), arg)

    /**
     * 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) {
        //将当前线程封装为node结构，因为FIFO队列存储的是node数据结构
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        //获取当前队列尾部节点 ，如果不为空，则CAS进行入队列，将当前node更新为尾节点
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        //如果尾节点为空，则进入enq()初始化队列;
        enq(node);
        return node;
    }

我们紧跟着看些enq方法干了什么

 /**
     * 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) {
        //自旋
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
          //CAS方式初始化一个node作为队列的头节点，同时这个节点也是尾节点
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
            //自旋第二次就把当前node作为尾节点，并返回。
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

这个时候我们通过addWaiter返回了一个刚放入队列的node，acquireQueued方法会再次更新

  /**
     * 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();
                //如果node的前驱节点是head就进行尝试acquire，成功则把当前节点设置为head。将当前节点中的thread设置为null、将当前节点的prev设置为null。
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //否则将线程挂起
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

ReentrantLock的unlock方法。

//ReentrantLock.Sync
        protected final boolean tryRelease(int releases) {
            //定义state-1之后的值
            int c = getState() - releases;
            //如果当前线程不是独占的主线程，抛出异常
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            //定义free表示是否已释放
            boolean free = false;
            //如果state==0则表示可以释放。OwnerThread设置为空
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            //更新state
            setState(c);
            return free;

        }

//AbstractQueuedSynchronizer
 public final boolean release(int arg) {
        //释放成功后，获取队列头节点进行唤醒操作
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }


 /**
     * 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);
    }

以上就是非公平锁的源码实现

公平锁的源码实现

公平锁和非公平锁的区别就在于，获取资源的时候没有用当前的线程去操作，而是只从队列头部取节点进行尝试改变state.

/**
 * Sync object for fair locks
 */
static final class FairSync extends Sync {
    private static final long serialVersionUID = -3000897897090466540L;

    final void lock() {
        //没有用当前线程尝试改变state
        acquire(1);
    }

    /**
     * Fair version of tryAcquire.  Don't grant access unless
     * recursive call or no waiters or is first.
     */
    protected final boolean tryAcquire(int acquires) {
        final Thread current = Thread.currentThread();
        int c = getState();
        if (c == 0) {
            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;
    }
}

公平的方法

    public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
      //如果头节点没有next 返回false
       //如果头节点有next  并且不是当前线程返回true
      //如果头节点有next  并且是当前线程返回true
     
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }