PacketManager.java

/*
 * PacketManager.java October 2007
 *
 * Copyright (C) 2007, Niall Gallagher <niallg@users.sf.net>
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or 
 * implied. See the License for the specific language governing 
 * permissions and limitations under the License.
 */

package org.simpleframework.transport;

import java.nio.ByteBuffer;
import java.util.concurrent.LinkedBlockingQueue;

/**
 * The <code>PacketManager</code> object is used to create buffers used to
 * buffer output. Buffers are created lazily so that they are allocated only on
 * demand. Typically buffers are only created when small chunks of data are
 * written to the transport and the socket is blocking. This ensures that
 * writing can continue without waiting for the data to be fully drained.
 * 
 * @author Niall Gallagher
 */
class PacketManager implements Recycler {

    /**
     * This is the queue that is used to recycle the buffers.
     */
    private Queue queue;

    /**
     * Determines how many buffers can be lazily created.
     */
    private int allow;

    /**
     * Determines the size of the buffers that are created.
     */
    private int size;

    /**
     * Constructor for the <code>PacketManager</code> object. This requires the
     * size of the buffers that will be allocated and the number of buffers that
     * can be lazily created before it will block waiting for the next buffer to
     * be returned.
     */
    public PacketManager() {
        this(3);
    }

    /**
     * Constructor for the <code>PacketManager</code> object. This requires the
     * size of the buffers that will be allocated and the number of buffers that
     * can be lazily created before it will block waiting for the next buffer to
     * be returned.
     * 
     * @param allow
     *            this is the size of the buffers to be allocated
     */
    public PacketManager(int allow) {
        this(allow, 4096);
    }

    /**
     * Constructor for the <code>PacketManager</code> object. This requires the
     * size of the buffers that will be allocated and the number of buffers that
     * can be lazily created before it will block waiting for the next buffer to
     * be returned.
     * 
     * @param allow
     *            this is the number of buffers to be created
     * @param size
     *            this is the size of the buffers to be allocated
     */
    public PacketManager(int allow, int size) {
        this.queue = new Queue();
        this.allow = allow;
        this.size = size;
    }

    /**
     * This checks to see if there is a buffer ready within the queue. If there
     * is one ready then this returns it, if not then this checks how many
     * buffers have been created. If we can create one then return a newly
     * instantiated buffer, otherwise block and wait for one to be recycled.
     * 
     * @return this returns the next ready buffer within the manager
     */
    public ByteBuffer allocate() throws PacketException {
        ByteBuffer next = this.queue.poll();

        if (next != null) return next;
        return this.create();
    }

    /**
     * This checks to see if there is a buffer ready within the queue. If there
     * is one ready then this returns it, if not then this checks how many
     * buffers have been created. If we can create one then return a newly
     * instantiated buffer, otherwise block and wait for one to be enqueued.
     * 
     * @return this returns the next ready buffer within the queue
     */
    private ByteBuffer create() throws PacketException {
        if (this.allow-- >= 0) return this.build();
        try {
            return this.queue.take();
        } catch (Exception e) {
            throw new PacketException("Thread interrupt", e);
        }
    }

    /**
     * This method is used to recycle the buffer. Invoking this with a buffer
     * instance will pass the buffer back in to the pool. Once passed back in to
     * the pool the buffer should no longer be used as it may affect future uses
     * of the buffer.
     * 
     * @param buffer
     *            this is the buffer that is to be recycled
     */
    @Override
    public void recycle(ByteBuffer buffer) {
        buffer.clear();
        this.queue.offer(buffer);
    }

    /**
     * This is used to allocate a buffer if there is no buffer ready within the
     * queue. The size of the buffer is determined from the size specified when
     * the buffer queue is created.
     * 
     * @return this returns a newly allocated byte buffer
     */
    private ByteBuffer build() {
        try {
            return ByteBuffer.allocateDirect(this.size);
        } catch (Throwable e) {
            return ByteBuffer.allocate(this.size);
        }
    }

    /**
     * The <code>Queue</code> is used to transfer recycled buffers back in to
     * the main memory store. When recycled they can be allocated again and used
     * to transfer data to the socket.
     */
    private class Queue extends LinkedBlockingQueue<ByteBuffer> {

        /**
         * Constructor for the <code>Queue</code> object. This is used to create
         * a queue that will transfer buffers back in to the main memory store
         * so they can be reused.
         */
        public Queue() {
            super();
        }
    }
}