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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.DataByteArrayInputStream;
import org.fusesource.hawtbuf.DataByteArrayOutputStream;
import org.fusesource.mqtt.client.QoS;
import org.fusesource.mqtt.client.Topic;

import java.io.IOException;
import java.net.ProtocolException;
import java.util.ArrayList;
import java.util.Arrays;
import static org.fusesource.mqtt.codec.MessageSupport.*;

/**
 * 
 * 
 *
 * @author Hiram Chirino
 */
public class SUBSCRIBE extends MessageSupport.HeaderBase implements Message, Acked {

    public static final byte TYPE = 8;
    public static final Topic[] NO_TOPICS = new Topic[0];

    private short messageId;
    private Topic[] topics = NO_TOPICS;

    public SUBSCRIBE() {
        qos(QoS.AT_LEAST_ONCE);
    }

    public byte messageType() {
        return TYPE;
    }

    public SUBSCRIBE decode(MQTTFrame frame) throws ProtocolException {
        assert(frame.buffers.length == 1);
        header(frame.header());

        DataByteArrayInputStream is = new DataByteArrayInputStream(frame.buffers[0]);
        QoS qos = qos();
        if(qos != QoS.AT_MOST_ONCE) {
            messageId = is.readShort();
        }
        ArrayList list = new ArrayList();
        while(is.available() > 0) {
            Topic topic = new Topic(MessageSupport.readUTF(is), QoS.values()[is.readByte()]);
            list.add(topic);
        }
        topics = list.toArray(new Topic[list.size()]);
        return this;
    }
    
    public MQTTFrame encode() {
        try {
            DataByteArrayOutputStream os = new DataByteArrayOutputStream();
            QoS qos = qos();
            if(qos != QoS.AT_MOST_ONCE) {
                os.writeShort(messageId);
            }
            for(Topic topic: topics) {
                MessageSupport.writeUTF(os, topic.name());
                os.writeByte(topic.qos().ordinal());
            }

            MQTTFrame frame = new MQTTFrame();
            frame.header(header());
            frame.commandType(TYPE);
            return frame.buffer(os.toBuffer());
        } catch (IOException e) {
            throw new RuntimeException("The impossible happened");
        }
    }

    @Override
    public boolean dup() {
        return super.dup();
    }

    @Override
    public SUBSCRIBE dup(boolean dup) {
        return (SUBSCRIBE) super.dup(dup);
    }

    @Override
    public QoS qos() {
        return super.qos();
    }

    public short messageId() {
        return messageId;
    }

    public SUBSCRIBE messageId(short messageId) {
        this.messageId = messageId;
        return this;
    }

    public Topic[] topics() {
        return topics;
    }

    public SUBSCRIBE topics(Topic[] topics) {
        this.topics = topics;
        return this;
    }

    @Override
    public String toString() {
        return "SUBSCRIBE{" +
                "dup=" + dup() +
                ", qos=" + qos() +
                ", messageId=" + messageId +
                ", topics=" + (topics == null ? null : Arrays.asList(topics)) +
                '}';
    }
}