Java 公平锁与非公平锁学习研究

最近学习研究了一下Java中关于公平锁与非公平锁的底层实现原理,总结了一下.

首先呢,通过其字面意思,公平与非公平的评判标准就是付出与收获成正比(和社会中的含义差不多一个意思).放到程序中,尤其是

在多线程环境,线程获取资源(CPU\内存\网络等等)的时序与其所再此资源上消耗的等待时间成正比,亦即我等的时间越长,获取

该资源的时序越短.(和上学时排队打饭一个道理).公平锁这个样子,那非公平锁,简单来说也就是插队.(不过,Java实现还是有些差别的,

稍后会重点说明)

     Java中关于公平锁与非公平锁的实现,是基于AQS(AbstractQueuedSynchronizer)实现的,想要深入研究Java锁及同步技术,该类是

绕不过去,So,Let's fuck up this bitch!

从名字上来简单翻译一下,抽象排队同步器(绕口.....),顾名思义,就是为实现排队获取资源同步抽象类,按照其官方说明,一些比较重要的同步

工具类(Semaphore信号量,CountDownLatch闭锁,CyclicBarrier栅栏等)等底层都是由该类完成其核心功能实现的,是对资源同步抽象.

该抽象类主要的核心:排队\同步(从其名字上就可以看出),先来分析一下其排队实现

• 排队:该类是基于一种双向不循环列表来实现其队列功能的,大致数据结构如下草图,

  

  再通过将线程与必要的状态值进行封装成节点,再利用其同步原理来实现.

• 同步:基于sun.misc.Unsafe来实现CAS原子性操作(Compare and Swap)

/**
     * CAS head field. Used only by enq.
     */
    private final boolean compareAndSetHead(Node update) {
        return unsafe.compareAndSwapObject(this, headOffset, null, update);
    }

    /**
     * CAS tail field. Used only by enq.
     */
    private final boolean compareAndSetTail(Node expect, Node update) {
        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
    }

    /**
     * CAS waitStatus field of a node.
     */
    private static final boolean compareAndSetWaitStatus(Node node, int expect, int update) {
        return unsafe.compareAndSwapInt(node, waitStatusOffset, expect, update);
    }

    /**
     * CAS next field of a node.
     */
    private static final boolean compareAndSetNext(Node node, Node expect, Node update) {
        return unsafe.compareAndSwapObject(node, nextOffset, expect, update);  }

基于以上两个核心点,完成大多说同步操作.接下来,再看一下公平锁与非公平锁实现原理,废话不多说,直接上码:

/**
     * Base of synchronization control for this lock. Subclassed
     * into fair and nonfair versions below. Uses AQS state to
     * represent the number of holds on the lock.
     */
    abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -5179523762034025860L;

        /**
         * Performs {@link Lock#lock}. The main reason for subclassing
         * is to allow fast path for nonfair version.
         */
        abstract void lock();

        /**
         * 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;
                // overflow
                if (nextc < 0) {
                    throw new Error("Maximum lock count exceeded");
                }
                setState(nextc);
                return true;
            }
            return false;
        }

        @Override
        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);
            }
            setState(c);
            return free;
        }

        @Override
        protected final boolean isHeldExclusively() {
            // While we must in general read state before owner,
            // we don't need to do so to check if current thread is owner
            return getExclusiveOwnerThread() == Thread.currentThread();
        }

        final ConditionObject newCondition() {
            return new ConditionObject();
        }

        // Methods relayed from outer class

        final Thread getOwner() {
            return getState() == 0 ? null : getExclusiveOwnerThread();
        }

        final int getHoldCount() {
            return isHeldExclusively() ? getState() : 0;
        }

        final boolean isLocked() {
            return getState() != 0;
        }

        /**
         * Reconstitutes the instance from a stream (that is, deserializes it).
         */
        private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
            s.defaultReadObject();
            // reset to unlocked state
            setState(0);
        }
    }

    /**
     * Sync object for non-fair locks
     */
    static final class NonfairSync extends ReentrantLock.Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        @Override
        final void lock() {
            // CAS原子性操作
            if (compareAndSetState(0, 1)) {
                // 设置独占所有线程线程
                setExclusiveOwnerThread(Thread.currentThread());
            } else {
                acquire(1);
            }
        }

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

    /**
     * Sync object for fair locks
     */
    static final class FairSync extends ReentrantLock.Sync {
        private static final long serialVersionUID = -3000897897090466540L;

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

        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        @Override
        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;
        }
    }

这个就是Java中大名鼎鼎的公平锁与非公平锁的实现,两者最主要的区别在于其获取锁的方式,非公平锁获取时:

而公平锁:

最主要的区别就在于红线圈起来的地方.

举个很不雅但很贴切的例子,排队上厕所.公平锁就是,你内急了,就去乖乖的排队,到时候自然会轮到你.而非公平锁就是你内急了,

就可以尝试一下去进入厕所间并锁门,如果恰好成功的话,就捷足先登喽;不行的话就去排队.而且在获取锁时也不用考虑排在队列

前面的人,直接就可以尝试去占有厕所,成功了就万事大吉.