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mqtt-client provides an ASL 2.0 licensed API to MQTT. It takes care of automatically reconnecting to your MQTT server and restoring your client session if any network failures occur. Applications can use a blocking API style, a futures based API, or a callback/continuations passing API style.

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/**
 * Copyright (C) 2010-2012, FuseSource Corp.  All rights reserved.
 *
 *     http://fusesource.com
 *
 * 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.fusesource.mqtt.codec;

import org.fusesource.hawtbuf.Buffer;
import org.fusesource.hawtdispatch.transport.AbstractProtocolCodec;
import org.fusesource.hawtdispatch.util.BufferPools;

import java.io.IOException;
import java.nio.ByteBuffer;

/**
 * 
 * 
 *
 * @author Hiram Chirino
 */
public class MQTTProtocolCodec extends AbstractProtocolCodec {

    private static final BufferPools BUFFER_POOLS = new BufferPools();

    private int maxMessageLength = 1024*1024*100;

    public MQTTProtocolCodec() {
        this.bufferPools = BUFFER_POOLS;
    }

    public int getMaxMessageLength() {
        return maxMessageLength;
    }

    public void setMaxMessageLength(int maxMessageLength) {
        this.maxMessageLength = maxMessageLength;
    }

    @Override
    protected void encode(Object value) throws IOException {
        MQTTFrame frame = (MQTTFrame) value;
        nextWriteBuffer.write(frame.header());

        int remaining = 0;
        for(Buffer buffer : frame.buffers) {
            remaining += buffer.length;
        }
        do {
            byte digit = (byte) (remaining & 0x7F);
            remaining >>>= 7;
            if (remaining > 0) {
                digit |= 0x80;
            }
            nextWriteBuffer.write(digit);
        } while (remaining > 0);
        for(Buffer buffer : frame.buffers) {
            nextWriteBuffer.write(buffer.data, buffer.offset, buffer.length);
        }
    }

    @Override
    protected Action initialDecodeAction() {
        return readHeader;
    }

    private final Action readHeader = new Action() {
        public MQTTFrame apply() throws IOException {
            int length = readLength();
            if( length >= 0 ) {
                if( length > maxMessageLength) {
                    throw new IOException("The maximum message length was exceeded");
                }
                byte header = readBuffer.get(readStart);
                readStart = readEnd;
                if( length > 0 ) {
                    nextDecodeAction = readBody(header, length);
                } else {
                    return new MQTTFrame().header(header);
                }
            }
            return null;
        }
    };

    private int readLength() throws IOException {
        readEnd = readStart+2; // Header is at least 2 bytes..
        int limit = readBuffer.position();
        int length = 0;
        int multiplier = 1;
        byte digit;

        while (readEnd-1 < limit) {
            // last byte is part of the encoded length..
            digit = readBuffer.get(readEnd-1);
            length += (digit & 0x7F) * multiplier;
            if( (digit & 0x80) == 0 ) {
                return length;
            }

            // length extends out one more byte..
            multiplier <<= 7;
            readEnd++;
        }
        return -1;
    }

    Action readBody(final byte header, final int length) {
        return new Action() {
            public MQTTFrame apply() throws IOException {
                int limit = readBuffer.position();
                if ((limit - readStart) < length) {
                    readEnd = limit;
                    return null;
                } else {
                    Buffer body = new Buffer(readBuffer.array(), readStart, length);
                    readEnd = readStart = readStart + length;
                    nextDecodeAction = readHeader;
                    return new MQTTFrame(body).header(header);
                }
            }
        };
    }
}