CODEWITHSUNDEEP
javaspringapache-kafka
jpganz18
jpganz18

Reputation: 5878

Kafka listeners, not working with Strings?

I am following a simple guide documentation to use Kafka streams with spring boot (Spring guide)

It is clear for me how come in and out the messages and then in the middle I am able to do some processing, replacing the @KafkaListener and kafkaTemplate.send()

So I made a super simple basic class like this:

@EnableBinding(Processor.class)
public static class UppercaseTransformer {

  @StreamListener
  @Input(Processor.INPUT)
  public void receive(String input) {
    System.out.println(input);
  }
}

and then (and maybe this is my error), from a controller I do this:

 template.send("my-topic","hello world");

I am using spring cloud streams with a configuration like this:

spring:
  cloud:
    stream:
      bindings:
        input:
          destination: my-topic
          group: ${spring.application.name}
          consumer:
            concurrency: ${KAFKA_CONSUMER_CONCURRENCY:3}
        output:
          destination: my-topic
      kafka:
        binder:
          brokers: localhost:9092
          auto-create-topics: false
          required-acks: all
          transaction:
            transaction-id-prefix: ${spring.application.name}-
            producer:
              configuration:
                retries: 3
        bindings:
          input:
            consumer:
              configuration:
                isolation.level: read_committed
              enable-dlq: true
              dlq-name: some-name

Also tried this with the consumer and listener

key-deserializer: org.apache.kafka.common.serialization.StringDeserializer
              value-deserializer: org.apache.kafka.common.serialization.StringDeserializer

and everytime I try to send a message I get this:

class java.lang.String cannot be cast to class [B (java.lang.String and [B are in module java.base of loader 'bootstrap')

Have no idea what is wrong and why so much change from the normal listener to this version... ideas?

Upvotes: 0

Views: 323

Answers (1)

sobychacko
sobychacko

Reputation: 5924

I just created an application from start.spring.io and selected "Cloud Stream" and "Kafka". Generated the project and added this in the main class (Used the same configuration you provided above).

@SpringBootApplication
@EnableBinding(Processor.class)
public class So54408906Application {

    public static void main(String[] args) {
        SpringApplication.run(So54408906Application.class, args);
    }

    @StreamListener(Processor.INPUT)
    public void receive(String input) {
        System.out.println(input);
    }

}

Then ran the kafka console producer script.

kafka-console-producer.sh --broker-list localhost:9092 --topic my-topic

The text provided at the script was getting logged on the application's console.

Upvotes: 1

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