RocketMQ中Producer的启动源码分析

RocketMQ中通过DefaultMQProducer创建Producer

DefaultMQProducer定义如下：

public class DefaultMQProducer extends ClientConfig implements MQProducer {
    protected final transient DefaultMQProducerImpl defaultMQProducerImpl;
    
    private String createTopicKey = MixAll.AUTO_CREATE_TOPIC_KEY_TOPIC = "TBW102";
    
    private volatile int defaultTopicQueueNums = 4;
    
    private int sendMsgTimeout = 3000;
    
    private int compressMsgBodyOverHowmuch = 1024 * 4;
    
    private int retryTimesWhenSendFailed = 2;
    
    private int retryTimesWhenSendAsyncFailed = 2;
    
    private boolean retryAnotherBrokerWhenNotStoreOK = false;
    
    private int maxMessageSize = 1024 * 1024 * 4; // 4M
}

其中defaultMQProducerImpl成员是Producer的具体实现，其余的一些成员是对一些参数的设置：
createTopicKey：是一个Topic值，在创建时使用，后面会说明
defaultTopicQueueNums ：默认的Topic队列个数
sendMsgTimeout：发送消息超时时间
compressMsgBodyOverHowmuch：消息容量限制，超过需要进行压缩
retryTimesWhenSendFailed：同步消息发送失败的允许重发次数
retryTimesWhenSendAsyncFailed：异步消息发送失败的允许重发次数
retryAnotherBrokerWhenNotStoreOK：是否允许发送给Broker失败后，重新选择Broker发送
maxMessageSize：消息最大大小
这些属性可以通过DefaultMQProducer提供的get、set方法进行相应操作

常用的构造方法如下：

public DefaultMQProducer() {
    this(MixAll.DEFAULT_PRODUCER_GROUP, null);
}

public DefaultMQProducer(final String producerGroup) {
    this(producerGroup, null);
}

public DefaultMQProducer(final String producerGroup, RPCHook rpcHook) {
    this.producerGroup = producerGroup;
    defaultMQProducerImpl = new DefaultMQProducerImpl(this, rpcHook);
}

DefaultMQProducer继承自ClientConfig，首先会设置ClientConfig提供的更底层的参数配置：

public class ClientConfig {
    public static final String SEND_MESSAGE_WITH_VIP_CHANNEL_PROPERTY = "com.rocketmq.sendMessageWithVIPChannel";
    
    private String namesrvAddr = System.getProperty(MixAll.NAMESRV_ADDR_PROPERTY, System.getenv(MixAll.NAMESRV_ADDR_ENV));
    
    private String clientIP = RemotingUtil.getLocalAddress();
    
    private String instanceName = System.getProperty("rocketmq.client.name", "DEFAULT");
    
    private int clientCallbackExecutorThreads = Runtime.getRuntime().availableProcessors();
    
    private int pollNameServerInterval = 1000 * 30;
    
    private int heartbeatBrokerInterval = 1000 * 30;
    
    private boolean vipChannelEnabled = Boolean.parseBoolean(System.getProperty(SEND_MESSAGE_WITH_VIP_CHANNEL_PROPERTY, "true"));
}

其中namesrvAddr是非常重要的成员，其保存着名称服务器（Name Server）的地址，在一开始构造时会根据系统属性进行设置，若是没有设置系统属性就是null，则需要在后面通过set方法进行设置
clientIP：Producer端的本地IP
instanceName：Producer的实例名称
pollNameServerInterval ：轮询NameServer的时间间隔
heartbeatBrokerInterval ：向Broker发送心跳包的时间间隔
SEND_MESSAGE_WITH_VIP_CHANNEL_PROPERTY 和vipChannelEnabled：决定是否使用VIP通道，即高优先级

回到DefaultMQProducer的构造方法，其会创建DefaultMQProducerImpl实例

private final Random random = new Random();
private final ConcurrentMap<String/* topic */, TopicPublishInfo> topicPublishInfoTable =
        new ConcurrentHashMap<String, TopicPublishInfo>();
private final ArrayList<SendMessageHook> sendMessageHookList = new ArrayList<SendMessageHook>();
private final RPCHook rpcHook;
protected BlockingQueue<Runnable> checkRequestQueue;
protected ExecutorService checkExecutor;
private ServiceState serviceState = ServiceState.CREATE_JUST;
private MQClientInstance mQClientFactory;
private ArrayList<CheckForbiddenHook> checkForbiddenHookList = new ArrayList<CheckForbiddenHook>();
private int zipCompressLevel = Integer.parseInt(System.getProperty(MixAll.MESSAGE_COMPRESS_LEVEL, "5"));
private MQFaultStrategy mqFaultStrategy = new MQFaultStrategy();
private final BlockingQueue<Runnable> asyncSenderThreadPoolQueue;
private final ExecutorService defaultAsyncSenderExecutor;
private ExecutorService asyncSenderExecutor;

public DefaultMQProducerImpl(final DefaultMQProducer defaultMQProducer, RPCHook rpcHook) {
    this.defaultMQProducer = defaultMQProducer;
    this.rpcHook = rpcHook;

    this.asyncSenderThreadPoolQueue = new LinkedBlockingQueue<Runnable>(50000);
    this.defaultAsyncSenderExecutor = new ThreadPoolExecutor(
        Runtime.getRuntime().availableProcessors(),
        Runtime.getRuntime().availableProcessors(),
        1000 * 60,
        TimeUnit.MILLISECONDS,
        this.asyncSenderThreadPoolQueue,
        new ThreadFactory() {
            private AtomicInteger threadIndex = new AtomicInteger(0);

            @Override
            public Thread newThread(Runnable r) {
                return new Thread(r, "AsyncSenderExecutor_" + this.threadIndex.incrementAndGet());
            }
        });
}

在构造方法中会创建一个线程池，用来处理异步消息的发送
其中有一个topicPublishInfoTable成员很重要，是一个map，保存了不同top和消息队列之间的映射，在后面详细介绍

DefaultMQProducer创建完成后，接着来看DefaultMQProducer的start方法：

public void start() throws MQClientException {
    this.defaultMQProducerImpl.start();
    if (null != traceDispatcher) {
        try {
            traceDispatcher.start(this.getNamesrvAddr());
        } catch (MQClientException e) {
            log.warn("trace dispatcher start failed ", e);
        }
    }
}

首先交给了defaultMQProducerImpl的start方法去处理

defaultMQProducerImpl的start方法：

public void start() throws MQClientException {
    this.start(true);
}

public void start(final boolean startFactory) throws MQClientException {
    switch (this.serviceState) {
        case CREATE_JUST:
            this.serviceState = ServiceState.START_FAILED;

            this.checkConfig();

            if (!this.defaultMQProducer.getProducerGroup().equals(MixAll.CLIENT_INNER_PRODUCER_GROUP)) {
                this.defaultMQProducer.changeInstanceNameToPID();
            }

            this.mQClientFactory = MQClientManager.getInstance().getAndCreateMQClientInstance(this.defaultMQProducer, rpcHook);

            boolean registerOK = mQClientFactory.registerProducer(this.defaultMQProducer.getProducerGroup(), this);
            if (!registerOK) {
                this.serviceState = ServiceState.CREATE_JUST;
                throw new MQClientException("The producer group[" + this.defaultMQProducer.getProducerGroup()
                    + "] has been created before, specify another name please." + FAQUrl.suggestTodo(FAQUrl.GROUP_NAME_DUPLICATE_URL),
                    null);
            }

            this.topicPublishInfoTable.put(this.defaultMQProducer.getCreateTopicKey(), new TopicPublishInfo());

            if (startFactory) {
                mQClientFactory.start();
            }

            log.info("the producer [{}] start OK. sendMessageWithVIPChannel={}", this.defaultMQProducer.getProducerGroup(),
                this.defaultMQProducer.isSendMessageWithVIPChannel());
            this.serviceState = ServiceState.RUNNING;
            break;
        case RUNNING:
        case START_FAILED:
        case SHUTDOWN_ALREADY:
            throw new MQClientException("The producer service state not OK, maybe started once, "
                + this.serviceState
                + FAQUrl.suggestTodo(FAQUrl.CLIENT_SERVICE_NOT_OK),
                null);
        default:
            break;
    }

    this.mQClientFactory.sendHeartbeatToAllBrokerWithLock();
}

在一开始DefaultMQProducerImpl实例化的时候，serviceState初始化为CREATE_JUST状态，这是一个枚举值，一共有如下几种状态：

public enum ServiceState {
    CREATE_JUST,
    RUNNING,
    SHUTDOWN_ALREADY,
    START_FAILED;

    private ServiceState() {
    }
}

这几个状态值很容易理解，在后面MQClientInstance中还会使用到

回到start方法，根据serviceState进行判断，只有当是CREATE_JUST状态时正常执行，防止在其他状态下错误调用start

直接看到CREATE_JUST的case部分：

this.serviceState = ServiceState.START_FAILED;

this.checkConfig();

if (!this.defaultMQProducer.getProducerGroup().equals(MixAll.CLIENT_INNER_PRODUCER_GROUP)) {
this.defaultMQProducer.changeInstanceNameToPID();
}

this.mQClientFactory = MQClientManager.getInstance().getAndCreateMQClientInstance(this.defaultMQProducer, rpcHook);

boolean registerOK = mQClientFactory.registerProducer(this.defaultMQProducer.getProducerGroup(), this);
if (!registerOK) {
this.serviceState = ServiceState.CREATE_JUST;
throw new MQClientException("The producer group[" + this.defaultMQProducer.getProducerGroup()
+ "] has been created before, specify another name please." + FAQUrl.suggestTodo(FAQUrl.GROUP_NAME_DUPLICATE_URL),
null);
}

this.topicPublishInfoTable.put(this.defaultMQProducer.getCreateTopicKey(), new TopicPublishInfo());

if (startFactory) {
mQClientFactory.start();
}

log.info("the producer [{}] start OK. sendMessageWithVIPChannel={}", this.defaultMQProducer.getProducerGroup(),
this.defaultMQProducer.isSendMessageWithVIPChannel());
this.serviceState = ServiceState.RUNNING;
break;

首先更改serviceState状态为START_FAILED，防止中途的失败

checkConfig方法是用来进行ProducerGroup命名检查：

private void checkConfig() throws MQClientException {
    Validators.checkGroup(this.defaultMQProducer.getProducerGroup());

    if (null == this.defaultMQProducer.getProducerGroup()) {
        throw new MQClientException("producerGroup is null", null);
    }

    if (this.defaultMQProducer.getProducerGroup().equals(MixAll.DEFAULT_PRODUCER_GROUP)) {
        throw new MQClientException("producerGroup can not equal " + MixAll.DEFAULT_PRODUCER_GROUP + ", please specify another one.",
            null);
    }
}

主要是检查命名的合法性，以及防止和默认的producerGroup生产者组名DEFAULT_PRODUCER_GROUP产生冲突

public static final String DEFAULT_PRODUCER_GROUP = "DEFAULT_PRODUCER";

接下来实例化mQClientFactory，这其实是生产者客户端的实例，其中MQClientManager采用单例模式，getInstance是获取MQClientManager的单例，根据ClientConfig的类型，通过getAndCreateMQClientInstance方法实例化不同属性的生产者客户端

MQClientManager：

public class MQClientManager {
    private final static InternalLogger log = ClientLogger.getLog();
    private static MQClientManager instance = new MQClientManager();
    private AtomicInteger factoryIndexGenerator = new AtomicInteger();
    private ConcurrentMap<String/* clientId */, MQClientInstance> factoryTable =
        new ConcurrentHashMap<String, MQClientInstance>();

    private MQClientManager() {
    }

    public static MQClientManager getInstance() {
        return instance;
    }
}

其中factoryTable是所有生产者客户端实例的map缓存，factoryIndexGenerator 是创建的每个客户端实例的流水号

getAndCreateMQClientInstance方法：

public MQClientInstance getAndCreateMQClientInstance(final ClientConfig clientConfig, RPCHook rpcHook) {
    String clientId = clientConfig.buildMQClientId();
    MQClientInstance instance = this.factoryTable.get(clientId);
    if (null == instance) {
        instance =
            new MQClientInstance(clientConfig.cloneClientConfig(),
                this.factoryIndexGenerator.getAndIncrement(), clientId, rpcHook);
        MQClientInstance prev = this.factoryTable.putIfAbsent(clientId, instance);
        if (prev != null) {
            instance = prev;
            log.warn("Returned Previous MQClientInstance for clientId:[{}]", clientId);
        } else {
            log.info("Created new MQClientInstance for clientId:[{}]", clientId);
        }
    }

    return instance;
}

首先通过buildMQClientId方法创建clientId：

public String buildMQClientId() {
    StringBuilder sb = new StringBuilder();
    sb.append(this.getClientIP());

    sb.append("@");
    sb.append(this.getInstanceName());
    if (!UtilAll.isBlank(this.unitName)) {
        sb.append("@");
        sb.append(this.unitName);
    }

    return sb.toString();
}

clientId主要由生产者客户端的ip地址以及实例名称，根据unitName的有无，附加unitName

通过生成的clientId，在factoryTable缓存中先去获取是否创建过客户端实例
若是没有获取到，就需要实例化一个MQClientInstance
这里在实例化MQClientInstance时，并没有直接传入clientConfig，而是通过cloneClientConfig方法复制了一份，来保证安全性：

public ClientConfig cloneClientConfig() {
    ClientConfig cc = new ClientConfig();
    cc.namesrvAddr = namesrvAddr;
    cc.clientIP = clientIP;
    cc.instanceName = instanceName;
    cc.clientCallbackExecutorThreads = clientCallbackExecutorThreads;
    cc.pollNameServerInterval = pollNameServerInterval;
    cc.heartbeatBrokerInterval = heartbeatBrokerInterval;
    cc.persistConsumerOffsetInterval = persistConsumerOffsetInterval;
    cc.unitMode = unitMode;
    cc.unitName = unitName;
    cc.vipChannelEnabled = vipChannelEnabled;
    cc.useTLS = useTLS;
    cc.language = language;
    return cc;
}

创建MQClientInstance实例：

public MQClientInstance(ClientConfig clientConfig, int instanceIndex, String clientId, RPCHook rpcHook) {
    this.clientConfig = clientConfig;
    this.instanceIndex = instanceIndex;
    this.nettyClientConfig = new NettyClientConfig();
    this.nettyClientConfig.setClientCallbackExecutorThreads(clientConfig.getClientCallbackExecutorThreads());
    this.nettyClientConfig.setUseTLS(clientConfig.isUseTLS());
    this.clientRemotingProcessor = new ClientRemotingProcessor(this);
    this.mQClientAPIImpl = new MQClientAPIImpl(this.nettyClientConfig, this.clientRemotingProcessor, rpcHook, clientConfig);

    if (this.clientConfig.getNamesrvAddr() != null) {
        this.mQClientAPIImpl.updateNameServerAddressList(this.clientConfig.getNamesrvAddr());
        log.info("user specified name server address: {}", this.clientConfig.getNamesrvAddr());
    }

    this.clientId = clientId;

    this.mQAdminImpl = new MQAdminImpl(this);

    this.pullMessageService = new PullMessageService(this);

    this.rebalanceService = new RebalanceService(this);

    this.defaultMQProducer = new DefaultMQProducer(MixAll.CLIENT_INNER_PRODUCER_GROUP);
    this.defaultMQProducer.resetClientConfig(clientConfig);

    this.consumerStatsManager = new ConsumerStatsManager(this.scheduledExecutorService);

    log.info("Created a new client Instance, InstanceIndex:{}, ClientID:{}, ClientConfig:{}, ClientVersion:{}, SerializerType:{}",
        this.instanceIndex,
        this.clientId,
        this.clientConfig,
        MQVersion.getVersionDesc(MQVersion.CURRENT_VERSION), RemotingCommand.getSerializeTypeConfigInThisServer());
}

可以看到MQClientInstance的构造方法创建了很多东西，就不一一说明，主要说几个重要的
其中nettyClientConfig，就很清楚的说明了RocketMQ通过Netty来进行网络之间的I/O，其保存了对Netty的一些配置
clientRemotingProcessor，用来进行消息的处理

mQClientAPIImpl则是一个非常重要的部分，直接实例化了一个MQClientAPIImpl对象：

public MQClientAPIImpl(final NettyClientConfig nettyClientConfig,
    final ClientRemotingProcessor clientRemotingProcessor,
    RPCHook rpcHook, final ClientConfig clientConfig) {
    this.clientConfig = clientConfig;
    topAddressing = new TopAddressing(MixAll.getWSAddr(), clientConfig.getUnitName());
    this.remotingClient = new NettyRemotingClient(nettyClientConfig, null);
    this.clientRemotingProcessor = clientRemotingProcessor;
    
    this.remotingClient.registerRPCHook(rpcHook);
    this.remotingClient.registerProcessor(RequestCode.CHECK_TRANSACTION_STATE, this.clientRemotingProcessor, null);
    
    this.remotingClient.registerProcessor(RequestCode.NOTIFY_CONSUMER_IDS_CHANGED, this.clientRemotingProcessor, null);
    
    this.remotingClient.registerProcessor(RequestCode.RESET_CONSUMER_CLIENT_OFFSET, this.clientRemotingProcessor, null);
    
    this.remotingClient.registerProcessor(RequestCode.GET_CONSUMER_STATUS_FROM_CLIENT, this.clientRemotingProcessor, null);
    
    this.remotingClient.registerProcessor(RequestCode.GET_CONSUMER_RUNNING_INFO, this.clientRemotingProcessor, null);
    
    this.remotingClient.registerProcessor(RequestCode.CONSUME_MESSAGE_DIRECTLY, this.clientRemotingProcessor, null);
}

可以看到在这个构造方法里，首先创建了一个TopAddressing，用于以后的名称服务的寻址，其默认地址是：

http://jmenv.tbsite.net:8080/rocketmq/nsaddr

需要通过系统属性来完成更改

接着创建了一个NettyRemotingClient，这个就是实实在在的Netty客户端

private final Bootstrap bootstrap = new Bootstrap();
// 名称服务列表
private final AtomicReference<List<String>> namesrvAddrList = new AtomicReference<List<String>>();

public NettyRemotingClient(final NettyClientConfig nettyClientConfig,
    final ChannelEventListener channelEventListener) {
    super(nettyClientConfig.getClientOnewaySemaphoreValue(), nettyClientConfig.getClientAsyncSemaphoreValue());
    this.nettyClientConfig = nettyClientConfig;
    this.channelEventListener = channelEventListener;

    int publicThreadNums = nettyClientConfig.getClientCallbackExecutorThreads();
    if (publicThreadNums <= 0) {
        publicThreadNums = 4;
    }

    this.publicExecutor = Executors.newFixedThreadPool(publicThreadNums, new ThreadFactory() {
        private AtomicInteger threadIndex = new AtomicInteger(0);

        @Override
        public Thread newThread(Runnable r) {
            return new Thread(r, "NettyClientPublicExecutor_" + this.threadIndex.incrementAndGet());
        }
    });

    this.eventLoopGroupWorker = new NioEventLoopGroup(1, new ThreadFactory() {
        private AtomicInteger threadIndex = new AtomicInteger(0);

        @Override
        public Thread newThread(Runnable r) {
            return new Thread(r, String.format("NettyClientSelector_%d", this.threadIndex.incrementAndGet()));
        }
    });

    if (nettyClientConfig.isUseTLS()) {
        try {
            sslContext = TlsHelper.buildSslContext(true);
            log.info("SSL enabled for client");
        } catch (IOException e) {
            log.error("Failed to create SSLContext", e);
        } catch (CertificateException e) {
            log.error("Failed to create SSLContext", e);
            throw new RuntimeException("Failed to create SSLContext", e);
        }
    }
}

此时Netty的客户端仅仅完成了对Bootstrap的初始化，以及对NioEventLoopGroup的设置和初始化

回到MQClientInstance的构造方法，在完成MQClientAPIImpl的创建后，会根据clientConfig的getNamesrvAddr判断是否设置了namesrvAddr名称服务地址，若是设置了，需要通过mQClientAPIImpl的updateNameServerAddressList方法，完成对名称服务地址的更新：

MQClientAPIImpl的updateNameServerAddressList方法：

public void updateNameServerAddressList(final String addrs) {
    String[] addrArray = addrs.split(";");
    List<String> list = Arrays.asList(addrArray);
    this.remotingClient.updateNameServerAddressList(list);
}

由于名称服务可以是集群的方式，所以在这里用“；”进行分割，得到所有的名称服务地址，再由remotingClient进行更新，而此时的remotingClient也就是刚才创建的NettyRemotingClient
NettyRemotingClient的updateNameServerAddressList方法：

public void updateNameServerAddressList(List<String> addrs) {
    List<String> old = this.namesrvAddrList.get();
    boolean update = false;

    if (!addrs.isEmpty()) {
        if (null == old) {
            update = true;
        } else if (addrs.size() != old.size()) {
            update = true;
        } else {
            for (int i = 0; i < addrs.size() && !update; i++) {
                if (!old.contains(addrs.get(i))) {
                    update = true;
                }
            }
        }

        if (update) {
            Collections.shuffle(addrs);
            log.info("name server address updated. NEW : {} , OLD: {}", addrs, old);
            this.namesrvAddrList.set(addrs);
        }
    }
}

这里逻辑比较简单，完成了名称服务列表的更新

回到MQClientInstance的构造方法，做完以上操作后，又在后面创建了MQAdminImpl、PullMessageService、RebalanceService、ConsumerStatsManager以及一个新的DefaultMQProducer，关于这几个在后面出现时再介绍

回到MQClientManager的getAndCreateMQClientInstance方法，在完成MQClientInstance的创建后，将其放入缓存中

再回到DefaultMQProducerImpl的start方法，在创建完MQClientInstance后，调用registerProducer方法
MQClientInstance的registerProducer方法：

public boolean registerProducer(final String group, final DefaultMQProducerImpl producer) {
    if (null == group || null == producer) {
        return false;
    }

    MQProducerInner prev = this.producerTable.putIfAbsent(group, producer);
    if (prev != null) {
        log.warn("the producer group[{}] exist already.", group);
        return false;
    }

    return true;
}

在MQClientInstance初始化时，会创建producerTable 、consumerTable 、topicRouteTable 、brokerAddrTable 这几个比较重要的map

private final ConcurrentMap<String/* group */, MQProducerInner> producerTable = new ConcurrentHashMap<String, MQProducerInner>();
private final ConcurrentMap<String/* group */, MQConsumerInner> consumerTable = new ConcurrentHashMap<String, MQConsumerInner>();
private final ConcurrentMap<String/* Topic */, TopicRouteData> topicRouteTable = new ConcurrentHashMap<String, TopicRouteData>();
private final ConcurrentMap<String/* Broker Name */, HashMap<Long/* brokerId */, String/* address */>> brokerAddrTable =
        new ConcurrentHashMap<String, HashMap<Long, String>>();

其中MQProducerInner是接口，DefaultMQProducerImpl是其实现类，完成了以group组名称为键值的DefaultMQProducerImpl的关联
在这里就是根据group，进行DefaultMQProducerImpl的缓存，MQConsumerInner同理
topicRouteTable 则记录与Topic对应的Broker以及消息队列信息
brokerAddrTable则记录与Broker Name对应的Broker的地址列表

还是回到start方法，在完成registerProducer方法后，根据返回值registerOK，判断接下来的操作
若是失败，将serviceState置为CREATE_JUST，并报出异常，方便下一次的正常start

若是成功，则先需要向topicPublishInfoTable中添加一条键值为createTopicKey（"TBW102"）的TopicPublishInfo记录
TopicPublishInfo：

public class TopicPublishInfo {
    private boolean orderTopic = false;
    private boolean haveTopicRouterInfo = false;
    private List<MessageQueue> messageQueueList = new ArrayList<MessageQueue>();
    private volatile ThreadLocalIndex sendWhichQueue = new ThreadLocalIndex();
}

其中messageQueueList存放消息队列MessageQueue，sendWhichQueue 是用来获取sendWhichQueue中的下标，也就是当前所要发送的具体的消息队列

MessageQueue：

public class MessageQueue implements Comparable<MessageQueue>, Serializable {
    private static final long serialVersionUID = 6191200464116433425L;
    private String topic;
    private String brokerName;
    private int queueId;

    public MessageQueue() {
    }

    public MessageQueue(String topic, String brokerName, int queueId) {
        this.topic = topic;
        this.brokerName = brokerName;
        this.queueId = queueId;
    }
}

可以看到这是一个简单的pojo，其封装了topic，brokerName以及queueId

ThreadLocalIndex :

public class ThreadLocalIndex {
    private final ThreadLocal<Integer> threadLocalIndex = new ThreadLocal<Integer>();
    private final Random random = new Random();

    public int getAndIncrement() {
        Integer index = this.threadLocalIndex.get();
        if (null == index) {
            index = Math.abs(random.nextInt());
            if (index < 0)
                index = 0;
            this.threadLocalIndex.set(index);
        }

        index = Math.abs(index + 1);
        if (index < 0)
            index = 0;

        this.threadLocalIndex.set(index);
        return index;
    }

    @Override
    public String toString() {
        return "ThreadLocalIndex{" +
            "threadLocalIndex=" + threadLocalIndex.get() +
            '}';
    }
}

通过ThreadLocal，赋予每个线程一个随机值，后面会根据这个随机值通过和messageQueueList的length取余运算，选取一个MessageQueue ，进而选取一条真正的消息队列进行消息发送

再次回到DefaultMQProducerImpl的start方法，在完成createTopicKey的Topic的记录添加后，根据startFactory判断是否需要调用mQClientFactory的start方法，这里默认startFactory是true，就需要调用mQClientFactory的start方法：

MQClientInstance的start方法：

public void start() throws MQClientException {
    synchronized (this) {
        switch (this.serviceState) {
            case CREATE_JUST:
                this.serviceState = ServiceState.START_FAILED;
                // If not specified,looking address from name server
                if (null == this.clientConfig.getNamesrvAddr()) {
                    this.mQClientAPIImpl.fetchNameServerAddr();
                }
                // Start request-response channel
                this.mQClientAPIImpl.start();
                // Start various schedule tasks
                this.startScheduledTask();
                // Start pull service
                this.pullMessageService.start();
                // Start rebalance service
                this.rebalanceService.start();
                // Start push service
                this.defaultMQProducer.getDefaultMQProducerImpl().start(false);
                log.info("the client factory [{}] start OK", this.clientId);
                this.serviceState = ServiceState.RUNNING;
                break;
            case RUNNING:
                break;
            case SHUTDOWN_ALREADY:
                break;
            case START_FAILED:
                throw new MQClientException("The Factory object[" + this.getClientId() + "] has been created before, and failed.", null);
            default:
                break;
        }
    }
}

MQClientInstance在创建时其serviceState状态也是CREATE_JUST

这里首先检查名称服务地址是否设置，若是没有设置，则通过MQClientAPIImpl的fetchNameServerAddr方法，尝试自动获取名称服务
MQClientAPIImpl的fetchNameServerAddr方法：

public String fetchNameServerAddr() {
    try {
        String addrs = this.topAddressing.fetchNSAddr();
        if (addrs != null) {
            if (!addrs.equals(this.nameSrvAddr)) {
                log.info("name server address changed, old=" + this.nameSrvAddr + ", new=" + addrs);
                this.updateNameServerAddressList(addrs);
                this.nameSrvAddr = addrs;
                return nameSrvAddr;
            }
        }
    } catch (Exception e) {
        log.error("fetchNameServerAddr Exception", e);
    }
    return nameSrvAddr;
}

这里首先根据topAddressing的fetchNSAddr方法获取名称服务地址，若是获取到了，则判断是否需要更新名称服务列表以及原来的nameSrvAddr

topAddressing在前面说过，MQClientAPIImpl构造方法中，创建TopAddressing实例
TopAddressing的fetchNSAddr方法：

public final String fetchNSAddr() {
    return fetchNSAddr(true, 3000);
}

public final String fetchNSAddr(boolean verbose, long timeoutMills) {
    String url = this.wsAddr;
    try {
        if (!UtilAll.isBlank(this.unitName)) {
            url = url + "-" + this.unitName + "?nofix=1";
        }
        HttpTinyClient.HttpResult result = HttpTinyClient.httpGet(url, null, null, "UTF-8", timeoutMills);
        if (200 == result.code) {
            String responseStr = result.content;
            if (responseStr != null) {
                return clearNewLine(responseStr);
            } else {
                log.error("fetch nameserver address is null");
            }
        } else {
            log.error("fetch nameserver address failed. statusCode=" + result.code);
        }
    } catch (IOException e) {
        if (verbose) {
            log.error("fetch name server address exception", e);
        }
    }

    if (verbose) {
        String errorMsg =
            "connect to " + url + " failed, maybe the domain name " + MixAll.getWSAddr() + " not bind in /etc/hosts";
        errorMsg += FAQUrl.suggestTodo(FAQUrl.NAME_SERVER_ADDR_NOT_EXIST_URL);

        log.warn(errorMsg);
    }
    return null;
}

首先根据wsAddr和unitName创建url，其中wsAddr在前面说过，默认是http://jmenv.tbsite.net:8080/rocketmq/nsaddr，需要通过系统属性来更改

然后通过HttpTinyClient的httpGet方法建立连接，进行GET请求，获取名称地址
HttpTinyClient的httpGet方法：

static public HttpResult httpGet(String url, List<String> headers, List<String> paramValues,
    String encoding, long readTimeoutMs) throws IOException {
    String encodedContent = encodingParams(paramValues, encoding);
    url += (null == encodedContent) ? "" : ("?" + encodedContent);

    HttpURLConnection conn = null;
    try {
        conn = (HttpURLConnection) new URL(url).openConnection();
        conn.setRequestMethod("GET");
        conn.setConnectTimeout((int) readTimeoutMs);
        conn.setReadTimeout((int) readTimeoutMs);
        setHeaders(conn, headers, encoding);

        conn.connect();
        int respCode = conn.getResponseCode();
        String resp = null;

        if (HttpURLConnection.HTTP_OK == respCode) {
            resp = IOTinyUtils.toString(conn.getInputStream(), encoding);
        } else {
            resp = IOTinyUtils.toString(conn.getErrorStream(), encoding);
        }
        return new HttpResult(respCode, resp);
    } finally {
        if (conn != null) {
            conn.disconnect();
        }
    }
}

这里就通过了JDK原生的HttpURLConnection ，完成了一次指定url的GET请求，返回请求数据，将请求到的数据以及状态码封装为HttpResult，返回给上一级调用，也就是TopAddressing的fetchNSAddr方法中，再调用clearNewLine方法，将状态码为200的数据处理（清除不必要的空客、换行、回车），得到名称地址，最后回到fetchNameServerAddr方法中，完成名称服务列表的更新，至此自动获取名称服务结束

回到MQClientInstance的start方法中：
在确定有名称服务的情况下，首先调用mQClientAPIImpl的start方法：
MQClientAPIImpl的start方法：

public void start() {
    this.remotingClient.start();
}

这里实际上调用了前面所创建的Nettt客户端的start方法：
NettyRemotingClient的start方法：

public void start() {
    this.defaultEventExecutorGroup = new DefaultEventExecutorGroup(
        nettyClientConfig.getClientWorkerThreads(),
        new ThreadFactory() {

            private AtomicInteger threadIndex = new AtomicInteger(0);

            @Override
            public Thread newThread(Runnable r) {
                return new Thread(r, "NettyClientWorkerThread_" + this.threadIndex.incrementAndGet());
            }
        });

    Bootstrap handler = this.bootstrap.group(this.eventLoopGroupWorker).channel(NioSocketChannel.class)
        .option(ChannelOption.TCP_NODELAY, true)
        .option(ChannelOption.SO_KEEPALIVE, false)
        .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, nettyClientConfig.getConnectTimeoutMillis())
        .option(ChannelOption.SO_SNDBUF, nettyClientConfig.getClientSocketSndBufSize())
        .option(ChannelOption.SO_RCVBUF, nettyClientConfig.getClientSocketRcvBufSize())
        .handler(new ChannelInitializer<SocketChannel>() {
            @Override
            public void initChannel(SocketChannel ch) throws Exception {
                ChannelPipeline pipeline = ch.pipeline();
                if (nettyClientConfig.isUseTLS()) {
                    if (null != sslContext) {
                        pipeline.addFirst(defaultEventExecutorGroup, "sslHandler", sslContext.newHandler(ch.alloc()));
                        log.info("Prepend SSL handler");
                    } else {
                        log.warn("Connections are insecure as SSLContext is null!");
                    }
                }
                pipeline.addLast(
                    defaultEventExecutorGroup,
                    new NettyEncoder(),
                    new NettyDecoder(),
                    new IdleStateHandler(0, 0, nettyClientConfig.getClientChannelMaxIdleTimeSeconds()),
                    new NettyConnectManageHandler(),
                    new NettyClientHandler());
            }
        });

    this.timer.scheduleAtFixedRate(new TimerTask() {
        @Override
        public void run() {
            try {
                NettyRemotingClient.this.scanResponseTable();
            } catch (Throwable e) {
                log.error("scanResponseTable exception", e);
            }
        }
    }, 1000 * 3, 1000);

    if (this.channelEventListener != null) {
        this.nettyEventExecutor.start();
    }
}

这里完成了Bootstrap对前面创建的EventLoopGroup以及handler的绑定

在完成mQClientAPIImpl的start方法后，调用startScheduledTask方法，启动定时任务
startScheduledTask方法：

private void startScheduledTask() {
    if (null == this.clientConfig.getNamesrvAddr()) {
        this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {

            @Override
            public void run() {
                try {
                    MQClientInstance.this.mQClientAPIImpl.fetchNameServerAddr();
                } catch (Exception e) {
                    log.error("ScheduledTask fetchNameServerAddr exception", e);
                }
            }
        }, 1000 * 10, 1000 * 60 * 2, TimeUnit.MILLISECONDS);
    }

    this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {

        @Override
        public void run() {
            try {
                MQClientInstance.this.updateTopicRouteInfoFromNameServer();
            } catch (Exception e) {
                log.error("ScheduledTask updateTopicRouteInfoFromNameServer exception", e);
            }
        }
    }, 10, this.clientConfig.getPollNameServerInterval(), TimeUnit.MILLISECONDS);

    this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {

        @Override
        public void run() {
            try {
                MQClientInstance.this.cleanOfflineBroker();
                MQClientInstance.this.sendHeartbeatToAllBrokerWithLock();
            } catch (Exception e) {
                log.error("ScheduledTask sendHeartbeatToAllBroker exception", e);
            }
        }
    }, 1000, this.clientConfig.getHeartbeatBrokerInterval(), TimeUnit.MILLISECONDS);

    this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {

        @Override
        public void run() {
            try {
                MQClientInstance.this.persistAllConsumerOffset();
            } catch (Exception e) {
                log.error("ScheduledTask persistAllConsumerOffset exception", e);
            }
        }
    }, 1000 * 10, this.clientConfig.getPersistConsumerOffsetInterval(), TimeUnit.MILLISECONDS);

    this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {

        @Override
        public void run() {
            try {
                MQClientInstance.this.adjustThreadPool();
            } catch (Exception e) {
                log.error("ScheduledTask adjustThreadPool exception", e);
            }
        }
    }, 1, 1, TimeUnit.MINUTES);
}

可以看到，一共设置了五个定时任务

①若是名称服务地址namesrvAddr不存在，则调用前面的fetchNameServerAddr方法，定时更新名称服务

②通过updateTopicRouteInfoFromNameServer方法定时更新Topic所对应的路由信息：

public void updateTopicRouteInfoFromNameServer() {
    Set<String> topicList = new HashSet<String>();

    // Consumer
    {
        Iterator<Entry<String, MQConsumerInner>> it = this.consumerTable.entrySet().iterator();
        while (it.hasNext()) {
            Entry<String, MQConsumerInner> entry = it.next();
            MQConsumerInner impl = entry.getValue();
            if (impl != null) {
                Set<SubscriptionData> subList = impl.subscriptions();
                if (subList != null) {
                    for (SubscriptionData subData : subList) {
                        topicList.add(subData.getTopic());
                    }
                }
            }
        }
    }

    // Producer
    {
        Iterator<Entry<String, MQProducerInner>> it = this.producerTable.entrySet().iterator();
        while (it.hasNext()) {
            Entry<String, MQProducerInner> entry = it.next();
            MQProducerInner impl = entry.getValue();
            if (impl != null) {
                Set<String> lst = impl.getPublishTopicList();
                topicList.addAll(lst);
            }
        }
    }

    for (String topic : topicList) {
        this.updateTopicRouteInfoFromNameServer(topic);
    }
}

将所有Consumer和Producer的Topic封装在topicList，交给updateTopicRouteInfoFromNameServer调用

updateTopicRouteInfoFromNameServer方法：

public boolean updateTopicRouteInfoFromNameServer(final String topic) {
    return updateTopicRouteInfoFromNameServer(topic, false, null);
}

public boolean updateTopicRouteInfoFromNameServer(final String topic, boolean isDefault,
        DefaultMQProducer defaultMQProducer) {
    try {
        if (this.lockNamesrv.tryLock(LOCK_TIMEOUT_MILLIS, TimeUnit.MILLISECONDS)) {
            try {
                TopicRouteData topicRouteData;
                if (isDefault && defaultMQProducer != null) {
                    topicRouteData = this.mQClientAPIImpl.getDefaultTopicRouteInfoFromNameServer(defaultMQProducer.getCreateTopicKey(),
                        1000 * 3);
                    if (topicRouteData != null) {
                        for (QueueData data : topicRouteData.getQueueDatas()) {
                            int queueNums = Math.min(defaultMQProducer.getDefaultTopicQueueNums(), data.getReadQueueNums());
                            data.setReadQueueNums(queueNums);
                            data.setWriteQueueNums(queueNums);
                        }
                    }
                } else {
                    topicRouteData = this.mQClientAPIImpl.getTopicRouteInfoFromNameServer(topic, 1000 * 3);
                }
                if (topicRouteData != null) {
                    TopicRouteData old = this.topicRouteTable.get(topic);
                    boolean changed = topicRouteDataIsChange(old, topicRouteData);
                    if (!changed) {
                        changed = this.isNeedUpdateTopicRouteInfo(topic);
                    } else {
                        log.info("the topic[{}] route info changed, old[{}] ,new[{}]", topic, old, topicRouteData);
                    }

                    if (changed) {
                        TopicRouteData cloneTopicRouteData = topicRouteData.cloneTopicRouteData();

                        for (BrokerData bd : topicRouteData.getBrokerDatas()) {
                            this.brokerAddrTable.put(bd.getBrokerName(), bd.getBrokerAddrs());
                        }

                        // Update Pub info
                        {
                            TopicPublishInfo publishInfo = topicRouteData2TopicPublishInfo(topic, topicRouteData);
                            publishInfo.setHaveTopicRouterInfo(true);
                            Iterator<Entry<String, MQProducerInner>> it = this.producerTable.entrySet().iterator();
                            while (it.hasNext()) {
                                Entry<String, MQProducerInner> entry = it.next();
                                MQProducerInner impl = entry.getValue();
                                if (impl != null) {
                                    impl.updateTopicPublishInfo(topic, publishInfo);
                                }
                            }
                        }

                        // Update sub info
                        {
                            Set<MessageQueue> subscribeInfo = topicRouteData2TopicSubscribeInfo(topic, topicRouteData);
                            Iterator<Entry<String, MQConsumerInner>> it = this.consumerTable.entrySet().iterator();
                            while (it.hasNext()) {
                                Entry<String, MQConsumerInner> entry = it.next();
                                MQConsumerInner impl = entry.getValue();
                                if (impl != null) {
                                    impl.updateTopicSubscribeInfo(topic, subscribeInfo);
                                }
                            }
                        }
                        log.info("topicRouteTable.put. Topic = {}, TopicRouteData[{}]", topic, cloneTopicRouteData);
                        this.topicRouteTable.put(topic, cloneTopicRouteData);
                        return true;
                    }
                } else {
                    log.warn("updateTopicRouteInfoFromNameServer, getTopicRouteInfoFromNameServer return null, Topic: {}", topic);
                }
            } catch (Exception e) {
                if (!topic.startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX) && !topic.equals(MixAll.AUTO_CREATE_TOPIC_KEY_TOPIC)) {
                    log.warn("updateTopicRouteInfoFromNameServer Exception", e);
                }
            } finally {
                this.lockNamesrv.unlock();
            }
        } else {
            log.warn("updateTopicRouteInfoFromNameServer tryLock timeout {}ms", LOCK_TIMEOUT_MILLIS);
        }
    } catch (InterruptedException e) {
        log.warn("updateTopicRouteInfoFromNameServer Exception", e);
    }

    return false;
}

这里首先由mQClientAPIImpl的getTopicRouteInfoFromNameServer方法，从名称服务器上获取其Topic所对应的路由信息

其中Topic的路由信息由TopicRouteData进行封装：

public class TopicRouteData extends RemotingSerializable {
    private String orderTopicConf;
    private List<QueueData> queueDatas;
    private List<BrokerData> brokerDatas;
    private HashMap<String/* brokerAddr */, List<String>/* Filter Server */> filterServerTable;
}

QueueData：

public class QueueData implements Comparable<QueueData> {
    private String brokerName;
    private int readQueueNums;
    private int writeQueueNums;
    private int perm;
    private int topicSynFlag;
}

BrokerData：

public class BrokerData implements Comparable<BrokerData> {
    private String cluster;
    private String brokerName;
    private HashMap<Long/* brokerId */, String/* broker address */> brokerAddrs;
}

getTopicRouteInfoFromNameServer方法：

public TopicRouteData getTopicRouteInfoFromNameServer(final String topic, final long timeoutMillis)
        throws RemotingException, MQClientException, InterruptedException {
    return getTopicRouteInfoFromNameServer(topic, timeoutMillis, true);
}

    public TopicRouteData getTopicRouteInfoFromNameServer(final String topic, final long timeoutMillis,
        boolean allowTopicNotExist) throws MQClientException, InterruptedException, RemotingTimeoutException, RemotingSendRequestException, RemotingConnectException {
    GetRouteInfoRequestHeader requestHeader = new GetRouteInfoRequestHeader();
    requestHeader.setTopic(topic);

    RemotingCommand request = RemotingCommand.createRequestCommand(RequestCode.GET_ROUTEINTO_BY_TOPIC, requestHeader);

    RemotingCommand response = this.remotingClient.invokeSync(null, request, timeoutMillis);
    assert response != null;
    switch (response.getCode()) {
        case ResponseCode.TOPIC_NOT_EXIST: {
            if (allowTopicNotExist && !topic.equals(MixAll.AUTO_CREATE_TOPIC_KEY_TOPIC)) {
                log.warn("get Topic [{}] RouteInfoFromNameServer is not exist value", topic);
            }

            break;
        }
        case ResponseCode.SUCCESS: {
            byte[] body = response.getBody();
            if (body != null) {
                return TopicRouteData.decode(body, TopicRouteData.class);
            }
        }
        default:
            break;
    }

    throw new MQClientException(response.getCode(), response.getRemark());
}

这里主要通过remotingClient即Netty客户端的invokeSync方法向名称服务器发送封装好的request请求来获取response
通过名称服务器寻找与Topic相关的Broker有关路由信息，将这些信息作为response返回，在这里接收到进行处理，封装成TopicRouteData

在invokeSync方法中采用懒加载的方式，尝试获取已经建立好连接的Channel，若是没有，则需要通过bootstrap的connect方法先建立连接产生ChannelFuture，进而获取并缓存Channel

回到updateTopicRouteInfoFromNameServer，通过名称服务器获取到了有关Topic的路由信息，调用topicRouteDataIsChange方法和原来topicRouteTable保存的路由信息进行比较
topicRouteDataIsChange方法：

private boolean topicRouteDataIsChange(TopicRouteData olddata, TopicRouteData nowdata) {
    if (olddata == null || nowdata == null)
        return true;
    TopicRouteData old = olddata.cloneTopicRouteData();
    TopicRouteData now = nowdata.cloneTopicRouteData();
    Collections.sort(old.getQueueDatas());
    Collections.sort(old.getBrokerDatas());
    Collections.sort(now.getQueueDatas());
    Collections.sort(now.getBrokerDatas());
    return !old.equals(now);
}

若是没有发生改变，任然要调用isNeedUpdateTopicRouteInfo方法检查是否有需要更新

isNeedUpdateTopicRouteInfo方法：

private boolean isNeedUpdateTopicRouteInfo(final String topic) {
    boolean result = false;
    {
        Iterator<Entry<String, MQProducerInner>> it = this.producerTable.entrySet().iterator();
        while (it.hasNext() && !result) {
            Entry<String, MQProducerInner> entry = it.next();
            MQProducerInner impl = entry.getValue();
            if (impl != null) {
                result = impl.isPublishTopicNeedUpdate(topic);
            }
        }
    }

    {
        Iterator<Entry<String, MQConsumerInner>> it = this.consumerTable.entrySet().iterator();
        while (it.hasNext() && !result) {
            Entry<String, MQConsumerInner> entry = it.next();
            MQConsumerInner impl = entry.getValue();
            if (impl != null) {
                result = impl.isSubscribeTopicNeedUpdate(topic);
            }
        }
    }

    return result;
}

分别对所有的消费者和生产者进行检查是否有需要更新有关该Topic的路由信息

当存在需要跟新的情况时，在updateTopicRouteInfoFromNameServer中
首先从topicRouteData中取出BrokerData，即Broker的路由信息，进行更新
再根据topicRouteData从中获取消费者生产者的消息路由信息，分别进行更新

③定时清除离线的Broker，以及向当前在线的Broker发送心跳包
cleanOfflineBroker清除离线的Broker：

private void cleanOfflineBroker() {
    try {
        if (this.lockNamesrv.tryLock(LOCK_TIMEOUT_MILLIS, TimeUnit.MILLISECONDS))
            try {
                ConcurrentHashMap<String, HashMap<Long, String>> updatedTable = new ConcurrentHashMap<String, HashMap<Long, String>>();

                Iterator<Entry<String, HashMap<Long, String>>> itBrokerTable = this.brokerAddrTable.entrySet().iterator();
                while (itBrokerTable.hasNext()) {
                    Entry<String, HashMap<Long, String>> entry = itBrokerTable.next();
                    String brokerName = entry.getKey();
                    HashMap<Long, String> oneTable = entry.getValue();

                    HashMap<Long, String> cloneAddrTable = new HashMap<Long, String>();
                    cloneAddrTable.putAll(oneTable);

                    Iterator<Entry<Long, String>> it = cloneAddrTable.entrySet().iterator();
                    while (it.hasNext()) {
                        Entry<Long, String> ee = it.next();
                        String addr = ee.getValue();
                        if (!this.isBrokerAddrExistInTopicRouteTable(addr)) {
                            it.remove();
                            log.info("the broker addr[{} {}] is offline, remove it", brokerName, addr);
                        }
                    }

                    if (cloneAddrTable.isEmpty()) {
                        itBrokerTable.remove();
                        log.info("the broker[{}] name's host is offline, remove it", brokerName);
                    } else {
                        updatedTable.put(brokerName, cloneAddrTable);
                    }
                }

                if (!updatedTable.isEmpty()) {
                    this.brokerAddrTable.putAll(updatedTable);
                }
            } finally {
                this.lockNamesrv.unlock();
            }
    } catch (InterruptedException e) {
        log.warn("cleanOfflineBroker Exception", e);
    }
}

这里的brokerAddrTable是会通过②中的定时任务来更新，遍历其中的所有Broker信息，通过isBrokerAddrExistInTopicRouteTable方法，进行检查：

private boolean isBrokerAddrExistInTopicRouteTable(final String addr) {
    Iterator<Entry<String, TopicRouteData>> it = this.topicRouteTable.entrySet().iterator();
    while (it.hasNext()) {
        Entry<String, TopicRouteData> entry = it.next();
        TopicRouteData topicRouteData = entry.getValue();
        List<BrokerData> bds = topicRouteData.getBrokerDatas();
        for (BrokerData bd : bds) {
            if (bd.getBrokerAddrs() != null) {
                boolean exist = bd.getBrokerAddrs().containsValue(addr);
                if (exist)
                    return true;
            }
        }
    }

    return false;
}

通过比对topicRouteTable中的所有TopicRouteData保存的BrokerAddrs来判断，若是Broker不存在，需要进行清除，进而更新brokerAddrTable

sendHeartbeatToAllBrokerWithLock定时向Broker发送心跳包：

public void sendHeartbeatToAllBrokerWithLock() {
    if (this.lockHeartbeat.tryLock()) {
        try {
            this.sendHeartbeatToAllBroker();
            this.uploadFilterClassSource();
        } catch (final Exception e) {
            log.error("sendHeartbeatToAllBroker exception", e);
        } finally {
            this.lockHeartbeat.unlock();
        }
    } else {
        log.warn("lock heartBeat, but failed.");
    }
}

这一部分就不详细介绍，主要还是通过Netty客户端完成心跳包的发送

④定时持久化消费者队列的消费进度，这个在分析消费者时再详细说明

⑤定时调整消费者端的线程池的大小，还是在分析消费者时再详细说明

startScheduledTask创建的五个定时任务结束，回到MQClientInstance的start方法
接着开启pullMessageService服务，为消费者拉取消息
然后开启rebalanceService服务，用来均衡消息队列
这两个服务在有关消费者时再介绍

接着通过：

this.defaultMQProducer.getDefaultMQProducerImpl().start(false);

开启push service服务
其中defaultMQProducer是在前面MQClientInstance构造方法中创建的

this.defaultMQProducer = new DefaultMQProducer(MixAll.CLIENT_INNER_PRODUCER_GROUP);

只不过他调用的start方法，参数为false，也就是没有调用mQClientFactory的start方法
后续会介绍其用途

到这DefaultMQProducerImpl的start方法已经基本完毕，只不过在最后，会通过mQClientFactory的sendHeartbeatToAllBrokerWithLock方法，给所有Broker发送一次心跳包

到此，Producer的启动结束