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ReentrantLock的公平锁与非公平锁

2021/6/21 18:10:07 人评论

ReentrantLock类中包含FairLock和NonfairLock两个内部类继承Sync,实现了公平锁和非公平锁的方法。它们在获取不到锁时都会加入FIFO队列排队等待,唯一不同的是新来一个线程要来获取锁时,公平锁会判断队列头部是否有在等待的线程。 NonfairLock…

        ReentrantLock类中包含FairLock和NonfairLock两个内部类继承Sync,实现了公平锁和非公平锁的方法。它们在获取不到锁时都会加入FIFO队列排队等待,唯一不同的是新来一个线程要来获取锁时,公平锁会判断队列头部是否有在等待的线程。

       NonfairLock

     

/**
         * 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;
        }

    FairLock

/**
         * 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;
        }

 关键区别就是非公平锁只会cas比较,不会有

!hasQueuedPredecessors()这个判断,这个判断的源码如下:
/**
     * Queries whether any threads have been waiting to acquire longer
     * than the current thread.
     *
     * <p>An invocation of this method is equivalent to (but may be
     * more efficient than):
     *  <pre> {@code
     * getFirstQueuedThread() != Thread.currentThread() &&
     * hasQueuedThreads()}</pre>
     *
     * <p>Note that because cancellations due to interrupts and
     * timeouts may occur at any time, a {@code true} return does not
     * guarantee that some other thread will acquire before the current
     * thread.  Likewise, it is possible for another thread to win a
     * race to enqueue after this method has returned {@code false},
     * due to the queue being empty.
     *
     * <p>This method is designed to be used by a fair synchronizer to
     * avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
     * Such a synchronizer's {@link #tryAcquire} method should return
     * {@code false}, and its {@link #tryAcquireShared} method should
     * return a negative value, if this method returns {@code true}
     * (unless this is a reentrant acquire).  For example, the {@code
     * tryAcquire} method for a fair, reentrant, exclusive mode
     * synchronizer might look like this:
     *
     *  <pre> {@code
     * protected boolean tryAcquire(int arg) {
     *   if (isHeldExclusively()) {
     *     // A reentrant acquire; increment hold count
     *     return true;
     *   } else if (hasQueuedPredecessors()) {
     *     return false;
     *   } else {
     *     // try to acquire normally
     *   }
     * }}</pre>
     *
     * @return {@code true} if there is a queued thread preceding the
     *         current thread, and {@code false} if the current thread
     *         is at the head of the queue or the queue is empty
     * @since 1.7
     */
    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;
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }

 

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