ArrayBlockingQueue

ArrayBlockingQueue是由数组实现的有界阻塞队列，数据遵循FIFO先进先出的顺序，一旦创建容量固定无法修改。尝试put一个元素到满队列则阻塞等待，尝试
take一个元素到满队列亦阻塞等待。支持可选的公平策略控制等待的生产者和消费者获取数据的顺序，默认顺序是无法保证的。public ArrayBlockingQueue(int capacity, boolean fair)
通过构造函数将fair设置为true能保证线程访问的顺序按照FIFO，这种模式下会降低吞吐量但也能减少可变性、避免饥饿。

public class ArrayBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {

    /**
     * Serialization ID. This class relies on default serialization
     * even for the items array, which is default-serialized, even if
     * it is empty. Otherwise it could not be declared final, which is
     * necessary here.
     */
    private static final long serialVersionUID = -817911632652898426L;

    /** 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;//控制线程的数据访问顺序

    /** Condition for waiting takes */
    private final Condition notEmpty;//条件等待：消费者等待获取数据

    /** Condition for waiting puts */
    private final Condition notFull;//条件等待：生产者等待放入数据
    ... ...
/**
 * Creates an {@code ArrayBlockingQueue} with the given (fixed)
 * capacity and the specified access policy.
 *
 * @param capacity the capacity of this queue
 * @param fair if {@code true} then queue accesses for threads blocked
 *        on insertion or removal, are processed in FIFO order;
 *        if {@code false} the access order is unspecified.
 * @throws IllegalArgumentException if {@code capacity < 1}
 */
public ArrayBlockingQueue(int capacity, boolean fair) {
    if (capacity <= 0)
        throw new IllegalArgumentException();
    this.items = new Object[capacity];
    lock = new ReentrantLock(fair);
    notEmpty = lock.newCondition();
    notFull =  lock.newCondition();
}