Failed to construct kafka consumer with Spring Boot

Question

I have implemented a simple consumer application to consume messages from the topics. When I run the kafka-consumer application then the following error has occurred.

StackTrace

org.springframework.context.ApplicationContextException: Failed to start bean 'org.springframework.kafka.config.internalKafkaListenerEndpointRegistry'; nested exception is org.apache.kafka.common.KafkaException: Failed to construct kafka consumer
    at org.springframework.context.support.DefaultLifecycleProcessor.doStart(DefaultLifecycleProcessor.java:185) ~[spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]

org.apache.kafka.common.KafkaException: class org.apache.kafka.common.serialization.StringSerializer is not an instance of org.apache.kafka.common.serialization.Deserializer

Config class

@Configuration
@EnableKafka
public class KafkaConfig {

    private ConsumerFactory<String,String> consumerFactory()
    {
        Map<String,Object> config=new ConcurrentHashMap<>();

        config.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG,"127.0.0.1:9092");
        config.put(ConsumerConfig.GROUP_ID_CONFIG,"group_string");
        config.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringSerializer.class);
        config.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG,StringSerializer.class);

        return new DefaultKafkaConsumerFactory<>(config);
    }

    @Bean
    public ConcurrentKafkaListenerContainerFactory<String,String> kafkaListenerContainerFactory()
    {
        ConcurrentKafkaListenerContainerFactory<String,String> factory
                =new ConcurrentKafkaListenerContainerFactory<>();
        factory.setConsumerFactory(consumerFactory());

        return factory;
    }
}

Listener class

@Component
public class KafkaConsumer {

    @KafkaListener(topics = {"Kafka_Example"},groupId = "group_string")
    public void consume(String message)
    {
        System.out.println("Consumed Message "+message);
    }
}

pom.xml

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.kafka</groupId>
            <artifactId>spring-kafka</artifactId>
            <version>2.6.0</version>
        </dependency>
 <dependency>
            <groupId>org.springframework.kafka</groupId>
            <artifactId>spring-kafka-test</artifactId>
            <version>2.6.0</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.11.1</version>
        </dependency>

Note - My Kafka version is 2.13-2.6.0

Answer 1

You are using a StringSerializer but should use a StringDeserializer , one serializes, the other de serializes.
And since you set them for ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG and ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG you apparently want to de serialize.

config.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
config.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);