ReentrantLock实现-学习笔记

    线程安全往往需要靠互斥锁机制来保证,ReentrantLock是比关键字sychronized更灵活的可重入互斥锁。ReentrantLock在不使用锁(它自己本身就是Lock)和synchronized关键字的前提下,是怎么保证线程安全的呢。

    看一下不公平锁实现的lock方法(调用的是Sync(NonfairSync)的lock方法

    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

    1. 先通过CAS原子操作来试图获取锁(当前状态==0并且成功的将状态改为1),如果CAS返回true,说明成功获取到锁,那么简单的将该锁的拥有线程设为当前线程即可返回。锁获取成功。

    2. 如果CAS返回false,则调用acquire方法,其中先调用tryAcquire方法,在不公平锁的实现中,tryAcuqire方法首先做的操作与lock方法的第一步操作基本重复,也是先通过CAS原子操作尝试获取锁,成功则设置所谓当前线程所有。如果当前的状态不为0(说明有线程已经获取到该锁),则判断持有该锁的是不是当前线程自身,如果是自身,由于是可重入锁,则简单的将当前状态计数+1就可返回,锁获取成功。

        

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

 

       //tryAcquire 
       /**
         * 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;
        }

      3. 如果不是上面这2种情况,则说明锁已经被其他线程排他占用,则调用addWaiter先将当前线程放入等待队列中,然后在acquireQueued方法中,循环判断是否已经轮到当前节点(即该节点前面没有可用节点),如果没有有效节点,则再尝试调用tryAcquire方法,失败则继续循环,否则成功获取到锁并返回。如果前面有其他节点,则将当前线程park,当前线程阻塞,可能直到其他线程unlock调用unpark,然后才恢复执行并重新试图获取该锁。

     //this method is implemented in AbstractQueuedSynchronizer
    /**
     * 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);
        }
    }

 

你可能感兴趣的:(ReentrantLock实现-学习笔记)