ReentranReadWritetLock的ReadLock源码解析

//ReentrantReadWriteLock的ReadLock
//ReadLock的lock方法

public void lock() {
            sync.acquireShared(1);
        }

public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
	   //入队挂起当前线程
            doAcquireShared(arg);
    }


protected final int tryAcquireShared(int unused) {
           
            Thread current = Thread.currentThread();
	    //获取状态
            int c = getState();
	    //如果当前写锁被占用直接排队进入队列
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;
	   //获取写锁的数量
            int r = sharedCount(c);
	    //判断线程是否可以直接获取锁
	    //该线程是否需要阻塞,如果当前队列中没有写线程就可以直接获取锁
            if (!readerShouldBlock() &&
                r < MAX_COUNT &&
                compareAndSetState(c, c + SHARED_UNIT)) {
                if (r == 0) {
                    firstReader = current;
                    firstReaderHoldCount = 1;
                } else if (firstReader == current) {
                    firstReaderHoldCount++;
                } else {
                    HoldCounter rh = cachedHoldCounter;
                    if (rh == null || rh.tid != current.getId())
                        cachedHoldCounter = rh = readHolds.get();
                    else if (rh.count == 0)
                        readHolds.set(rh);
                    rh.count++;
                }
                return 1;
            }
                 //这个方法用于重试写线程是否已经释放
            return fullTryAcquireShared(current);
        }

static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }

static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }

 final boolean readerShouldBlock() {
           
            return apparentlyFirstQueuedIsExclusive();
        }
 //判断当前队列中是否有写线程在等待
 final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) != null &&
            (s = h.next)  != null &&
            !s.isShared()         &&
            s.thread != null;
    }


    final int fullTryAcquireShared(Thread current) {
            
            HoldCounter rh = null;
            for (;;) {
                int c = getState();
		//被写锁占用
                if (exclusiveCount(c) != 0) {
                    if (getExclusiveOwnerThread() != current)
                        return -1;
                  //队列中有写线程在排队
                } else if (readerShouldBlock()) {
                    
                    if (firstReader == current) {
                        // assert firstReaderHoldCount > 0;
                    } else {
                        if (rh == null) {
                            rh = cachedHoldCounter;
                            if (rh == null || rh.tid != current.getId()) {
                                rh = readHolds.get();
                                if (rh.count == 0)
                                    readHolds.remove();
                            }
                        }
                        if (rh.count == 0)
                            return -1;
                    }
                }

		//下面的方法直接获得锁

                if (sharedCount(c) == MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                if (compareAndSetState(c, c + SHARED_UNIT)) {
                    if (sharedCount(c) == 0) {
                        firstReader = current;
                        firstReaderHoldCount = 1;
                    } else if (firstReader == current) {
                        firstReaderHoldCount++;
                    } else {
                        if (rh == null)
                            rh = cachedHoldCounter;
                        if (rh == null || rh.tid != current.getId())
                            rh = readHolds.get();
                        else if (rh.count == 0)
                            readHolds.set(rh);
                        rh.count++;
                        cachedHoldCounter = rh; // cache for release
                    }
                    return 1;
                }
            }
        }


 private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
		       //这里注意在被唤醒以后如果下一个节点不是write节点会释放锁唤醒node中的下一个节点
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
   

    private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; // Record old head for check below
        setHead(node);
       
        if (propagate > 0 || h == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }

 //那几个变量的用法没整明白。
 /**
 共享锁的流程是：判断当前等待队列中是否有排他锁在等待。如果没有直接运行不会锁住任何线程。
 有则入队等待唤醒。在自身被唤醒以后会去尝试获取锁如果获取锁成功并且下一个节点不是write节点则会唤醒下一个节点（这会提升效率）不用等到unlock的时候再释放,当然unlock也会释放锁。

 */



//ReadLock的unlock方法：

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


public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }


 protected final boolean tryReleaseShared(int unused) {
            Thread current = Thread.currentThread();
	    //如果当前是firstReader
            if (firstReader == current) {
               //如果读锁个数是一
                if (firstReaderHoldCount == 1)
		
                    firstReader = null;
                else
                    firstReaderHoldCount--;
            } else {
                HoldCounter rh = cachedHoldCounter;
                if (rh == null || rh.tid != current.getId())
                    rh = readHolds.get();
                int count = rh.count;
                if (count <= 1) {
                    readHolds.remove();
                    if (count <= 0)
                        throw unmatchedUnlockException();
                }
                --rh.count;
            }
	    //无限循环设置状态
            for (;;) {
                int c = getState();
                int nextc = c - SHARED_UNIT;
                if (compareAndSetState(c, nextc))
               
                    return nextc == 0;
            }
        }


private void doReleaseShared() {
       
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue; 
		    //没有等到unlock就释放了锁
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }




//lockInterruptibly获取读取锁除非线程被中断和lock方法基本差不多就是增加了判断线程如果中断就抛出异常
  public void lockInterruptibly() throws InterruptedException {
            sync.acquireSharedInterruptibly(1);
        }

 public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
		    //如果中断就抛出异常
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }



//当线程没有被写入锁占用则一直尝试获取读取锁相当于插队
public  boolean tryLock() {
            return sync.tryReadLock();
        }

  final boolean tryReadLock() {
            Thread current = Thread.currentThread();
            for (;;) {
                int c = getState();
		//被写入锁占用了
                if (exclusiveCount(c) != 0 &&
                    getExclusiveOwnerThread() != current)
                    return false;
                int r = sharedCount(c);
                if (r == MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                if (compareAndSetState(c, c + SHARED_UNIT)) {
                    if (r == 0) {
                        firstReader = current;
                        firstReaderHoldCount = 1;
                    } else if (firstReader == current) {
                        firstReaderHoldCount++;
                    } else {
                        HoldCounter rh = cachedHoldCounter;
                        if (rh == null || rh.tid != current.getId())
                            cachedHoldCounter = rh = readHolds.get();
                        else if (rh.count == 0)
                            readHolds.set(rh);
                        rh.count++;
                    }
                    return true;
                }
            }
        }


//tryLock(long timeout, TimeUnit unit)在指定时间内如果没有获取到锁则锁定失败
  public boolean tryLock(long timeout, TimeUnit unit)
                throws InterruptedException {
            return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
        }

public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
	 //立即去获取一次锁
        return tryAcquireShared(arg) >= 0 ||
	    //指定时间内获取锁
            doAcquireSharedNanos(arg, nanosTimeout);
    }


private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
        throws InterruptedException {

        long lastTime = System.nanoTime();
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return true;
                    }
                }
                if (nanosTimeout <= 0)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                long now = System.nanoTime();
                nanosTimeout -= now - lastTime;
                lastTime = now;
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }