Nicksxs's Blog

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

class BoundedBuffer {
    final Lock lock = new ReentrantLock();
    // condition 依赖于 lock 来产生
    final Condition notFull = lock.newCondition();
    final Condition notEmpty = lock.newCondition();

    // 对象池子，put 跟 take 的就是这里的
    final Object[] items = new Object[100];
    int putptr, takeptr, count;

    // 生产
    public void put(Object x) throws InterruptedException {
        // 这里也说明了，需要先拥有锁
        lock.lock();
        try {
            while (count == items.length)
                notFull.await();  // 队列已满，等待，直到 not full 才能继续生产
            items[putptr] = x;
            if (++putptr == items.length) putptr = 0;
            ++count;
            notEmpty.signal(); // 生产成功，队列已经 not empty 了，发个通知出去
        } finally {
            lock.unlock();
        }
    }

    // 消费
    public Object take() throws InterruptedException {
        lock.lock();
        try {
            while (count == 0)
                notEmpty.await(); // 队列为空，等待，直到队列 not empty，才能继续消费
            Object x = items[takeptr];
            if (++takeptr == items.length) takeptr = 0;
            --count;
            notFull.signal(); // 被我消费掉一个，队列 not full 了，发个通知出去
            return x;
        } finally {
            lock.unlock();
        }
    }
}

public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /** First node of condition queue. */
        private transient Node firstWaiter;
        /** Last node of condition queue. */
        private transient Node lastWaiter;

/**
 * Implements interruptible condition wait.
 * <ol>
 * <li> If current thread is interrupted, throw InterruptedException.
 * <li> Save lock state returned by {@link #getState}.
 * <li> Invoke {@link #release} with saved state as argument,
 *      throwing IllegalMonitorStateException if it fails.
 * <li> Block until signalled or interrupted.
 * <li> Reacquire by invoking specialized version of
 *      {@link #acquire} with saved state as argument.
 * <li> If interrupted while blocked in step 4, throw InterruptedException.
 * </ol>
 */
public final void await() throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();

    // 将当前节点包装成一个  condition waiter node 节点
    Node node = addConditionWaiter();

    // 完全释放占有的锁，这里需要是占有锁的线程
    int savedState = fullyRelease(node);
    int interruptMode = 0;

    // 判断下是否在阻塞队列中，因为有可能被其他节点从等待队列移动到阻塞队列
    while (!isOnSyncQueue(node)) {
        // park等待，等待被唤醒
        LockSupport.park(this);
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
            break;
    }

    // 被唤醒后进入阻塞队列，等待获取锁，这里继续用了fullyRelease返回的 state
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
        interruptMode = REINTERRUPT;
    if (node.nextWaiter != null) // clean up if cancelled
        unlinkCancelledWaiters();
    if (interruptMode != 0)
        reportInterruptAfterWait(interruptMode);
}

/**
 * Adds a new waiter to wait queue.
 * @return its new wait node
 */
private Node addConditionWaiter() {
    Node t = lastWaiter;
    // If lastWaiter is cancelled, clean out.
    // 如果节点已经不是 CONDITION 状态了，表示已经取消了
    if (t != null && t.waitStatus != Node.CONDITION) {
        // 把等待队列中取消的节点清理出去
        unlinkCancelledWaiters();
        t = lastWaiter;
    }
    // 把当前线程包装成waitStatus=CONDITION 的节点
    Node node = new Node(Thread.currentThread(), Node.CONDITION);
    // 没有 lastWaiter 节点，直接是 firstWaiter
    if (t == null)
        firstWaiter = node;
    else
    // 不然就接在 lastWaiter 后面
        t.nextWaiter = node;
    // 当前节点就会变成新的 lastWaiter
    lastWaiter = node;
    return node;
}

/**
 * Unlinks cancelled waiter nodes from condition queue.
 * Called only while holding lock. This is called when
 * cancellation occurred during condition wait, and upon
 * insertion of a new waiter when lastWaiter is seen to have
 * been cancelled. This method is needed to avoid garbage
 * retention in the absence of signals. So even though it may
 * require a full traversal, it comes into play only when
 * timeouts or cancellations occur in the absence of
 * signals. It traverses all nodes rather than stopping at a
 * particular target to unlink all pointers to garbage nodes
 * without requiring many re-traversals during cancellation
 * storms.
 */
private void unlinkCancelledWaiters() {
    Node t = firstWaiter;
    Node trail = null;
    // 循环遍历单向链表的节点，如果状态不是 CONDITION 就清出去
    while (t != null) {
        Node next = t.nextWaiter;
        // 循环链表操作，清掉取消的节点
        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;
    }
}

/**
 * Invokes release with current state value; returns saved state.
 * Cancels node and throws exception on failure.
 * @param node the condition node for this wait
 * @return previous sync state
 */
final int fullyRelease(Node node) {
    boolean failed = true;
    try {
        // 获取下当前的 state 值，因为是可重入的，所以这个值要保存下来
        int savedState = getState();
        // 这里还包含比较多操作，不过跟前面分析 AQS 的释放比较类似，不深入了
        if (release(savedState)) {
            failed = false;
            // 返回这个值
            return savedState;
        } else {
            throw new IllegalMonitorStateException();
        }
    } finally {
        if (failed)
            node.waitStatus = Node.CANCELLED;
    }
}

/**
 * Returns true if a node, always one that was initially placed on
 * a condition queue, is now waiting to reacquire on sync queue.
 * @param node the node
 * @return true if is reacquiring
 */
final boolean isOnSyncQueue(Node node) {
    // 如果waitStatus 是 CONDITION 或者没有 prev 前置节点肯定就不在
    if (node.waitStatus == Node.CONDITION || node.prev == null)
        return false;
    // 这里就是我前面提到的 next 的作用
    if (node.next != null) // If has successor, it must be on queue
        return true;
    // 从 tail 开始找，是否在阻塞队列中
    /*
     * node.prev can be non-null, but not yet on queue because
     * the CAS to place it on queue can fail. So we have to
     * traverse from tail to make sure it actually made it.  It
     * will always be near the tail in calls to this method, and
     * unless the CAS failed (which is unlikely), it will be
     * there, so we hardly ever traverse much.
     */
    return findNodeFromTail(node);
}
/**
 * Returns true if node is on sync queue by searching backwards from tail.
 * Called only when needed by isOnSyncQueue.
 * @return true if present
 */
private boolean findNodeFromTail(Node node) {
    Node t = tail;
    // 从 tail 开始，从后往前找
    for (;;) {
        if (t == node)
            return true;
        if (t == null)
            return false;
        t = t.prev;
    }
}

/**
 * Moves the longest-waiting thread, if one exists, from the
 * wait queue for this condition to the wait queue for the
 * owning lock.
 *
 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
 *         returns {@code false}
 */
public final void signal() {
    if (!isHeldExclusively())
        throw new IllegalMonitorStateException();
    // firstWaiter 肯定是最早开始等待的
    Node first = firstWaiter;
    // 如果不为空就唤醒
    if (first != null)
        doSignal(first);
}
/**
 * Removes and transfers nodes until hit non-cancelled one or
 * null. Split out from signal in part to encourage compilers
 * to inline the case of no waiters.
 * @param first (non-null) the first node on condition queue
 */
private void doSignal(Node first) {
    do {
        // 因为要去唤醒 first 节点了，firstWaiter 需要再从后面找一个
        // 并且判断是否为空，如果是空的话就直接可以把 lastWaiter 设置成空了
        if ( (firstWaiter = first.nextWaiter) == null)
            lastWaiter = null;
        //  first 不需要继续保存后面的 waiter 了，因为 firstWaiter 已经是 first 的后置节点了
        first.nextWaiter = null;
    // 如果 first 节点转移不成功，并且 firstWaiter 节点不为空，则继续进入循环
    } while (!transferForSignal(first) &&
             (first = firstWaiter) != null);
}
/**
 * Transfers a node from a condition queue onto sync queue.
 * Returns true if successful.
 * @param node the node
 * @return true if successfully transferred (else the node was
 * cancelled before signal)
 */
final boolean transferForSignal(Node node) {
    /*
     * If cannot change waitStatus, the node has been cancelled.
     */
    // 如果状态已经不是 CONDITION 就不会设置成功，返回 false
    if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
        return false;
    /*
     * Splice onto queue and try to set waitStatus of predecessor to
     * indicate that thread is (probably) waiting. If cancelled or
     * attempt to set waitStatus fails, wake up to resync (in which
     * case the waitStatus can be transiently and harmlessly wrong).
     */
    // 调用跟aqs 第一篇中一样的 enq 方法进入阻塞队列，返回入队后的前一节点
    Node p = enq(node);
    int ws = p.waitStatus;
    // 将前置节点状态设置成SIGNAL，表示后面有节点在等了
    if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
        LockSupport.unpark(node.thread);
    // 返回 true，上一个方法的循环就退出了
    return true;
}

/*
  File: BoundedBuffer.java

  Originally written by Doug Lea and released into the public domain.
  This may be used for any purposes whatsoever without acknowledgment.
  Thanks for the assistance and support of Sun Microsystems Labs,
  and everyone contributing, testing, and using this code.

  History:
  Date       Who                What
  11Jun1998  dl               Create public version
  17Jul1998  dl               Simplified by eliminating wait counts
  25aug1998  dl               added peek
   5May1999  dl               replace % with conditional (slightly faster)
*/

package EDU.oswego.cs.dl.util.concurrent;

/**
 * Efficient array-based bounded buffer class.
 * Adapted from CPJ, chapter 8, which describes design.
 * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>] <p>
 **/

public class BoundedBuffer implements BoundedChannel {

  protected final Object[]  array_;      // the elements

  protected int takePtr_ = 0;            // circular indices
  protected int putPtr_ = 0;       

  protected int usedSlots_ = 0;          // length
  protected int emptySlots_;             // capacity - length

  /**
   * Helper monitor to handle puts. 
   **/
  protected final Object putMonitor_ = new Object();

  /**
   * Create a BoundedBuffer with the given capacity.
   * @exception IllegalArgumentException if capacity less or equal to zero
   **/
  public BoundedBuffer(int capacity) throws IllegalArgumentException {
    if (capacity <= 0) throw new IllegalArgumentException();
    array_ = new Object[capacity];
    emptySlots_ = capacity;
  }

  /**
   * Create a buffer with the current default capacity
   **/

  public BoundedBuffer() { 
    this(DefaultChannelCapacity.get()); 
  }

  /** 
   * Return the number of elements in the buffer.
   * This is only a snapshot value, that may change
   * immediately after returning.
   **/
  public synchronized int size() { return usedSlots_; }

  public int capacity() { return array_.length; }

  protected void incEmptySlots() {
    synchronized(putMonitor_) {
      ++emptySlots_;
      putMonitor_.notify();
    }
  }

  protected synchronized void incUsedSlots() {
    ++usedSlots_;
    notify();
  }

  protected final void insert(Object x) { // mechanics of put
    --emptySlots_;
    array_[putPtr_] = x;
    if (++putPtr_ >= array_.length) putPtr_ = 0;
  }

  protected final Object extract() { // mechanics of take
    --usedSlots_;
    Object old = array_[takePtr_];
    array_[takePtr_] = null;
    if (++takePtr_ >= array_.length) takePtr_ = 0;
    return old;
  }

  public Object peek() {
    synchronized(this) {
      if (usedSlots_ > 0)
        return array_[takePtr_];
      else
        return null;
    }
  }


  public void put(Object x) throws InterruptedException {
    if (x == null) throw new IllegalArgumentException();
    if (Thread.interrupted()) throw new InterruptedException();

    synchronized(putMonitor_) {
      while (emptySlots_ <= 0) {
	try { putMonitor_.wait(); }
        catch (InterruptedException ex) {
          putMonitor_.notify();
          throw ex;
        }
      }
      insert(x);
    }
    incUsedSlots();
  }

  public boolean offer(Object x, long msecs) throws InterruptedException {
    if (x == null) throw new IllegalArgumentException();
    if (Thread.interrupted()) throw new InterruptedException();

    synchronized(putMonitor_) {
      long start = (msecs <= 0)? 0 : System.currentTimeMillis();
      long waitTime = msecs;
      while (emptySlots_ <= 0) {
        if (waitTime <= 0) return false;
	try { putMonitor_.wait(waitTime); }
        catch (InterruptedException ex) {
          putMonitor_.notify();
          throw ex;
        }
        waitTime = msecs - (System.currentTimeMillis() - start);
      }
      insert(x);
    }
    incUsedSlots();
    return true;
  }



  public  Object take() throws InterruptedException { 
    if (Thread.interrupted()) throw new InterruptedException();
    Object old = null; 
    synchronized(this) { 
      while (usedSlots_ <= 0) {
        try { wait(); }
        catch (InterruptedException ex) {
          notify();
          throw ex; 
        }
      }
      old = extract();
    }
    incEmptySlots();
    return old;
  }

  public  Object poll(long msecs) throws InterruptedException { 
    if (Thread.interrupted()) throw new InterruptedException();
    Object old = null; 
    synchronized(this) { 
      long start = (msecs <= 0)? 0 : System.currentTimeMillis();
      long waitTime = msecs;
      
      while (usedSlots_ <= 0) {
        if (waitTime <= 0) return null;
        try { wait(waitTime); }
        catch (InterruptedException ex) {
          notify();
          throw ex; 
        }
        waitTime = msecs - (System.currentTimeMillis() - start);

      }
      old = extract();
    }
    incEmptySlots();
    return old;
  }

}

/**
         * 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();
            // 这里如果state还是0说明锁还空着
            if (c == 0) {
                // 因为是公平锁版本的，先去看下是否阻塞队列里有排着队的
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    // 没有排队的，并且state使用cas设置成功的就标记当前占有锁的线程是我
                    setExclusiveOwnerThread(current);
                    // 然后其实就返回了，包括阻塞队列的head和tail节点和waitStatus都没有设置
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            // 这里就是第二个线程会返回false
            return false;
        }
    }

/**
     * Acquires in exclusive mode, ignoring interrupts.  Implemented
     * by invoking at least once {@link #tryAcquire},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquire} until success.  This method can be used
     * to implement method {@link Lock#lock}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     */
    public final void acquire(int arg) {
        // 前面第一种情况是tryAcquire直接成功了，这个if判断第一个条件就是false，就不往下执行了
        // 如果是第二个线程，第一个条件获取锁不成功，条件判断!tryAcquire(arg) == true，就会走
        // acquireQueued(addWaiter(Node.EXCLUSIVE), arg)
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

/**
     * Creates and enqueues node for current thread and given mode.
     *
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     */
    private Node addWaiter(Node mode) {
        // 这里是包装成一个node
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        // 最快的方式就是把当前线程的节点放在阻塞队列的最后
        Node pred = tail;
        // 只有当tail，也就是pred不为空的时候可以直接接上
        if (pred != null) {
            node.prev = pred;
            // 如果这里cas成功了，就直接接上返回了
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        // 不然就会继续走到这里
        enq(node);
        return node;
    }

/**
     * Inserts node into queue, initializing if necessary. See picture above.
     * @param node the node to insert
     * @return node's predecessor
     */
    private Node enq(final Node node) {
        for (;;) {
            // 如果状态没变化的话，tail这时还是null的
            Node t = tail;
            if (t == null) { // Must initialize
                // 这里就会初始化头结点，就是个空节点
                if (compareAndSetHead(new Node()))
                    // tail也赋值成head
                    tail = head;
            } else {
                // 这里就设置tail了
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

/**
     * Attempts to release this lock.
     *
     * <p>If the current thread is the holder of this lock then the hold
     * count is decremented.  If the hold count is now zero then the lock
     * is released.  If the current thread is not the holder of this
     * lock then {@link IllegalMonitorStateException} is thrown.
     *
     * @throws IllegalMonitorStateException if the current thread does not
     *         hold this lock
     */
    public void unlock() {
        // 释放锁
        sync.release(1);
    }
/**
     * Releases in exclusive mode.  Implemented by unblocking one or
     * more threads if {@link #tryRelease} returns true.
     * This method can be used to implement method {@link Lock#unlock}.
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryRelease} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @return the value returned from {@link #tryRelease}
     */
    public final boolean release(int arg) {
        // 尝试去释放
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
    		// 判断是否完全释放锁，因为可重入
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }
// 这段代码和上面的一致，只是为了顺序性，又拷下来看下

public final boolean release(int arg) {
        // 尝试去释放，如果是完全释放，返回的就是true，否则是false
        if (tryRelease(arg)) {
            Node h = head;
            // 这里判断头结点是否为空以及waitStatus的状态，前面说了head节点其实是
            // 在第二个线程进来的时候初始化的，如果是空的话说明没后续节点，并且waitStatus
            // 也表示了后续的等待状态
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

/**
     * Wakes up node's successor, if one exists.
     *
     * @param node the node
     */
// 唤醒后继节点
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        // 如果后继节点是空或者当前节点取消等待了
        if (s == null || s.waitStatus > 0) {
            s = null;
            // 从后往前找，找到非取消的节点，注意这里不是找到就退出，而是一直找到头
            // 所以不必担心中间有取消的
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            // 将其唤醒
            LockSupport.unpark(s.thread);
    }