Kafka java Producer - Connection to localhost:9092 could not be established

Question

I am playing with a Kafka for Java tutorial. I have concocted the following code:

Producer class

public class ProducerDemo {
    static String bootstrapServers = "127.0.0.1:9092";

    public static void main(String[] args) {

        // create producer properties
        Properties properties = new Properties();
        properties.setProperty(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
        properties.setProperty(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class.getName());
        properties.setProperty(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, StringSerializer.class.getName());

        // create producer
        KafkaProducer<String, String> producer = new KafkaProducer<>(properties);

        // create ProducerRecord
        ProducerRecord<String, String> record = new ProducerRecord<>("first_topic", "hello world!");

        // send data
        producer.send(record);

        producer.close();
    }
}

Gradle file

plugins {
    id 'java'
}

group 'com.greem666.kafka'
version '1.0-SNAPSHOT'

repositories {
    mavenCentral()
}

dependencies {
    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.6.0'
    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine'

    compileOnly 'org.projectlombok:lombok:1.18.20'
    annotationProcessor 'org.projectlombok:lombok:1.18.20'

    testCompileOnly 'org.projectlombok:lombok:1.18.20'
    testAnnotationProcessor 'org.projectlombok:lombok:1.18.20'

    implementation group: 'org.apache.kafka', name: 'kafka-clients', version: '2.8.0'

    implementation group: 'org.slf4j', name: 'slf4j-simple', version: '1.7.30'
}

I have a kafka_2.13-2.8.0 Zookeeper and one Broker running in a WSL2 instance on the same Win10 machine. When trying to play with console Consumer and console Producer in WSL2, it all works fine.

However, when I try to run the above Java class, I get this:

Java logs

[main] INFO org.apache.kafka.common.utils.AppInfoParser - Kafka version: 2.8.0
[main] INFO org.apache.kafka.common.utils.AppInfoParser - Kafka commitId: ebb1d6e21cc92130
[main] INFO org.apache.kafka.common.utils.AppInfoParser - Kafka startTimeMs: 1623940023993
[kafka-producer-network-thread | producer-1] WARN org.apache.kafka.clients.NetworkClient - [Producer clientId=producer-1] Connection to node -1 (/127.0.0.1:9092) could not be established. Broker may not be available.
[kafka-producer-network-thread | producer-1] WARN org.apache.kafka.clients.NetworkClient - [Producer clientId=producer-1] Bootstrap broker 127.0.0.1:9092 (id: -1 rack: null) disconnected

Any ideas why this is happening? Does it have something to do with Kafka Zookeeper and Broker running in a WSL2?

localhost is not exactly the same as 127.0.0.1. Did you try actually using localhost? — Felix
– Felix, Commented Jun 17, 2021 at 14:40
Yes, have tried both 127.0.0.1:9092, localhost:9092. When these did not work, even tried 0.0.0.0:9092, but to same result. — Greem666
– Greem666, Commented Jun 17, 2021 at 14:43

OneCricketeer · Accepted Answer · 2021-06-17 14:58:43Z

1

You need to run your code in WSL2, or you need to setup a port forward from the hypervisor to your Windows local network adapter in order for 127.0.0.1 or localhost to work (assuming you didn't modify any host file)

answered Jun 17, 2021 at 14:58

OneCricketeer

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