先把幾個比較重要的方法列出來

// 入口，處理 message及messages
  def handle(events: Seq[KeyedMessage[K,V]])

//處理序列化的KeyedMessage資料
  private def dispatchSerializedData(messages: Seq[KeyedMessage[K,Message]]): Seq[KeyedMessage[K, Message]]

// 為messages分割槽
  def partitionAndCollate(messages: Seq[KeyedMessage[K,Message]]): Option[Map[Int, collection.mutable.Map[TopicAndPartition, Seq[KeyedMessage[K,Message]]]]]

  /**
   * Constructs and sends the produce request based on a map from (topic, partition) -> messages
   *
   * @param brokerId the broker that will receive the request
   * @param messagesPerTopic the messages as a map from (topic, partition) -> messages
   * @return the set (topic, partitions) messages which incurred an error sending or processing
   */
  private def send(brokerId: Int, messagesPerTopic: collection.mutable.Map[TopicAndPartition, ByteBufferMessageSet])

kafka在sync模式下發送訊息時，是通過DefaultEventHandler的handle方法把message傳送給broker，可以是一條訊息也可以是多條訊息組成的List。

用List舉例：
DefaultEventHandler得到messages（可能包含多個topic的messages）後，呼叫dispatchSerializedData方法，遍歷message為各個topic的每個message進行partition分割槽。如果message中key!=null, 則呼叫配置的partition.class定義的分割槽規則類，得到partitionIndex，否則在sendPartitionPerTopicCache的記錄中去獲取topic的partitionId,然後找到partitionIndex對應的leaderBrokerId。所以1個message會分配給1個topic的1個partition的leaderBrokerId.

package kafka.producer.async

import kafka.common._
import kafka.message.{NoCompressionCodec, Message, ByteBufferMessageSet}
import kafka.producer._
import kafka.serializer.Encoder
import kafka.utils.{Utils, Logging, SystemTime}
import scala.util.Random
import scala.collection.{Seq, Map}
import scala.collection.mutable.{ArrayBuffer, HashMap, Set}
import java.util.concurrent.atomic._
import kafka.api.{TopicMetadata, ProducerRequest}

class DefaultEventHandler[K,V](config: ProducerConfig,
                               private val partitioner: Partitioner[K],
                               private val encoder: Encoder[V],
                               private val keyEncoder: Encoder[K],
                               private val producerPool: ProducerPool,
                               private val topicPartitionInfos: HashMap[String, TopicMetadata] = new HashMap[String, TopicMetadata])
  extends EventHandler[K,V] with Logging {
  val isSync = ("sync" == config.producerType)

  val correlationId = new AtomicInteger(0)
  val brokerPartitionInfo = new BrokerPartitionInfo(config, producerPool, topicPartitionInfos)

  private val topicMetadataRefreshInterval = config.topicMetadataRefreshIntervalMs
  private var lastTopicMetadataRefreshTime = 0L
  private val topicMetadataToRefresh = Set.empty[String]
  private val sendPartitionPerTopicCache = HashMap.empty[String, Int]

  private val producerStats = ProducerStatsRegistry.getProducerStats(config.clientId)
  private val producerTopicStats = ProducerTopicStatsRegistry.getProducerTopicStats(config.clientId)
  // 入口，處理 message及messages
  def handle(events: Seq[KeyedMessage[K,V]]) {
    val serializedData = serialize(events)
    serializedData.foreach {
      keyed =>
        val dataSize = keyed.message.payloadSize
        producerTopicStats.getProducerTopicStats(keyed.topic).byteRate.mark(dataSize)
        producerTopicStats.getProducerAllTopicsStats.byteRate.mark(dataSize)
    }
    var outstandingProduceRequests = serializedData
    var remainingRetries = config.messageSendMaxRetries + 1
    val correlationIdStart = correlationId.get()
    debug("Handling %d events".format(events.size))
    while (remainingRetries > 0 && outstandingProduceRequests.size > 0) {
      topicMetadataToRefresh ++= outstandingProduceRequests.map(_.topic)
      if (topicMetadataRefreshInterval >= 0 &&
          SystemTime.milliseconds - lastTopicMetadataRefreshTime > topicMetadataRefreshInterval) {
        Utils.swallowError(brokerPartitionInfo.updateInfo(topicMetadataToRefresh.toSet, correlationId.getAndIncrement))
        sendPartitionPerTopicCache.clear()
        topicMetadataToRefresh.clear
        lastTopicMetadataRefreshTime = SystemTime.milliseconds
      }
      // 處理序列化的資料
      outstandingProduceRequests = dispatchSerializedData(outstandingProduceRequests)
      if (outstandingProduceRequests.size > 0) {
        info("Back off for %d ms before retrying send. Remaining retries = %d".format(config.retryBackoffMs, remainingRetries-1))
        // back off and update the topic metadata cache before attempting another send operation
        Thread.sleep(config.retryBackoffMs)
        // get topics of the outstanding produce requests and refresh metadata for those
        Utils.swallowError(brokerPartitionInfo.updateInfo(outstandingProduceRequests.map(_.topic).toSet, correlationId.getAndIncrement))
        sendPartitionPerTopicCache.clear()
        remainingRetries -= 1
        producerStats.resendRate.mark()
      }
    }
    if(outstandingProduceRequests.size > 0) {
      producerStats.failedSendRate.mark()
      val correlationIdEnd = correlationId.get()
      error("Failed to send requests for topics %s with correlation ids in [%d,%d]"
        .format(outstandingProduceRequests.map(_.topic).toSet.mkString(","),
        correlationIdStart, correlationIdEnd-1))
      throw new FailedToSendMessageException("Failed to send messages after " + config.messageSendMaxRetries + " tries.", null)
    }
  }
  //處理序列化的資料
  private def dispatchSerializedData(messages: Seq[KeyedMessage[K,Message]]): Seq[KeyedMessage[K, Message]] = {
    //按規則為messages分配分割槽
    val partitionedDataOpt = partitionAndCollate(messages)
    partitionedDataOpt match {
      case Some(partitionedData) =>
        val failedProduceRequests = new ArrayBuffer[KeyedMessage[K,Message]]
        try {
          //遍歷每個leaderBrokerId，傳送messages
          for ((brokerid, messagesPerBrokerMap) <- partitionedData) {
            if (logger.isTraceEnabled)
              messagesPerBrokerMap.foreach(partitionAndEvent =>
                trace("Handling event for Topic: %s, Broker: %d, Partitions: %s".format(partitionAndEvent._1, brokerid, partitionAndEvent._2)))
            val messageSetPerBroker = groupMessagesToSet(messagesPerBrokerMap)
            //傳送訊息到brokerid
            val failedTopicPartitions = send(brokerid, messageSetPerBroker)
            failedTopicPartitions.foreach(topicPartition => {
              messagesPerBrokerMap.get(topicPartition) match {
                case Some(data) => failedProduceRequests.appendAll(data)
                case None => // nothing
              }
            })
          }
        } catch {
          case t: Throwable => error("Failed to send messages", t)
        }
        failedProduceRequests
      case None => // all produce requests failed
        messages
    }
  }

  def serialize(events: Seq[KeyedMessage[K,V]]): Seq[KeyedMessage[K,Message]] = {
    val serializedMessages = new ArrayBuffer[KeyedMessage[K,Message]](events.size)
    events.map{e =>
      try {
        if(e.hasKey)
          serializedMessages += KeyedMessage[K,Message](topic = e.topic, key = e.key, message = new Message(key = keyEncoder.toBytes(e.key), bytes = encoder.toBytes(e.message)))
        else
          serializedMessages += KeyedMessage[K,Message](topic = e.topic, key = null.asInstanceOf[K], message = new Message(bytes = encoder.toBytes(e.message)))
      } catch {
        case t: Throwable =>
          producerStats.serializationErrorRate.mark()
          if (isSync) {
            throw t
          } else {
            // currently, if in async mode, we just log the serialization error. We need to revisit
            // this when doing kafka-496
            error("Error serializing message for topic %s".format(e.topic), t)
          }
      }
    }
    serializedMessages
  }
  // 為messages分割槽
  def partitionAndCollate(messages: Seq[KeyedMessage[K,Message]]): Option[Map[Int, collection.mutable.Map[TopicAndPartition, Seq[KeyedMessage[K,Message]]]]] = {
    //定義一個leaderId ==> (topic,messages)的對映
    val ret = new HashMap[Int, collection.mutable.Map[TopicAndPartition, Seq[KeyedMessage[K,Message]]]]
    try {
      for (message <- messages) {
        //獲取topic的partition集合
        val topicPartitionsList = getPartitionListForTopic(message)
        //為message分配partition index，如果message的key不等於null,則會呼叫配置的partitioner.class 
        val partitionIndex = getPartition(message.topic, message.key, topicPartitionsList)
        //根據partitionIndex獲取對應的partition-broker資訊
        val brokerPartition = topicPartitionsList(partitionIndex)

        // postpone the failure until the send operation, so that requests for other brokers are handled correctly
        // 獲取partitionIndex的leaderBrokerId
        val leaderBrokerId = brokerPartition.leaderBrokerIdOpt.getOrElse(-1)
        // 定義leaderBrokerId需要傳送的資料集合<topic,messages>
        //最終傳送messages時是按照  每個leaderBrokerId的每個Topic的每個partitionId傳送的
        var dataPerBroker: HashMap[TopicAndPartition, Seq[KeyedMessage[K,Message]]] = null
        ret.get(leaderBrokerId) match {
          case Some(element) =>
            dataPerBroker = element.asInstanceOf[HashMap[TopicAndPartition, Seq[KeyedMessage[K,Message]]]]
          case None =>
            dataPerBroker = new HashMap[TopicAndPartition, Seq[KeyedMessage[K,Message]]]
            ret.put(leaderBrokerId, dataPerBroker)
        }
        
        val topicAndPartition = TopicAndPartition(message.topic, brokerPartition.partitionId)
        // 定義topic的配個Partition傳送的messages
        var dataPerTopicPartition: ArrayBuffer[KeyedMessage[K,Message]] = null
        dataPerBroker.get(topicAndPartition) match {
          case Some(element) =>
            dataPerTopicPartition = element.asInstanceOf[ArrayBuffer[KeyedMessage[K,Message]]]
          case None =>
            dataPerTopicPartition = new ArrayBuffer[KeyedMessage[K,Message]]
            dataPerBroker.put(topicAndPartition, dataPerTopicPartition)
        }
        // 為brokerid的topic的partition 追加需要傳送的訊息
        dataPerTopicPartition.append(message)
      }//end for
      Some(ret)
    }catch {    // Swallow recoverable exceptions and return None so that they can be retried.
      case ute: UnknownTopicOrPartitionException => warn("Failed to collate messages by topic,partition due to: " + ute.getMessage); None
      case lnae: LeaderNotAvailableException => warn("Failed to collate messages by topic,partition due to: " + lnae.getMessage); None
      case oe: Throwable => error("Failed to collate messages by topic, partition due to: " + oe.getMessage); None
    }
  }

  private def getPartitionListForTopic(m: KeyedMessage[K,Message]): Seq[PartitionAndLeader] = {
    val topicPartitionsList = brokerPartitionInfo.getBrokerPartitionInfo(m.topic, correlationId.getAndIncrement)
    debug("Broker partitions registered for topic: %s are %s"
      .format(m.topic, topicPartitionsList.map(p => p.partitionId).mkString(",")))
    val totalNumPartitions = topicPartitionsList.length
    if(totalNumPartitions == 0)
      throw new NoBrokersForPartitionException("Partition key = " + m.key)
    topicPartitionsList
  }

  /**
   * Retrieves the partition id and throws an UnknownTopicOrPartitionException if
   * the value of partition is not between 0 and numPartitions-1
   * @param key the partition key
   * @param topicPartitionList the list of available partitions
   * @return the partition id
   */
  private def getPartition(topic: String, key: K, topicPartitionList: Seq[PartitionAndLeader]): Int = {
    val numPartitions = topicPartitionList.size
    if(numPartitions <= 0)
      throw new UnknownTopicOrPartitionException("Topic " + topic + " doesn't exist")
    val partition =
      if(key == null) {
        // If the key is null, we don't really need a partitioner
        // So we look up in the send partition cache for the topic to decide the target partition
        val id = sendPartitionPerTopicCache.get(topic)
        id match {
          case Some(partitionId) =>
            // directly return the partitionId without checking availability of the leader,
            // since we want to postpone the failure until the send operation anyways
            partitionId
          case None =>
            val availablePartitions = topicPartitionList.filter(_.leaderBrokerIdOpt.isDefined)
            if (availablePartitions.isEmpty)
              throw new LeaderNotAvailableException("No leader for any partition in topic " + topic)
            val index = Utils.abs(Random.nextInt) % availablePartitions.size
            val partitionId = availablePartitions(index).partitionId
            sendPartitionPerTopicCache.put(topic, partitionId)
            partitionId
        }
      } else
        partitioner.partition(key, numPartitions)
    if(partition < 0 || partition >= numPartitions)
      throw new UnknownTopicOrPartitionException("Invalid partition id: " + partition + " for topic " + topic +
        "; Valid values are in the inclusive range of [0, " + (numPartitions-1) + "]")
    trace("Assigning message of topic %s and key %s to a selected partition %d".format(topic, if (key == null) "[none]" else key.toString, partition))
    partition
  }

  /**
   * Constructs and sends the produce request based on a map from (topic, partition) -> messages
   *
   * @param brokerId the broker that will receive the request
   * @param messagesPerTopic the messages as a map from (topic, partition) -> messages
   * @return the set (topic, partitions) messages which incurred an error sending or processing
   */
  private def send(brokerId: Int, messagesPerTopic: collection.mutable.Map[TopicAndPartition, ByteBufferMessageSet]) = {
    if(brokerId < 0) {
      warn("Failed to send data since partitions %s don't have a leader".format(messagesPerTopic.map(_._1).mkString(",")))
      messagesPerTopic.keys.toSeq
    } else if(messagesPerTopic.size > 0) {
      val currentCorrelationId = correlationId.getAndIncrement
      val producerRequest = new ProducerRequest(currentCorrelationId, config.clientId, config.requestRequiredAcks,
        config.requestTimeoutMs, messagesPerTopic)
      var failedTopicPartitions = Seq.empty[TopicAndPartition]
      try {
        val syncProducer = producerPool.getProducer(brokerId)
        debug("Producer sending messages with correlation id %d for topics %s to broker %d on %s:%d"
          .format(currentCorrelationId, messagesPerTopic.keySet.mkString(","), brokerId, syncProducer.config.host, syncProducer.config.port))
        val response = syncProducer.send(producerRequest)
        debug("Producer sent messages with correlation id %d for topics %s to broker %d on %s:%d"
          .format(currentCorrelationId, messagesPerTopic.keySet.mkString(","), brokerId, syncProducer.config.host, syncProducer.config.port))
        if(response != null) {
          if (response.status.size != producerRequest.data.size)
            throw new KafkaException("Incomplete response (%s) for producer request (%s)".format(response, producerRequest))
          if (logger.isTraceEnabled) {
            val successfullySentData = response.status.filter(_._2.error == ErrorMapping.NoError)
            successfullySentData.foreach(m => messagesPerTopic(m._1).foreach(message =>
              trace("Successfully sent message: %s".format(Utils.readString(message.message.payload)))))
          }
          val failedPartitionsAndStatus = response.status.filter(_._2.error != ErrorMapping.NoError).toSeq
          failedTopicPartitions = failedPartitionsAndStatus.map(partitionStatus => partitionStatus._1)
          if(failedTopicPartitions.size > 0) {
            val errorString = failedPartitionsAndStatus
              .sortWith((p1, p2) => p1._1.topic.compareTo(p2._1.topic) < 0 ||
                                    (p1._1.topic.compareTo(p2._1.topic) == 0 && p1._1.partition < p2._1.partition))
              .map{
                case(topicAndPartition, status) =>
                  topicAndPartition.toString + ": " + ErrorMapping.exceptionFor(status.error).getClass.getName
              }.mkString(",")
            warn("Produce request with correlation id %d failed due to %s".format(currentCorrelationId, errorString))
          }
          failedTopicPartitions
        } else
          Seq.empty[TopicAndPartition]
      } catch {
        case t: Throwable =>
          warn("Failed to send producer request with correlation id %d to broker %d with data for partitions %s"
            .format(currentCorrelationId, brokerId, messagesPerTopic.map(_._1).mkString(",")), t)
          messagesPerTopic.keys.toSeq
      }
    } else {
      List.empty
    }
  }

  private def groupMessagesToSet(messagesPerTopicAndPartition: collection.mutable.Map[TopicAndPartition, Seq[KeyedMessage[K,Message]]]) = {
    /** enforce the compressed.topics config here.
      *  If the compression codec is anything other than NoCompressionCodec,
      *    Enable compression only for specified topics if any
      *    If the list of compressed topics is empty, then enable the specified compression codec for all topics
      *  If the compression codec is NoCompressionCodec, compression is disabled for all topics
      */

    val messagesPerTopicPartition = messagesPerTopicAndPartition.map { case (topicAndPartition, messages) =>
      val rawMessages = messages.map(_.message)
      ( topicAndPartition,
        config.compressionCodec match {
          case NoCompressionCodec =>
            debug("Sending %d messages with no compression to %s".format(messages.size, topicAndPartition))
            new ByteBufferMessageSet(NoCompressionCodec, rawMessages: _*)
          case _ =>
            config.compressedTopics.size match {
              case 0 =>
                debug("Sending %d messages with compression codec %d to %s"
                  .format(messages.size, config.compressionCodec.codec, topicAndPartition))
                new ByteBufferMessageSet(config.compressionCodec, rawMessages: _*)
              case _ =>
                if(config.compressedTopics.contains(topicAndPartition.topic)) {
                  debug("Sending %d messages with compression codec %d to %s"
                    .format(messages.size, config.compressionCodec.codec, topicAndPartition))
                  new ByteBufferMessageSet(config.compressionCodec, rawMessages: _*)
                }
                else {
                  debug("Sending %d messages to %s with no compression as it is not in compressed.topics - %s"
                    .format(messages.size, topicAndPartition, config.compressedTopics.toString))
                  new ByteBufferMessageSet(NoCompressionCodec, rawMessages: _*)
                }
            }
        }
        )
    }
    messagesPerTopicPartition
  }

  def close() {
    if (producerPool != null)
      producerPool.close
  }
}