未完待更
ReentrantLock是公平锁、可重入锁
ReentrantLock是通过AQS实现的，内部类Sync继承AbstractQueuedSynchronizer，几个内部类的关系图如下

image.png image.png

FairSync和NonfairSync继承Sync，
初始化ReentrantLock时，如果不指定公平锁，ReentrantLock默认为非公平锁

一、参数

    //独占模式的线程拥有者，位于AbstractOwnableSynchronizer类中
    private transient Thread exclusiveOwnerThread;
     /**FIFO队列中的头Node 位于AbstractQueuedSynchronizer类中**/
    private transient volatile Node head;
     /***FIFO队列中的尾Node 位于AbstractQueuedSynchronizer类中**/
    private transient volatile Node tail;
   /***当前锁状态 位于AbstractQueuedSynchronizer类中**/
    private volatile int state;

二、内部类

1、AbstractQueuedSynchronizer内部类Node

    static final class Node {
       //表示Node处于共享模式
        static final Node SHARED = new Node();
        //独占模式 始终为空
        static final Node EXCLUSIVE = null;
        //因为异常情况需要清除node队列状态
        static final int CANCELLED =  1;
        //线程处于阻塞状态 ，等待被唤醒，一般先将waitStatus=-1然后LockSupport.park()
        static final int SIGNAL    = -1;
       //调用condition.await 存放条件等待队列状态
        static final int CONDITION = -2;

        static final int PROPAGATE = -3;

        volatile int waitStatus;
       //链表头部
        volatile Node prev;
       //链表尾部
        volatile Node next;

        volatile Thread thread;

        Node nextWaiter;

        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }

        Node() {    
        }

        Node(Thread thread, Node mode) {     
            this.nextWaiter = mode;
            this.thread = thread;
        }

        Node(Thread thread, int waitStatus) { 
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }

2、AbstractQueuedSynchronizer内部类ConditionObject

public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /**条件队列的第一个节点 */
        private transient Node firstWaiter;
        /** 条件队列的最后一个节点 */
        private transient Node lastWaiter;

        public ConditionObject() { }
        //添加新的等待节点到等待条件队列队尾，并且返回等待条件队列
        private Node addConditionWaiter() {
            Node t = lastWaiter;
            //检查等待条件队列
            if (t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            if (t == null)
                firstWaiter = node;
            else
                t.nextWaiter = node;
            lastWaiter = node;
            return node;
        }

        private void doSignal(Node first) {
            do {
                if ( (firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }

        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }
       /*condition.await调用
         *循环遍历等待条件队列，排除队列中不在等待的节点（waitStatus 不等于-2）**/
        private void unlinkCancelledWaiters() {
            Node t = firstWaiter;
            Node trail = null;
            while (t != null) {
                Node next = t.nextWaiter;
                if (t.waitStatus != Node.CONDITION) {
                    t.nextWaiter = null;
                    if (trail == null)
                        firstWaiter = next;
                    else
                        trail.nextWaiter = next;
                    if (next == null)
                        lastWaiter = trail;
                }
                else
                    trail = t;
                t = next;
            }
        }

        public final void signal() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignal(first);
        }

        public final void signalAll() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignalAll(first);
        }

        public final void awaitUninterruptibly() {
             //添加条件等待节点到条件等待队列列尾
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean interrupted = false;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if (Thread.interrupted())
                    interrupted = true;
            }
            if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }

        /** Mode meaning to reinterrupt on exit from wait */
        private static final int REINTERRUPT =  1;
        /** Mode meaning to throw InterruptedException on exit from wait */
        private static final int THROW_IE    = -1;
·
        private int checkInterruptWhileWaiting(Node node) {
            return Thread.interrupted() ?
                (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
                0;
        }

        private void reportInterruptAfterWait(int interruptMode)
            throws InterruptedException {
            if (interruptMode == THROW_IE)
                throw new InterruptedException();
            else if (interruptMode == REINTERRUPT)
                selfInterrupt();
        }

        public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
           //添加到条件等待队列列尾
            Node node = addConditionWaiter();
           //释放锁
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

        public final long awaitNanos(long nanosTimeout)
                throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return deadline - System.nanoTime();
        }

        public final boolean awaitUntil(Date deadline)
                throws InterruptedException {
            long abstime = deadline.getTime();
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (System.currentTimeMillis() > abstime) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                LockSupport.parkUntil(this, abstime);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }

        public final boolean await(long time, TimeUnit unit)
                throws InterruptedException {
            long nanosTimeout = unit.toNanos(time);
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }

        final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
            return sync == AbstractQueuedSynchronizer.this;
        }

        protected final boolean hasWaiters() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    return true;
            }
            return false;
        }

        protected final int getWaitQueueLength() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int n = 0;
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    ++n;
            }
            return n;
        }

        protected final Collection<Thread> getWaitingThreads() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION) {
                    Thread t = w.thread;
                    if (t != null)
                        list.add(t);
                }
            }
            return list;
        }
    }

三、方法实现

1.lock()非公平锁实现

lock()实现，如果获取到锁则state+1，获取不到则放入到双向链表，

final void lock() {
    /**如果state初始值为0（代表没有加锁），则更新为1，并且将当前线程对象
     **保存exclusiveOwnerThread中**/
    if (compareAndSetState(0, 1))
        setExclusiveOwnerThread(Thread.currentThread());
    else
        acquire(1);
}
//AbstractQueuedSynchronizer类中
public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
       //当前线程中断
        selfInterrupt();
}  
//NonfairSync内部类中
protected final boolean tryAcquire(int acquires) {
    return nonfairTryAcquire(acquires);
}

final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        /**有可能存在上一个线程占用，但是当前线程没在lock()里面获取到锁
         **运行到这里发现锁空闲，则直接加锁*/
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    /**如果锁已经被占用，并且当前线程和加锁线程一致，则重复加锁（可重入
     **锁），state+1
     **getExclusiveOwnerThread()获取到的是exclusiveOwnerThread的值*/
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    //其他情况返回失败
    return false;
}
       
    private Node addWaiter(Node mode) {
        /**构建当前线程的Node对象，Node.EXCLUSIVE始终为null所以mode也为null
         **   node .nextWaiter = null;
         **  node .thread = Thread.currentThread();**/
        Node node = new Node(Thread.currentThread(), mode);
        //第一个进入队列时，tail为null
        Node pred = tail;
       //也是实现和enq一样的队列
       if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { 
                //第一次进入队列的线程 会将AQS的 head和 tail 指向new node();
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
               /**下面这一段代码比较绕
                ** t指向tail地址
                ** node 中prev 指向了t,
                ** compareAndSetTail(t, node) 其实相当于将 tail 指向了node的地址
                ** t中next 指向node
                ** 最终结果为 head中next 指向node
                **  tail 指向 node
                ** node的prev指向head
                **/
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

多线程情况下addWaiter最终形成链表队列如下 node1即为返回的node节点

image.png

承接acquireQueued()
这个方法主要尝试去获取锁，如果获取不到则将线程阻塞。等待被唤醒

    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                //获取node prev节点
                final Node p = node.predecessor();
                //如果这个双向链表第一个元素指向head ，并且可以正常获取到锁
                if (p == head && tryAcquire(arg)) {
                   //即head变为链表第一个元素，
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                /**链表中waitStatus初始状态都为0，这里存在几种情况
                 ** 1.链表队列waitStatus初始为0，进入shouldParkAfterFailedAcquire(p,                                 
                **  node) waitStatus为-1，第二次进入shouldParkAfterFailedAcquire，返回              
                **true，并且执行parkAndCheckInterrupt()，阻塞当前线程，interrupted =         
                **true,也意味着尝试几次获取不到锁，当前线程阻塞
                **
                **/
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            //如果出错或者链表没有节点，将node节点清空
            if (failed)
                cancelAcquire(node);
        }
    }

    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            return true;
        if (ws > 0) {
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /**
             ** 将pred节点的状态值置为 -1 有可能失败*/
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

    private final boolean parkAndCheckInterrupt() {
       //这里this指的是ReentrantLock类中FairSync或NonfairSync，park确保线程挂起
        LockSupport.park(this);
        //确保中断标识被清除
        return Thread.interrupted();
    }

     private void cancelAcquire(Node node) {
        if (node == null)
            return;

        node.thread = null;

        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        Node predNext = pred.next;

        node.waitStatus = Node.CANCELLED;

        if (node == tail && compareAndSetTail(node, pred)) {
            compareAndSetNext(pred, predNext, null);
        } else {
            int ws;
            if (pred != head &&
                ((ws = pred.waitStatus) == Node.SIGNAL ||
                 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                pred.thread != null) {
                Node next = node.next;
                if (next != null && next.waitStatus <= 0)
                    compareAndSetNext(pred, predNext, next);
            } else {
                unparkSuccessor(node);
            }

            node.next = node; // help GC
        }
    }

2.unLock()非公平实现

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

  public final boolean release(int arg) {
       //重入锁最后一次解锁 tryRelease(arg)才会返回true，所以去唤醒其他等待的线程
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
              //唤醒线程
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

  private void unparkSuccessor(Node node) {
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);
        Node s = node.next;
      //找出需要被唤醒的节点
        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);
    }

3.lock()公平实现

     final void lock() {
           acquire(1);
     }
     public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

tryAcquire调用FairSync中tryAcquire()方法，这个方法加锁成功后返回true

    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;
                }
            }
            //重入锁
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }
   /**返回false有几种情况:
     **1、head=tail 队列中没有线程节点
     **2、head的next节点不为null并且next节点中线程为当前线程**/
    public final boolean hasQueuedPredecessors() {

        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());
    }

剩下调用和非公平锁相同

4.unLock()公平实现

unlock公平锁和非公平锁实现相同

5.lockInterruptibly()

lockInterruptibly()

public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
 }
/**AbstractQueuedSynchronizer类中**/
public final void acquireInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        //如果当前线程加锁失败
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }

    private void doAcquireInterruptibly(int arg)
        throws InterruptedException {
        //添加到FIFO 链表队列中
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
                //获取node prev节点
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
              //
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
           //置空队列
            if (failed)
                cancelAcquire(node);
        }
    }