Kafka consumer showing numbers in unreadable format

I am trying out the kafka streaming. I am reading messages from one topic and doing groupByKey and then doing the count of groups. But the problem is that the messages count is coming as unreadable “boxes”.

If I run the console consumer these are coming as empty strings

This is the WordCount code I wrote

package streams;

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 java.util.Arrays;
import java.util.Properties;

public class WordCount {
    public static void main(String[] args) {

        Properties properties = new Properties();
        properties.setProperty(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
        properties.setProperty(StreamsConfig.APPLICATION_ID_CONFIG, "streams-demo-2");
        properties.setProperty(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
        properties.setProperty(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.StringSerde.class.getName());
        properties.setProperty(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.StringSerde.class.getName());

        // topology
        StreamsBuilder builder = new StreamsBuilder();
        KStream<String, String> input = builder.stream("temp-in");
        KStream<String, Long> fil = input.flatMapValues(val -> Arrays.asList(val.split(" "))) // making stream of text line to stream of words
                .selectKey((k, v) -> v) // changing the key
                .groupByKey().count().toStream(); // getting count after groupBy

        fil.to("temp-out");

        KafkaStreams streams = new KafkaStreams(builder.build(), properties);
        streams.start();

        System.out.println(streams.toString());

        Runtime.getRuntime().addShutdownHook(new Thread(streams::close));
    }
}

This is the output I am getting in the consumer. It is there on the right side in image

I had tried casting the long to long again to see if it works. But it’s not working

I am attaching the consumer code too if it helps.

package tutorial;

import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.clients.consumer.ConsumerRecords;
import org.apache.kafka.clients.consumer.KafkaConsumer;
import org.apache.kafka.common.serialization.StringDeserializer;

import java.time.Duration;
import java.util.Collections;
import java.util.Properties;

public class Consumer {
    public static void main(String[] args) {
        Properties properties = new Properties();
        properties.setProperty(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
        properties.setProperty(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
        properties.setProperty(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());

        // Once the consumer starts running it keeps running even after we stop in console
        // We should create new consumer to read from earliest because the previous one had already consumed until certain offset
        // when we run the same consumer in two consoles kafka detects it and re balances
        // In this case the consoles split the partitions they consume forming a consumer group
        properties.setProperty(ConsumerConfig.GROUP_ID_CONFIG, "consumer-application-1"); // -> consumer id
        properties.setProperty(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest"); // -> From when consumer gets data

        KafkaConsumer<String, String> consumer = new KafkaConsumer<>(properties);
        consumer.subscribe(Collections.singleton("temp-out"));

        while (true) {
            ConsumerRecords<String, String> consumerRecords = consumer.poll(Duration.ofMillis(1000));
            for (ConsumerRecord<String, String> record: consumerRecords) {
                System.out.println(record.key() + " " + record.value());
                System.out.println(record.partition() + " " + record.offset());
            }
        }
    }
}

Any help is appreciated. Thanks in advance.

Answer

The message value you’re writing with Kafka Streams is a Long, and you’re consuming it as a String.

If you make the following changes to your Consumer class, you’ll be able to see the count printed correctly to stdout:

        // Change this from StringDeserializer to LongDeserializer.
        properties.setProperty(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, LongDeserializer.class.getName());

        ...

        // The value you're consuming here is a Long, not a String.
        KafkaConsumer<String, Long> consumer = new KafkaConsumer<>(properties);
        consumer.subscribe(Collections.singleton("temp-out"));

        while (true) {
            ConsumerRecords<String, Long> consumerRecords = consumer.poll(Duration.ofMillis(1000));
            for (ConsumerRecord<String, Long> record : consumerRecords) {
                System.out.println(record.key() + " " + record.value());
                System.out.println(record.partition() + " " + record.offset());
            }
        }