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title: Migrating from Streams Scala to Java API description: weight: 16 tags: [‘kafka’, ‘docs’] aliases: keywords: type: docs

⚠️ DEPRECATION NOTICE: The kafka-streams-scala library is deprecated as of Kafka 4.3 and will be removed in Kafka 5.0. This guide will help you migrate your Scala applications to use the Java Streams API directly. For more information, see KIP-1244.

Migration Overview

The Java Streams API works well from Scala with minimal adjustments. The main differences are:

1. Use Java types directly instead of Scala wrapper classes
2. Configure Serdes explicitly via StreamsConfig or pass them to methods

Example: Word Count Application

Scala Wrapper Approach (Deprecated)

import java.util.Properties

import org.apache.kafka.streams.scala.ImplicitConversions._
import org.apache.kafka.streams.scala._
import org.apache.kafka.streams.scala.kstream._
import org.apache.kafka.streams.{KafkaStreams, StreamsConfig}
import org.apache.kafka.streams.scala.serialization.Serdes._

object WordCountScala extends App {
  val props = new Properties()
  props.put(StreamsConfig.APPLICATION_ID_CONFIG, "wordcount")
  props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092")

  val builder = new StreamsBuilder  // Scala wrapper
  val textLines: KStream[String, String] = builder.stream[String, String]("input-topic")

  val wordCounts: KTable[String, Long] = textLines
    .flatMapValues(line => line.toLowerCase.split("\\W+"))
    .groupBy((_, word) => word)
    .count()

  wordCounts.toStream.to("output-topic")

  val streams = new KafkaStreams(builder.build(), props)
  streams.start()
}

Java API Approach

import java.util.Properties

import org.apache.kafka.streams.{KafkaStreams, StreamsBuilder, StreamsConfig}
import org.apache.kafka.streams.kstream.{KStream, KTable, Produced}
import org.apache.kafka.common.serialization.Serdes
import scala.jdk.CollectionConverters._

object WordCountJava extends App {
  val props = new Properties()
  props.put(StreamsConfig.APPLICATION_ID_CONFIG, "wordcount")
  props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092")
  // Configure default serdes
  props.put(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, classOf[Serdes.StringSerde])
  props.put(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, classOf[Serdes.StringSerde])

  val builder = new StreamsBuilder  // Java StreamsBuilder
  val textLines = builder.stream[String, String]("input-topic")

  val wordCounts = textLines
    .flatMapValues(_.toLowerCase.split("\\W+"))
    .groupBy((_, word) => word)
    .count()

  wordCounts.toStream.to("output-topic", Produced.`with`(Serdes.String(), Serdes.Long()))

  val streams = new KafkaStreams(builder.build(), props)
  streams.start()
}