Java源码解析阻塞队列ArrayBlockingQueue常用方法

本文基于jdk1.8进行分析

ArrayBlockingQueue的功能简介参考https://www.jb51.net/article/154211.htm。

首先看一下ArrayBlockingQueue的成员变量。如下图。最主要的成员变量是items，它是一个Object类型的数组用于保存阻塞队列中的元素。其次是takeIndex，putIndex，count，分别表示了从队列获取元素的位置，往队列里放元素的位置和队列中元素的个数。然后是lock，notEmpty和notFull三个和锁相关的成员变量。lock是一个可重入锁，而notEmpty和notFull是和lock绑定的2个Condition。对可重入锁不是很了解的同学，可以参考这篇文章https://www.jb51.net/article/154207.htm。对可重入锁的理解，是理解ArrayBlockingQueue的基础。也可以这么说，理解了可重入锁，那么在理解ArrayBlockingQueue就很顺利了。

  /** The queued items **/
  final Object[] items;
  /** items index for next take, poll, peek or remove **/
  int takeIndex;
  /** items index for next put, offer, or add **/
  int putIndex;
  /** Number of elements in the queue **/
  int count;
  /**
   * Concurrency control uses the classic two-condition algorithm
   * found in any textbook.
   **/
  /** Main lock guarding all access **/
  final ReentrantLock lock;
  /** Condition for waiting takes **/
  private final Condition notEmpty;
  /** Condition for waiting puts **/
  private final Condition notFull;
  /**
   * Shared state for currently active iterators, or null if there
   * are known not to be any. Allows queue operations to update
   * iterator state.
   **/
  transient Itrs itrs = null;

接下来介绍ArrayBlockingQueue的主要方法。首先是入队方法。ArrayBlockingQueue的入队方法有好几个，功能略有差异，下面我们逐一介绍各个入队方法。首先看一下put方法，如下图。put方法的功能是，往队列尾部插入指定元素，如果队列已满，那么就等待可用空间。方法的实现过程是，首先判断元素是否非空。然后，进行加锁，加锁后判断队列是否已满。如果已满，则等待不满条件。当被唤醒后，进行入队操作。入队方法中，会唤醒在notEmpty条件上等待的线程。

  /**
   * Inserts the specified element at the tail of this queue, waiting
   * for space to become available if the queue is full.
   * @throws InterruptedException {@inheritDoc}
   * @throws NullPointerException {@inheritDoc}
   **/
  public void put(E e) throws InterruptedException {
    checkNotNull(e);
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    try {
      while (count == items.length)
        notFull.await();
      enqueue(e);
    } finally {
      lock.unlock();
    }
  }
  /**
   * Inserts element at current put position, advances, and signals.
   * Call only when holding lock.
   **/
  private void enqueue(E x) {
    // assert lock.getHoldCount() == 1;
    // assert items[putIndex] == null;
    final Object[] items = this.items;
    items[putIndex] = x;
    if (++putIndex == items.length)
      putIndex = 0;
    count++;
    notEmpty.signal();
  }

另一个入队方法是offer，代码如下。这个方法与add方法的区别是，offer方法是立刻返回的，它并不像add方法那样，当队列满时会一直等待。

  /**
   * Inserts the specified element at the tail of this queue if it is
   * possible to do so immediately without exceeding the queue's capacity,
   * returning {@code true} upon success and {@code false} if this queue
   * is full. This method is generally preferable to method {@link #add},
   * which can fail to insert an element only by throwing an exception.
   * @throws NullPointerException if the specified element is null
   **/
  public boolean offer(E e) {
    checkNotNull(e);
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      if (count == items.length)
        return false;
      else {
        enqueue(e);
        return true;
      }
    } finally {
      lock.unlock();
    }
  }

接下来看一下出队方法take，代码如下。首先对可重入锁加锁，然后判断元素个数是否为0.如果为0，则等待不空条件，否则进行出队操作。

  public E take() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    try {
      while (count == 0)
        notEmpty.await();
      return dequeue();
    } finally {
      lock.unlock();
    }
  }

ArrayListBlockingqueue中还有其他相关方法，这里就不一一介绍了。

总结

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