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

title, linkTitle, description, weight

title	linkTitle	description	weight
Write tests with Testcontainers	Write tests	Test the Spring Boot 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 Spring context. Testcontainers spins up both services in Docker containers and @DynamicPropertySource connects them to Spring.

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 java.math.BigDecimal;
import java.time.Duration;
import java.util.Optional;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.test.context.DynamicPropertyRegistry;
import org.springframework.test.context.DynamicPropertySource;
import org.springframework.test.context.TestPropertySource;
import org.testcontainers.kafka.ConfluentKafkaContainer;
import org.testcontainers.junit.jupiter.Container;
import org.testcontainers.junit.jupiter.Testcontainers;

@SpringBootTest
@TestPropertySource(
  properties = {
    "spring.kafka.consumer.auto-offset-reset=earliest",
    "spring.datasource.url=jdbc:tc:mysql:8.0.32:///db",
  }
)
@Testcontainers
class ProductPriceChangedEventHandlerTest {

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

  @DynamicPropertySource
  static void overrideProperties(DynamicPropertyRegistry registry) {
    registry.add("spring.kafka.bootstrap-servers", kafka::getBootstrapServers);
  }

  @Autowired
  private KafkaTemplate<String, Object> kafkaTemplate;

  @Autowired
  private ProductRepository productRepository;

  @BeforeEach
  void setUp() {
    Product product = new Product(null, "P100", "Product One", BigDecimal.TEN);
    productRepository.save(product);
  }

  @Test
  void shouldHandleProductPriceChangedEvent() {
    ProductPriceChangedEvent event = new ProductPriceChangedEvent(
      "P100",
      new BigDecimal("14.50")
    );

    kafkaTemplate.send("product-price-changes", 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"));
      });
  }
}

Here's what the test does:

• @SpringBootTest starts the full Spring application context.
• The Testcontainers special JDBC URL (jdbc:tc:mysql:8.0.32:///db) in @TestPropertySource spins up a MySQL container and configures it as the datasource automatically.
• @Testcontainers and @Container manage the lifecycle of the Kafka container. @DynamicPropertySource registers the Kafka bootstrap servers with Spring so that the producer and consumer connect to the test container.
• @BeforeEach creates a Product record in the database before each test.
• The test sends a ProductPriceChangedEvent to the product-price-changes topic using KafkaTemplate. Spring Boot converts the object to JSON using JsonSerializer.
• 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.
• The property spring.kafka.consumer.auto-offset-reset is set to earliest so that the listener consumes messages even if they're sent to the topic before the listener is ready. This setting is helpful when running tests.