Sample Test Case with Embedded Kafka https://blog.mimacom.com/testing-apache-kafka-with-spring-boot-junit5/

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.2.5.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>
	<groupId>com.example</groupId>
	<artifactId>demo</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>demo</name>
	<description>Demo project for Spring Boot</description>

	<properties>
		<java.version>1.8</java.version>
	</properties>

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

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
			<exclusions>
				<exclusion>
					<groupId>org.junit.vintage</groupId>
					<artifactId>junit-vintage-engine</artifactId>
				</exclusion>
			</exclusions>
		</dependency>
		<dependency>
			<groupId>org.springframework.kafka</groupId>
			<artifactId>spring-kafka-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>

</project>

SimpleKafkaTest.java

package com.example.demo;

import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.clients.producer.Producer;
import org.apache.kafka.clients.producer.ProducerRecord;
import org.apache.kafka.common.serialization.StringDeserializer;
import org.apache.kafka.common.serialization.StringSerializer;
import org.junit.jupiter.api.AfterAll;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.TestInstance;
import org.junit.jupiter.api.extension.ExtendWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.kafka.core.DefaultKafkaConsumerFactory;
import org.springframework.kafka.core.DefaultKafkaProducerFactory;
import org.springframework.kafka.listener.ContainerProperties;
import org.springframework.kafka.listener.KafkaMessageListenerContainer;
import org.springframework.kafka.listener.MessageListener;
import org.springframework.kafka.test.EmbeddedKafkaBroker;
import org.springframework.kafka.test.context.EmbeddedKafka;
import org.springframework.kafka.test.utils.ContainerTestUtils;
import org.springframework.kafka.test.utils.KafkaTestUtils;
import org.springframework.test.context.junit.jupiter.SpringExtension;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;

import static org.assertj.core.api.Assertions.assertThat;

@EmbeddedKafka
@ExtendWith(SpringExtension.class)
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
public class SimpleKafkaTest {

    private static final String TOPIC = "domain-events";

    @Autowired
    private EmbeddedKafkaBroker embeddedKafkaBroker;

    BlockingQueue<ConsumerRecord<String, String>> records;

    KafkaMessageListenerContainer<String, String> container;

    @BeforeAll
    void setUp() {
        Map<String, Object> configs = new HashMap<>(KafkaTestUtils.consumerProps("consumer", "false", embeddedKafkaBroker));
        DefaultKafkaConsumerFactory<String, String> consumerFactory = new DefaultKafkaConsumerFactory<>(configs, new StringDeserializer(), new StringDeserializer());
        ContainerProperties containerProperties = new ContainerProperties(TOPIC);
        container = new KafkaMessageListenerContainer<>(consumerFactory, containerProperties);
        records = new LinkedBlockingQueue<>();
        container.setupMessageListener((MessageListener<String, String>) records::add);
        container.start();
        ContainerTestUtils.waitForAssignment(container, embeddedKafkaBroker.getPartitionsPerTopic());
    }

    @AfterAll
    void tearDown() {
        container.stop();
    }

    @Test
    public void kafkaSetup_withTopic_ensureSendMessageIsReceived() throws Exception {
        // Arrange
        Map<String, Object> configs = new HashMap<>(KafkaTestUtils.producerProps(embeddedKafkaBroker));
        Producer<String, String> producer = new DefaultKafkaProducerFactory<>(configs, new StringSerializer(), new StringSerializer()).createProducer();

        // Act
        producer.send(new ProducerRecord<>(TOPIC, "my-aggregate-id", "{\"event\":\"Test Event\"}"));
        producer.flush();

        // Assert
        ConsumerRecord<String, String> singleRecord = records.poll(100, TimeUnit.MILLISECONDS);
        assertThat(singleRecord).isNotNull();
        assertThat(singleRecord.key()).isEqualTo("my-aggregate-id");
        assertThat(singleRecord.value()).isEqualTo("{\"event\":\"Test Event\"}");
    }

}