src/java/org/apache/turbine/util/pool/BoundedBuffer.java

package org.apache.turbine.util.pool;

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 * reserved.
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 *    if and wherever such third-party acknowledgments normally appear.
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 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * <http://www.apache.org/>.
 */

/**
 * Efficient array-based bounded buffer class.
 * Adapted from CPJ, chapter 8, which describes design.
 * Originally written by Doug Lea and released into the public domain.
 * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>] <p>
 *
 * @author <a href="mailto:ilkka.priha@simsoft.fi">Ilkka Priha</a>
 * @version $Id$
 */
public class BoundedBuffer
{
    /** The default capacity. */
    public static final int DEFAULT_CAPACITY = 1024;

    protected final Object[] buffer;      // the elements

    protected int takePtr = 0;            // circular indices
    protected int putPtr = 0;

    protected int usedSlots = 0;          // length
    protected int emptySlots;             // capacity - length

    /**
     * Creates a buffer with the given capacity.
     *
     * @param capacity the capacity.
     * @throws IllegalArgumentException if capacity less or equal to zero.
     */
    public BoundedBuffer(int capacity)
            throws IllegalArgumentException
    {
        if (capacity <= 0)
        {
            throw new IllegalArgumentException(
                    "Bounded Buffer must have capacity > 0!");
        }

        buffer = new Object[capacity];
        emptySlots = capacity;
    }

    /**
     * Creates a buffer with the default capacity
     */
    public BoundedBuffer()
    {
        this(DEFAULT_CAPACITY);
    }

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

    /**
     * Returns the capacity of the buffer.
     *
     * @return the capacity.
     */
    public int capacity()
    {
        return buffer.length;
    }

    /**
     * Peeks, but does not remove the top item from the buffer.
     *
     * @return the object or null.
     */
    public synchronized Object peek()
    {
        return (usedSlots > 0)
                ? buffer[takePtr] : null;
    }

    /**
     * Puts an item in the buffer only if there is capacity available.
     *
     * @param item the item to be inserted.
     * @return true if accepted, else false.
     */
    public synchronized boolean offer(Object x)
    {
        if (x == null)
        {
            throw new IllegalArgumentException("Bounded Buffer cannot store a null object");
        }

        if (emptySlots > 0)
        {
            --emptySlots;
            buffer[putPtr] = x;
            if (++putPtr >= buffer.length)
            {
                putPtr = 0;
            }
            usedSlots++;
            return true;
        }
        else
        {
            return false;
        }
    }

    /**
     * Polls and removes the top item from the buffer if one is available.
     *
     * @return the oldest item from the buffer, or null if the buffer is empty.
     */
    public synchronized Object poll()
    {
        if (usedSlots > 0)
        {
            --usedSlots;
            Object old = buffer[takePtr];
            buffer[takePtr] = null;
            if (++takePtr >= buffer.length)
            {
                takePtr = 0;
            }
            emptySlots++;
            return old;
        }
        else
        {
            return null;
        }
    }
}