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Java并发编程之AbstractQueuedSynchronizer队列同步器与可重入锁ReentrantLock

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前言:之前有写过关于重入锁ReentrantLock的解析,而重入锁ReentrantLock的核心在于它的两个锁非公平锁公平锁的所继承的父类AbstractQueuedSynchronizer,接下来就是关于AbstractQueuedSynchronizer的详解,包括图文、源码。后文AbstractQueuedSynchronizer简称AQS
此文相比前文重入锁ReentrantLock多了源码与图解,并且从AQS的角度出发进行解析。

AQS流程图

源码解析

acquire方法源码

根据上面的AQS的执行流程图,AQS先执行acquire()方法。源码如下:

public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}
  1. 分别调用了,tryAcquire()(需要由子类实现);
  2. 调用addWaiter(Node.EXCLUSIVE)以独占模式创建结点Node,并将当先线程作为Nodethread变量,Node加入到AQS的队列中;
  3. addWaiter(Node.EXCLUSIVE)构建好的新Node参数,调用acquireQueued方法。

acquireQueued方法源码

线程队列的等待,由acquireQueued方法进行实现的:

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()) // 注释1
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
  1. 此方法对应上图中的圆形循环;
  2. 判断前置结点prev是否为头结点head
  3. 不是head结点,调用shouldParkAfterFailedAcquire判断是否应当park等待,并通过parkAndCheckInterrupt方法执行LockSupport.park(this)进行线程的等待,等待已经获得成功的线程release
  4. head结点,则调用tryAcquire方法,获取成功,则将当前结点设置为头结点head,线程获取成功,至此acquire执行完毕。

release释放

public final boolean release(int arg) {
    if (tryRelease(arg)) {
        Node h = head;
        if (h != null && h.waitStatus != 0)
            unparkSuccessor(h);
        return true;
    }
    return false;
}

看上述代码,可以发现,是以head结点为参数,调用unparkSuccessor方法:

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);
}
  1. 设置等待状态为0 - 初始状态
  2. waitStatus大于0的状态为CANCELLED,意为结点取消,需要将大于0的结点从队列中剔除
  3. 最后,调用LockSupport.unpark(s.thread),以让head的后结点线程取消park方法的阻塞,对应上面acquireQueued方法的注释1
  4. 下面是waitStatus可能出现的值:
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED =  1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL    = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/**
 * waitStatus value to indicate the next acquireShared should
 * unconditionally propagate
 */
static final int PROPAGATE = -3;

ReentrantLock的公平锁与非公平锁

从AQS的acquirerelease方法可以看到,都调用了tryAcquiretryRelease方法,这两个方法需要由子类实现。最典型的就是可重入锁ReentrantLockNonfairSync(非公平锁)与FairSync(公平锁)实现。

acquire方法的入口之lock()方法

非公平锁NonfairSync

final void lock() {
    if (compareAndSetState(0, 1))
        setExclusiveOwnerThread(Thread.currentThread());
    else
        acquire(1);
}

公平锁FairSync

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

可以发现,非公平锁在调用acquire方法之前,先调用了compareAndSetState方法,而公平锁是直接调用的acquire


非公平锁与公平锁的tryAcquire实现

非公平锁:

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

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;
}

公平锁:

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;
}

两者都有共同之处:

  • compareAndSetState更新状态
  • 同一线程多次调用lock,会累加state的值,于是也需要对应次数的release

但是公平锁有一个hasQueuedPredecessors判断方法:

public final boolean hasQueuedPredecessors() {
    Node t = tail; // Read fields in reverse initialization order
    Node h = head;
    Node s;
    return h != t &&
        ((s = h.next) == null || s.thread != Thread.currentThread());
}

从源码不难看出,这里的判断逻辑是,头尾结点不相等,头结点的后结点不为空或者头结点的后结点的线程不为当前线程。简单总结一下就是:队列里排在最前面的结点不是当前线程的结点。这也是公平锁的公平体现之处。

那么,非公平锁又是怎样体现的呢,可以从lock方法看出,在acquire方法之前,先进行compareAndSetState抢锁,这个时候有可能set成功,也有可能失败,失败的话就会进入AQS队列,然后顺序执行,而它也有可能被其它线程捷足先登,所以它是非公平的。

打个比方:

  • 公平锁:排队到食堂吃饭,所有人都严格根据顺序先来后到进行打饭,这个打饭窗口就好比锁,只能按照先来后到的顺序获得锁;
  • 非公平锁:同样排队吃饭,假设这个时候已经排起长队了,而后面每个人来的时候都会趁第一个人不注意进行插队,如果被发现了则乖乖地到后面排队,如果没被发现,就插队成功啦。
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