java锁ReentrantLock源码分析

[diaosichengxuyuan]

ReentrantLock是可重入的排它锁，支持公平和非公平模式，通过AbstractQueuedSynchronizer实现

lock()和unlock()

   public void lock() {
        sync.lock();
    }

    public void unlock() {
        sync.release(1);
    }

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();
            //获取状态
            int c = getState();
            //如果获取状态成功
            if (c == 0) {
                //通过CAS将状态从0改成1
                if (compareAndSetState(0, acquires)) {
                    //设置当前的独占线程
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            //如果当前状态不是0，但是当前独占线程是当前线程，表示是当前线程进行了重入
            else if (current == getExclusiveOwnerThread()) {
                //继续增加状态
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                //设置状态，不用CAS方式，因为只有当前线程获取了锁
                setState(nextc);
                return true;
            }
            return false;
        }

        protected final boolean tryRelease(int releases) {
            //状态每次减1
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            //如果状态已经释放完成
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
           //设置状态，c可能等于0也可能不等于0，如果等于0，则返回true；如果不等于0，则返回false
            setState(c);
            return free;
        }

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

    //非公平模式
    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() {
            //如果通过CAS获取状态0成功，将状态修改为1
            if (compareAndSetState(0, 1))
                //设置当前的独占线程为当前线程
                setExclusiveOwnerThread(Thread.currentThread());
            //如果获取状态不成功
            else
                //调用AbstractQueuedSynchronizer中的acquire
                acquire(1);
        }
        
        //尝试获取状态1
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

//公平模式
static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

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

        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        //公平模式获取锁的线程是在同步队列中排队的线程，谁先排队谁先获得，与非公平模式仅有下面一点 
        //不同
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                //公平模式比非公平模式多了hasQueuedPredecessors，这个方法判读当前线程是否有前驱
                //如果没有队列前驱，当前线程才可以获取状态；如果有队列前驱，优先队列头的线程获取状态
                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;
        }
    }

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
       
        public final void acquire(int arg) {
        //先判断tryAcquire(1)是否成功，如果成功，直接返回，表示获取到了锁
        //如果获取状态失败，addWaiter(Node.EXCLUSIVE)，将当前线程组装成Node添加到同步队列中
        //acquireQueued是将当前线程进行自循环
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
         }
        
        //添加到同步队列
        private Node addWaiter(Node mode) {
        //将当前线程组装成Node
        Node node = new Node(Thread.currentThread(), mode);
        Node pred = tail;
        //如果同步队列不为空
        if (pred != null) {
            node.prev = pred;
            //通过CAS将node添加到同步队列的队尾
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        //如果队列为空或者CAS添加到队尾时失败
        enq(node);
        return node;
    }
     
    //死循环方式将node添加到同步队列队尾
    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { 
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            //自循环
            for (;;) {
                final Node p = node.predecessor();
                //head是已经获取锁的线程，当前线程需要判断自己的前驱是否是head，如果是head，则尝试
                //获取状态1，如果获取状态成功，将自己设置成head，断开与前驱的关联
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //如果当前线程前驱不是head或者获取状态1失败，LockSupport.park(this)，阻塞当前线程
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    //释放状态
    public final boolean release(int arg) {
        //如果尝试释放状态成功
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                //唤醒同步队列中的下个线程
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
}

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