ReentrantLock原理分析

一 UML类图

通过类图ReentrantLock是同步锁，同一时间只能有一个线程获取到锁，其他获取该锁的线程会被阻塞而被放入AQS阻塞队列中。ReentrantLock类继承Lock接口；内部抽象类Sync实现抽象队列同步器AbstractQueuedSynchronizer；Sync类有两个子类NonfairSync（非公平锁）和FairSync（公平锁），默认构造方法使用非公平锁，可以使用带布尔参数的构造方法指定使用公平锁；ReentrantLock可以创建多个条件进行绑定。

二 原理分析

2.1 获取锁

2.1.1 void lock()方法

调用线程T调用该方法尝试获取当前锁。

①如果锁未被其他线程获取，则调用线程T成功获取到当前锁，然后设置当前锁的拥有者为调用线程T，并设置AQS的状态值state为1，然后直接返回。

②如果调用线程T之前已经获取当前锁，则只设置设置AQS的状态值state加1，然后返回。

③如果当前锁已被其他线程获取，则调用线程T放入AQS队列后阻塞挂起。

public void lock() {
    sync.lock();//委托内部公平锁和非公平锁获取锁
}

//非公平锁
final void lock() {
    if (compareAndSetState(0, 1))//设置AQS状态值为1
        setExclusiveOwnerThread(Thread.currentThread());//成功设置锁的线程拥有者
    else
        acquire(1);//失败加入到AQS队列阻塞挂起
}
//公平锁
final void lock() {
    acquire(1);
}

非公平锁分析：

//调用父类AbstractOwnableSynchronizer方法CAS设置state值，成功返回true，失败返回false
protected final boolean compareAndSetState(int expect, int update) {
    return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
//调用父类AbstractOwnableSynchronizer方法，设置当前线程为锁的拥有者
protected final void setExclusiveOwnerThread(Thread thread) {
    exclusiveOwnerThread = thread;
}

//调用AbstractQueuedSynchronizer父类方法，第一次获取锁失败
public final void acquire(int arg) {
    if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg))//排它锁类型
        selfInterrupt();
}
//NonfairSync子类，重写父类方法
protected final boolean tryAcquire(int acquires) {
    return nonfairTryAcquire(acquires);
}

//Sync类非公平锁尝试获取锁
final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {//二次获取锁
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {//当前线程已获取锁，AQS状态值加1
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

//AbstractQueuedSynchronizer类创建节点，添加到AQS队列后面
private Node addWaiter(Node mode) {
    Node node = new Node(Thread.currentThread(), mode);//创建AQS队列的节点，节点类型排它锁
    Node pred = tail;//尾结点
    if (pred != null) {
        node.prev = pred;//新节点的前一个节点是尾结点
        if (compareAndSetTail(pred, node)) {//CAS机制添加节点
            pred.next = node;//尾结点执行新的节点
            return node;
        }
    }
    enq(node);
    return node;
}

//插入节点到队列中
private Node enq(final Node node) {
    for (;;) {
        Node t = tail;//尾结点
        if (t == null) { // 尾结点为空，初始化
            if (compareAndSetHead(new Node()))//第一步：CAS创建头结点（哨兵节点）一个空节点
                tail = head;
        } else {
            node.prev = t;
            if (compareAndSetTail(t, node)) {//第二步：CAS设置尾结点
                t.next = node;
                return t;
            }
        }
    }
}

//
final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();//新节点的前节点
            if (p == head && tryAcquire(arg)) {//如果p节点的前节点是头结点，并且尝试给锁枷锁
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);//失败解锁
    }
}
//阻塞挂起当前线程
static void selfInterrupt() {
    Thread.currentThread().interrupt();
}

//
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;
}
//
private final boolean parkAndCheckInterrupt() {
    LockSupport.park(this);
    return Thread.interrupted();
}

公平锁分析：

protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (!hasQueuedPredecessors() && compareAndSetState(0, acquires)) {//与非公平锁相比，区别就在标红的位置
            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 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;
　　//①h != t：表示AQS队列头结点和尾结点不相同，队列不为空；
　　//②(s = h.next) == null || s.thread != Thread.currentThread()：头结点（哨兵节点）为空或者next节点不等于当前线程
    return h != t && ((s = h.next) == null || s.thread != Thread.currentThread());
}

2.1.2 void lockInterruptibly()方法

与2.2.1方法相似，不同之处在于：该方法对中断进行响应，其他线程调用当前线程中断方法，抛出InterruptedException。

2.1.3 boolean tryLock()方法

尝试获取锁。注意：该方法不会引起当前线程阻塞。

2.1.4 boolean tryLock(long timeout, TimeUnit unit)方法

与2.1.3方法相似，不同之处在于：设置了超时时间。

2.2 释放锁

尝试释放锁。

如果当前线程T已获取锁，则调用该方法更新AQS状态值减1。如果当前状态值为0，则释放锁；如果当前状态值部位0，则只是减1操作。

如果当前线程T未获取锁，则调用该方法是会抛出IllegalMonitorStateException异常。

2.2.1 void unlock()方法

public void unlock() {
    sync.release(1);
}
//调用AbstractQueuedSynchronizer方法
public final boolean release(int arg) {
    if (tryRelease(arg)) {
        Node h = head;
        if (h != null && h.waitStatus != 0)
            unparkSuccessor(h);//唤醒线程
        return true;
    }
    return false;
}
//回调Sync类释放锁
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);//设置锁的拥有线程为空
    }
    setState(c);
    return free;
}
//
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;//线程阻塞等待状态
    if (ws < 0)
        compareAndSetWaitStatus(node, ws, 0);//CAS设置状态
 
    /*
     * 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)//遍历AQS队列
            if (t.waitStatus <= 0)
                s = t;
    }
    if (s != null)
        LockSupport.unpark(s.thread);//唤醒线程
}

h != t