Adding some simple streams processor and listener for this.

spring-showcase-web/build.gradle

@@ -16,6 +16,7 @@ dependencies {
     implementation 'org.springframework.boot:spring-boot-starter-web'
     implementation 'org.postgresql:postgresql:42.2.18'
     implementation 'org.springframework.kafka:spring-kafka:2.6.4'
+    implementation 'org.apache.kafka:kafka-streams:2.6.0'
     providedRuntime 'org.springframework.boot:spring-boot-starter-tomcat'
     testImplementation 'org.springframework.boot:spring-boot-starter-test'
 }

spring-showcase-web/src/main/java/ch/polgrabia/springshowcaseweb/configs/KafkaConsumerConfig.java

@@ -1,6 +1,7 @@
 package ch.polgrabia.springshowcaseweb.configs;
 
 import org.apache.kafka.clients.consumer.ConsumerConfig;
+import org.apache.kafka.common.serialization.LongDeserializer;
 import org.apache.kafka.common.serialization.StringDeserializer;
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.context.annotation.Bean;
@@ -26,7 +27,7 @@ public class KafkaConsumerConfig {
     private String groupId;
 
     @Bean
-    public ConsumerFactory<String, String> consumerFactory() {
+    public ConsumerFactory<String, String> consumerStringFactory() {
         Map<String, Object> config = new HashMap<>();
         config.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapAddress);
         config.put(ConsumerConfig.GROUP_ID_CONFIG, groupId);
@@ -36,9 +37,26 @@ public class KafkaConsumerConfig {
     }
 
     @Bean
-    public ConcurrentKafkaListenerContainerFactory<String, String> kafkaListenerContainerFactory() {
+    public ConcurrentKafkaListenerContainerFactory<String, String> kafkaListenerStringContainerFactory() {
         var factory = new ConcurrentKafkaListenerContainerFactory<String, String>();
-        factory.setConsumerFactory(consumerFactory());
+        factory.setConsumerFactory(consumerStringFactory());
+        return factory;
+    }
+
+    @Bean
+    public ConsumerFactory<String, Long> consumerLongFactory() {
+        Map<String, Object> config = new HashMap<>();
+        config.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapAddress);
+        config.put(ConsumerConfig.GROUP_ID_CONFIG, groupId);
+        config.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
+        config.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, LongDeserializer.class);
+        return new DefaultKafkaConsumerFactory<>(config);
+    }
+
+    @Bean
+    public ConcurrentKafkaListenerContainerFactory<String, Long> kafkaListenerLongContainerFactory() {
+        var factory = new ConcurrentKafkaListenerContainerFactory<String, Long>();
+        factory.setConsumerFactory(consumerLongFactory());
         return factory;
     }
 }

spring-showcase-web/src/main/java/ch/polgrabia/springshowcaseweb/configs/KafkaStreamsConfig.java

@@ -0,0 +1,64 @@
+package ch.polgrabia.springshowcaseweb.configs;
+
+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.KTable;
+import org.apache.kafka.streams.kstream.Materialized;
+import org.apache.kafka.streams.kstream.Produced;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Qualifier;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.PropertySource;
+
+import java.util.Arrays;
+import java.util.Properties;
+
+@Configuration
+@PropertySource("classpath:application.properties")
+public class KafkaStreamsConfig {
+
+    private static final String APPLICATION_ID = "app_id";
+
+    @Value("${springshowcase.consumer.bootstrap.address}")
+    private String bootstrapAddress;
+
+    @Bean
+    @Qualifier("streamsConfig")
+    public Properties kStreamsConfigs() {
+        Properties props = new Properties();
+        props.put(StreamsConfig.APPLICATION_ID_CONFIG, APPLICATION_ID);
+        props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapAddress);
+        // props.put(StreamsConfig.STATE_DIR_CONFIG, stateStoreDir.toString());
+        props.put(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass());
+        props.put(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass());
+        return props;
+    }
+
+    @Bean
+    @Qualifier("streamsBuilder")
+    public StreamsBuilder streamsBuilder() {
+        StreamsBuilder sb = new StreamsBuilder();
+
+        KStream<String, String> textLines = sb.stream("topic1");
+        KTable<String, Long> wordCounts = textLines
+                .flatMapValues(textLine -> Arrays.asList(textLine.split("\\W+")))
+                .groupBy((key, word) -> word)
+                .count(Materialized.as("counts-store"));
+
+        wordCounts.toStream()
+                .to("counts-store-topic", Produced.with(Serdes.String(), Serdes.Long()));
+        return sb;
+    }
+
+    @Bean
+    public KafkaStreams kafkaStreams(
+            @Autowired @Qualifier("streamsBuilder") StreamsBuilder sb,
+            @Autowired @Qualifier("streamsConfig") Properties props) {
+        return new KafkaStreams(sb.build(), props);
+    }
+}

spring-showcase-web/src/main/java/ch/polgrabia/springshowcaseweb/controllers/KafkaController.java

@@ -1,5 +1,6 @@
 package ch.polgrabia.springshowcaseweb.controllers;
 
+import org.apache.kafka.clients.consumer.ConsumerRecord;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.context.annotation.PropertySource;
@@ -29,8 +30,13 @@ public class KafkaController {
         kafkaTemplate.send(topic1, message);
     }
 
-    @KafkaListener(topics = "topic1", groupId = "group1")
+    @KafkaListener(topics = "topic1", groupId = "group1", containerFactory = "kafkaListenerStringContainerFactory")
-    public void handleMessage(String message) {
+    public void handleMessage(ConsumerRecord<String, String> message) {
-        System.out.printf("Got message: %s", message);
+        System.out.printf("Got message: %s\n", message.value());
+    }
+
+    @KafkaListener(topics = "counts-store-topic", groupId = "group1", containerFactory = "kafkaListenerLongContainerFactory")
+    public void handleCountsStore(ConsumerRecord<String, Long> message) {
+        System.out.printf("Got counts store message: %s - with value %d\n", message.key(), message.value());
     }
 }

spring-showcase-web/src/main/java/ch/polgrabia/springshowcaseweb/services/KafkaService.java

@@ -0,0 +1,28 @@
+package ch.polgrabia.springshowcaseweb.services;
+
+import org.apache.kafka.streams.KafkaStreams;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.stereotype.Service;
+
+import javax.annotation.PostConstruct;
+import javax.annotation.PreDestroy;
+
+@Service
+public class KafkaService {
+    private final KafkaStreams kafkaStreams;
+
+    public KafkaService(
+            @Autowired KafkaStreams kafkaStreams) {
+        this.kafkaStreams = kafkaStreams;
+    }
+
+    @PostConstruct
+    public void init() {
+        kafkaStreams.start();
+    }
+
+    @PreDestroy
+    public void stop() {
+        kafkaStreams.close();
+    }
+}