docker-docs/content/guides/testcontainers-java-micronaut-kafka/write-tests.md

title, linkTitle, description, weight

title	linkTitle	description	weight
Write tests with Testcontainers	Write tests	Test the Micronaut Kafka listener using Testcontainers Kafka and MySQL modules with Awaitility.	20

To test the Kafka listener, you need a running Kafka broker and a MySQL database, plus a started Micronaut application context. Testcontainers spins up both services in Docker containers and the TestPropertyProvider interface connects them to Micronaut.

Create a Kafka client for testing

First, create a @KafkaClient interface to publish events in the test:

package com.testcontainers.demo;

import io.micronaut.configuration.kafka.annotation.KafkaClient;
import io.micronaut.configuration.kafka.annotation.KafkaKey;
import io.micronaut.configuration.kafka.annotation.Topic;

@KafkaClient
public interface ProductPriceChangesClient {

    @Topic("product-price-changes")
    void send(@KafkaKey String productCode, ProductPriceChangedEvent event);
}

Key details:

• The @KafkaClient annotation designates this interface as a Kafka producer.
• The @Topic annotation specifies the target topic.
• The @KafkaKey annotation marks the parameter used as the Kafka message key. If no such parameter exists, Micronaut sends the record with a null key.

Write the test

Create ProductPriceChangedEventHandlerTest.java:

package com.testcontainers.demo;

import static java.util.concurrent.TimeUnit.SECONDS;
import static org.assertj.core.api.Assertions.assertThat;
import static org.awaitility.Awaitility.await;

import io.micronaut.context.annotation.Property;
import io.micronaut.core.annotation.NonNull;
import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import io.micronaut.test.support.TestPropertyProvider;
import java.math.BigDecimal;
import java.time.Duration;
import java.util.Collections;
import java.util.Map;
import java.util.Optional;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.TestInstance;
import org.testcontainers.kafka.ConfluentKafkaContainer;
import org.testcontainers.junit.jupiter.Container;
import org.testcontainers.junit.jupiter.Testcontainers;

@MicronautTest(transactional = false)
@Property(name = "datasources.default.driver-class-name", value = "org.testcontainers.jdbc.ContainerDatabaseDriver")
@Property(name = "datasources.default.url", value = "jdbc:tc:mysql:8.0.32:///db")
@Testcontainers(disabledWithoutDocker = true)
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class ProductPriceChangedEventHandlerTest implements TestPropertyProvider {

    @Container
    static final ConfluentKafkaContainer kafka = new ConfluentKafkaContainer("confluentinc/cp-kafka:7.8.0");

    @Override
    public @NonNull Map<String, String> getProperties() {
        if (!kafka.isRunning()) {
            kafka.start();
        }
        return Collections.singletonMap("kafka.bootstrap.servers", kafka.getBootstrapServers());
    }

    @Test
    void shouldHandleProductPriceChangedEvent(
            ProductPriceChangesClient productPriceChangesClient, ProductRepository productRepository) {
        Product product = new Product(null, "P100", "Product One", BigDecimal.TEN);
        Long id = productRepository.save(product).getId();

        ProductPriceChangedEvent event = new ProductPriceChangedEvent("P100", new BigDecimal("14.50"));

        productPriceChangesClient.send(event.productCode(), event);

        await().pollInterval(Duration.ofSeconds(3)).atMost(10, SECONDS).untilAsserted(() -> {
            Optional<Product> optionalProduct = productRepository.findByCode("P100");
            assertThat(optionalProduct).isPresent();
            assertThat(optionalProduct.get().getCode()).isEqualTo("P100");
            assertThat(optionalProduct.get().getPrice()).isEqualTo(new BigDecimal("14.50"));
        });

        productRepository.deleteById(id);
    }
}

Here's what the test does:

• @MicronautTest initializes the Micronaut application context and the embedded server. Setting transactional to false prevents each test method from running inside a rolled-back transaction, which is necessary because the Kafka listener processes messages in a separate thread.
• The @Property annotations override the datasource driver and URL to use the Testcontainers special JDBC URL (jdbc:tc:mysql:8.0.32:///db). This spins up a MySQL container and configures it as the datasource automatically.
• @Testcontainers and @Container manage the Kafka container lifecycle. The TestPropertyProvider interface registers the Kafka bootstrap servers with Micronaut so that the producer and consumer connect to the test container.
• @TestInstance(TestInstance.Lifecycle.PER_CLASS) creates a single test instance for all test methods, which is required when implementing TestPropertyProvider.
• The test creates a Product record in the database, then sends a ProductPriceChangedEvent to the product-price-changes topic using the ProductPriceChangesClient.
• Because Kafka message processing is asynchronous, the test uses Awaitility to poll every 3 seconds (up to a maximum of 10 seconds) until the product price in the database matches the expected value.