# Merge 2 Kafka KStreams (Java)

Paul Tomoiu

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Kafka,

KStreams,

Java

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Published 2022-12-03

This tutorial explains to you how you can merge 2 Kafka KStreams in Java.

Kafka Streams is a library for building streaming applications, specifically applications that transform input Kafka topics into output Kafka topics (or call to external services, updates to databases, etc).

Sometimes you need to merge 2 input Kafka topics or 2 KStreams into one. This is done using merge command.

Here is the Java code of the application:

MergeKStreams.java
package org.example;
 
import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.common.serialization.Serdes;
import org.apache.kafka.streams.KafkaStreams;
import org.apache.kafka.streams.StreamsBuilder;
import org.apache.kafka.streams.StreamsConfig;
import org.apache.kafka.streams.kstream.KStream;
import org.apache.kafka.streams.kstream.Produced;
import java.util.Properties;
 
public class MergeKStreams {
    public static void main(String[] args) {
        System.out.println("Start");
 
        Properties config = new Properties();
        config.put(StreamsConfig.APPLICATION_ID_CONFIG, "my-application");
        config.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "kafka1:9092");
        config.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
        config.put(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass());
        config.put(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass());
 
        StreamsBuilder builder = new StreamsBuilder (); //Now StreamsBuilder replace KStreamBuilder
 
        KStream<string, string=""> inputKStream = builder.stream("input-kafka-topic");
        KStream<string, string=""> inputKStream2 = builder.stream("input-kafka-topic2");
        KStream<string, string=""> mergedKStream = inputKStream.merge(inputKStream2);
 
        KStream<string, string=""> outputKStream =
                mergedKStream.filterNot((key, value) -> value.substring(0,3).equals("ABC"))
                         .mapValues(value -> value.toLowerCase());
 
        //  - to in order to write the results back to kafka
        outputKStream.to("output-kafka-topic", Produced.with(Serdes.String(), Serdes.String()));
        KafkaStreams streams = new KafkaStreams(builder.build(), config);
 
        streams.start();
 
        // shutdown hook to correctly close the streams application
        Runtime.getRuntime().addShutdownHook(new Thread(streams::close));
 
    }
}