kakfa 2.4.1 java的生产者client在发送消息前分配消息属于哪个分区源码

标题是否不是很熟悉，面试不得必问啊

1、客户端暴露出来可以让开发人员调用的发送消息的方法send

 public Future<RecordMetadata> send(ProducerRecord<K, V> record) {
        return this.send(record, (Callback)null);
    }

    public Future<RecordMetadata> send(ProducerRecord<K, V> record, Callback callback) {
        ProducerRecord<K, V> interceptedRecord = this.interceptors.onSend(record);
        return this.doSend(interceptedRecord, callback);
    }

2、send实际调用私有方法doSend获取集群信息(并且得到这条数据写哪个分区)

private Future<RecordMetadata> doSend(ProducerRecord<K, V> record, Callback callback) {
            TopicPartition tp = null;
            //......删除干扰理解的代码行
            KafkaProducer.ClusterAndWaitTime clusterAndWaitTime;
            try {
                clusterAndWaitTime = this.waitOnMetadata(record.topic(), record.partition(), this.maxBlockTimeMs);
            } catch (KafkaException var20) {
             //.......删除干扰理解的代码行
            }
			//.......删除干扰理解的代码行
            Cluster cluster = clusterAndWaitTime.cluster;
			//.......删除干扰理解的代码行
            int partition = this.partition(record, serializedKey, serializedValue, cluster);
            tp = new TopicPartition(record.topic(), partition);
           //.......删除干扰理解的代码行
           //这里是把消息append，等待后续sender的线程被唤醒，统一发送，这篇文章不讲这里
            RecordAppendResult result = this.accumulator.append(tp, timestamp, serializedKey, serializedValue, headers, interceptCallback, remainingWaitMs, true);
       	//.......删除干扰理解的代码行
    }

2.1获取kafka服务端集群某个topic的元数据方法waitOnMetadata

 private KafkaProducer.ClusterAndWaitTime waitOnMetadata(String topic, Integer partition, long maxWaitMs) throws InterruptedException {
        	Cluster cluster = this.metadata.fetch();
       		//.......删除干扰理解的代码行
            this.metadata.add(topic);
            Integer partitionsCount = cluster.partitionCountForTopic(topic);
            //它检查partitionsCount是否为非空（不为null）
            //如果partition为null或者小于partitionsCount
            //两个都为真则返回cluster，否则请求服务端更新集群的元数据
            if (partitionsCount != null && (partition == null || partition < partitionsCount)) {
                return new KafkaProducer.ClusterAndWaitTime(cluster, 0L);
            } else {
            	//下面是请求服务端更新集群元数据的操作
                long begin = this.time.milliseconds();
                long remainingWaitMs = maxWaitMs;

                long elapsed;
                do {
                    do {
                        if (partition != null) {
                            this.log.trace("Requesting metadata update for partition {} of topic {}.", partition, topic);
                        } else {
                            this.log.trace("Requesting metadata update for topic {}.", topic);
                        }
                        this.metadata.add(topic);
                        int version = this.metadata.requestUpdate();
                        //唤醒发送者线程，这样就会发送更新请求
                        this.sender.wakeup();
                        //下面等待响应数据返回
                        try {
                            this.metadata.awaitUpdate(version, remainingWaitMs);
                        } catch (TimeoutException var15) {
                            throw new TimeoutException(String.format("Topic %s not present in metadata after %d ms.", topic, maxWaitMs));
                        }
						//集群topic最新元数据返回了，取出cluster
                        cluster = this.metadata.fetch();
                        elapsed = this.time.milliseconds() - begin;
                        if (elapsed >= maxWaitMs) {
                            throw new TimeoutException(partitionsCount == null ? String.format("Topic %s not present in metadata after %d ms.", topic, maxWaitMs) : String.format("Partition %d of topic %s with partition count %d is not present in metadata after %d ms.", partition, topic, partitionsCount, maxWaitMs));
                        }
                        this.metadata.maybeThrowExceptionForTopic(topic);
                        remainingWaitMs = maxWaitMs - elapsed;
                        partitionsCount = cluster.partitionCountForTopic(topic);
                    } while(partitionsCount == null);
                } while(partition != null && partition >= partitionsCount);
				//再把更新后的集群的元数据返回
                return new KafkaProducer.ClusterAndWaitTime(cluster, elapsed);
            }
    }

2.2根据消息是否指定分区或者实现Partitioner接口的实现类计算分区

 private int partition(ProducerRecord<K, V> record, byte[] serializedKey, byte[] serializedValue, Cluster cluster) {
 		//创建ProducerRecord时是否指定了分区，如果没有则嗲用org.apache.kafka.clients.producer.Partitioner的某个实现生成分区
        Integer partition = record.partition();
        return partition != null ? partition : this.partitioner.partition(record.topic(), record.key(), serializedKey, record.value(), serializedValue, cluster);
    }

其中org.apache.kafka.clients.producer.Partitioner接口现成的实现有三种，分别是DefaultPartitioner、RoundRobinPartitioner、UniformStickyPartitioner、具体实现可以去看

其中默认的是DefaultPartitioner

 KafkaProducer(Map<String, Object> configs, Serializer<K> keySerializer, Serializer<V> valueSerializer, ProducerMetadata metadata, KafkaClient kafkaClient, ProducerInterceptors interceptors, Time time) {
        ProducerConfig config = new ProducerConfig(ProducerConfig.addSerializerToConfig(configs, keySerializer, valueSerializer));
        //......删除干扰代码
        this.partitioner = (Partitioner)config.getConfiguredInstance("partitioner.class", Partitioner.class);
          //......删除干扰代码
    }

public class ProducerConfig extends AbstractConfig {
    private static final ConfigDef CONFIG;
     //......删除干扰代码
     static {
      //其他.define都删除了，防止理解有问题
        CONFIG = (new ConfigDef()).define("partitioner.class", Type.CLASS, DefaultPartitioner.class, Importance.MEDIUM, "Partitioner class that implements the org.apache.kafka.clients.producer.Partitioner interface.");
     }   
 }

当然也可以指定其他的，也可以自定义实现org.apache.kafka.clients.producer.Partitioner接口