AQS源码的简单理解

Posted 2021-03-22 expiator

概念

AQS全称 AbstractQueuedSynchronizer。

AQS是一个并发包的基础组件，用来实现各种锁，各种同步组件的。它包含了state变量、加锁线程、等待队列等并发中的核心组件。

ReentrantLock、Semaphore、CountDownLatch、FutrueTask，这些都是基于AQS构建的。

而AQS是基于volatile变量的读/写和CAS( 也就是compareAndSet()方法 )实现的。

volatile可以保证并发中的可见性，还可以禁止指令重排序。CAS，用于管理对共享数据的并发访问。

ReentrantLock和AQS的关系

在ReentrantLock中，state代表了加锁的状态。初始状态下，这个state的值是0。

另外，这个AQS内部还有一个关键变量，用来记录当前加锁的是哪个线程，初始化状态下，这个变量是null。

线程A跑过来调用ReentrantLock的lock()方法尝试进行加锁，这个加锁的过程，直接就是用CAS操作将state值从0变为1。
如果之前没人加过锁，那么state的值肯定是0，此时线程A就可以加锁成功。
一旦线程1加锁成功了之后，就可以设置当前加锁线程是自己。

主要流程

重要的变量

• state
  状态变量。

/**
 * The synchronization state.
 */
private volatile int state;

等待队列的节点Node

"CLH" (Craig, Landin, and Hagersten) lock queue。

CLH lock queue通常被用来处理并发的情况，它通过双向队列(FIFO)来完成同步状态。每个线程都会被封装成一个Node节点放到同步队列中。

队列的每个Node节点保存了当前线程的同步状态，等待状态，前驱和后继节点等。

static final class Node {
        //共享模式
        /** Marker to indicate a node is waiting in shared mode */
        static final Node SHARED = new Node();
        //独占模式
        /** Marker to indicate a node is waiting in exclusive mode */
        static final Node EXCLUSIVE = null;

        // 线程的等待状态 表示线程已经被取消
        static final int CANCELLED =  1;
        // 线程的等待状态 表示后继线程需要被唤醒
        static final int SIGNAL    = -1;
        // 线程的等待状态 表示线程在Condtion上
        static final int CONDITION = -2;
        // 表示下一个acquireShared需要无条件的传播
        static final int PROPAGATE = -3;

        /**
         *   等待状态有以下几种：
         *   
         *   SIGNAL:     当前节点的后继节点处于等待状态时,如果当前节点的同步状态被释放或者取消,
         *               必须唤起它的后继节点
         *         
         *   CANCELLED:  一个节点由于超时或者中断需要在CLH队列中取消等待状态,被取消的节点不会再次等待
         *               
         *   CONDITION:  当前节点在等待队列中,只有当节点的状态设为0的时候该节点才会被转移到同步队列
         *               
         *   PROPAGATE:  下一次的共享模式同步状态的获取将会无条件的传播
 
         *   waitStatus的初始值时0,使用CAS来修改节点的状态
         */
        volatile int waitStatus;

        /**
         * Link to predecessor node that current node/thread relies on
         * for checking waitStatus. Assigned during enqueuing, and nulled
         * out (for sake of GC) only upon dequeuing.  Also, upon
         * cancellation of a predecessor, we short-circuit while
         * finding a non-cancelled one, which will always exist
         * because the head node is never cancelled: A node becomes
         * head only as a result of successful acquire. A
         * cancelled thread never succeeds in acquiring, and a thread only
         * cancels itself, not any other node.
         */
        volatile Node prev;

        /**
         * Link to the successor node that the current node/thread
         * unparks upon release. Assigned during enqueuing, adjusted
         * when bypassing cancelled predecessors, and nulled out (for
         * sake of GC) when dequeued.  The enq operation does not
         * assign next field of a predecessor until after attachment,
         * so seeing a null next field does not necessarily mean that
         * node is at end of queue. However, if a next field appears
         * to be null, we can scan prev's from the tail to
         * double-check.  The next field of cancelled nodes is set to
         * point to the node itself instead of null, to make life
         * easier for isOnSyncQueue.
         */
        volatile Node next;

        /**
         * The thread that enqueued this node.  Initialized on
         * construction and nulled out after use.
         */
        volatile Thread thread;

        /**
         * Link to next node waiting on condition, or the special
         * value SHARED.  Because condition queues are accessed only
         * when holding in exclusive mode, we just need a simple
         * linked queue to hold nodes while they are waiting on
         * conditions. They are then transferred to the queue to
         * re-acquire. And because conditions can only be exclusive,
         * we save a field by using special value to indicate shared
         * mode.
         */
        Node nextWaiter;

        /**
         * Returns true if node is waiting in shared mode.
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        /**
         * Returns previous node, or throws NullPointerException if null.
         * Use when predecessor cannot be null.  The null check could
         * be elided, but is present to help the VM.
         *
         * @return the predecessor of this node
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }

        Node() {    // Used to establish initial head or SHARED marker
        }

        Node(Thread thread, Node mode) {     // Used by addWaiter
            this.nextWaiter = mode;
            this.thread = thread;
        }

        Node(Thread thread, int waitStatus) { // Used by Condition
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }

acquire()

线程在独占模式下获取state。

子类继承AQS后，经常会用到acquire() 、 release()。

acquire()获取状态、维护状态。

/**
 * Acquires in exclusive mode, ignoring interrupts.  Implemented
 * by invoking at least once {@link #tryAcquire},
 * returning on success.  Otherwise the thread is queued, possibly
 * repeatedly blocking and unblocking, invoking {@link
 * #tryAcquire} until success.  This method can be used
 * to implement method {@link Lock#lock}.
 *
 * @param arg the acquire argument.  This value is conveyed to
 *        {@link #tryAcquire} but is otherwise uninterpreted and
 *        can represent anything you like.
 */
public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}

release()

线程在独占模式下释放state。

/**
 * 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;
}

unparkSuccessor()

唤醒Node的后继节点。

    /**
     * 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);
    }

compareAndSetState()

通过CAS设置状态变量。当state值为指定的参数expect时，将其修改为另一个指定的值。

/**
 * Atomically sets synchronization state to the given updated
 * value if the current state value equals the expected value.
 * This operation has memory semantics of a {@code volatile} read
 * and write.
 *
 * @param expect the expected value
 * @param update the new value
 * @return {@code true} if successful. False return indicates that the actual
 *         value was not equal to the expected value.
 */
protected final boolean compareAndSetState(int expect, int update) {
    // See below for intrinsics setup to support this
    return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}

ConditionObject

而另一个内部类ConditionObject实现了Condition接口，并且实现了其中的await(),signal(),signalALL()等方法。
ConditionObject主要是为并发编程中的同步提供了等待通知的实现方式，可以在不满足某个条件的时候挂起线程等待。直到满足某个条件的时候在唤醒线程。

参考资料：

https://juejin.im/post/5c07e59cf265da617464a09c (大白话聊聊对AQS的理解)

http://ifeve.com/introduce-abstractqueuedsynchronizer/

https://blog.csdn.net/qq_30572275/article/details/80297047