spring-messaging-demo/kafka-basics/docs/07-testing.md

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7. Testing without installing Kafka

Two options, both real brokers, and the choice is less obvious than it looks.

@EmbeddedKafka — a real broker inside the JVM

@SpringBootTest
@EmbeddedKafka(topics = "orders", partitions = 3)
class KeysAndPartitionsTest { ... }

spring-kafka-test starts EmbeddedKafkaKraftBroker — the actual Apache Kafka broker classes, in KRaft mode, in-process. No ZooKeeper, no container, no daemon. It binds a random port and exposes it as ${spring.embedded.kafka.brokers}:

spring:
  kafka:
    bootstrap-servers: ${spring.embedded.kafka.brokers}

It starts in about three seconds and needs nothing installed, which is why every transcript in this module came from it and why ./scripts/run-all.sh works on a machine with no Docker.

Testcontainers — the image you actually deploy

@TestConfiguration(proxyBeanMethods = false)
public class TestcontainersConfiguration {

    @Bean
    @ServiceConnection
    KafkaContainer kafkaContainer() {
        return new KafkaContainer(DockerImageName.parse("apache/kafka:4.1.0"));
    }

}

@ServiceConnection registers the container's bootstrap servers as the application's, which removes the @DynamicPropertySource block that older examples all carry.

Two coordinates changed recently and both will bite you:

<!-- NOT org.testcontainers:kafka, which stopped at 1.21.4 -->
<dependency>
  <groupId>org.testcontainers</groupId>
  <artifactId>testcontainers-kafka</artifactId>
  <scope>test</scope>
</dependency>

Testcontainers 2.x prefixed every module artifact with testcontainers-, and Boot 4.1.1 imports testcontainers-bom 2.0.5, which manages only the new names. Using the old coordinate fails with a Maven error that does not mention the rename:

'dependencies.dependency.version' for org.testcontainers:kafka:jar is missing

The class moved too: use org.testcontainers.kafka.KafkaContainer (Apache Kafka, KRaft), not the older org.testcontainers.containers.KafkaContainer (Confluent images, ZooKeeper).

Which to use

	@EmbeddedKafka	Testcontainers
startup	~3s	~10s, plus image pull
needs Docker	no	yes
broker version	the client library's	whatever image you name
TLS, SASL, quotas, partitions	not modelled	real

Use @EmbeddedKafka for the bulk of a suite and Testcontainers for the handful of tests where the difference between "the broker classes" and "the broker you deploy" matters. The TestcontainersConfiguration in this module is compiled but not exercised by run-all.sh, because the machine that regenerates docs/output/ has no Docker daemon — which is itself the argument for keeping both paths.

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