Java Springboot

Messaging, caching, scheduling, async

Ask before adding a broker, a cache provider or a scheduler/async setup to an existing project: each one adds infrastructure and operational cost. When the user agrees, use the Boot-managed versions (no explicit version for Spring-managed artifacts). Verified on Boot 4.1.1 (JDK 26 runtime, --release 25); Boot 3 differences noted where known. Tests for these: spring-boot-testing; tracing/metrics for them: spring-observability.

Kafka (Spring for Apache Kafka)

• Boot 4: spring-boot-starter-kafka (+ spring-boot-starter-kafka-test). Boot 3: org.springframework.kafka:spring-kafka (no starter; version managed by the Boot BOM).

• Producer: KafkaTemplate<String, String>; consumer: @KafkaListener.

@Component
class Events {
	private final KafkaTemplate<String, String> template;
	Events(KafkaTemplate<String, String> template) { this.template = template; }

	void publish(String key, String payload) { template.send("orders", key, payload); }

	@KafkaListener(topics = "orders", groupId = "demo")
	void on(String payload) { /* must be idempotent: delivery is at-least-once */ }
}

spring.kafka.bootstrap-servers=${KAFKA_BOOTSTRAP_SERVERS:localhost:9092}
spring.kafka.consumer.auto-offset-reset=earliest

• Consumers must be idempotent (at-least-once delivery); decide on a retry/dead-letter strategy (DefaultErrorHandler with DeadLetterPublishingRecoverer) before going to production.

• Keep payloads as versioned DTOs (records); avoid sending JPA entities.

• Test with a real broker through Testcontainers (verified: send then receive in a @SpringBootTest):

import org.testcontainers.kafka.KafkaContainer;

@TestConfiguration(proxyBeanMethods = false)
public class TestcontainersConfiguration {
	@Bean @ServiceConnection
	KafkaContainer kafkaContainer() { return new KafkaContainer(DockerImageName.parse("apache/kafka-native:4.0.0")); }
}

Boot 4 uses Testcontainers 2.x (artifact org.testcontainers:testcontainers-kafka); Boot 3 uses 1.x (artifact org.testcontainers:kafka). The example uses org.testcontainers.kafka.KafkaContainer, available in recent 1.x releases as well as 2.x; check that the project's BOM-managed version supports apache/kafka-native. Do not substitute the legacy org.testcontainers.containers.KafkaContainer: it requires a confluentinc/cp-kafka image. If the project's older Testcontainers version lacks the modern class or native-image support, use the legacy class with a compatible pinned Confluent image instead, without implicitly upgrading dependencies. Pin the image tag, never latest. Wait with a CountDownLatch/Awaitility, not Thread.sleep.

RabbitMQ / AMQP (not verified in this skill's builds)

spring-boot-starter-amqp (Boot 3 and 4), RabbitTemplate to send, @RabbitListener to consume, @ServiceConnection with the Testcontainers RabbitMQ module for tests. Declare queues/exchanges as Queue/TopicExchange/Binding beans, use Jackson2JsonMessageConverter-style JSON converters only after checking the Jackson major in use (Boot 4 = Jackson 3), and configure a dead-letter exchange. Check the Spring AMQP reference for the exact converter class names for your version before generating code.

Transactional outbox

Publishing to a broker inside a database transaction is not atomic: a crash between commit and send loses the message. When messages must not be lost, write the event to an outbox table in the same transaction as the state change, and have a relay (scheduled poller or CDC such as Debezium) publish and mark it sent. Consumers stay idempotent. Do not implement it unless the user needs that guarantee; for events that can be lost, @TransactionalEventListener(phase = AFTER_COMMIT) is simpler.

Caching with Caffeine

• Dependencies: spring-boot-starter-cache (Boot 4 + test starter spring-boot-starter-cache-test; Boot 3 the same starter name) and com.github.ben-manes.caffeine:caffeine (BOM-managed).

@Configuration(proxyBeanMethods = false)
@EnableCaching
class CachingConfig {}   // a dedicated class, not the @SpringBootApplication class

@Service
class RateService {
	@Cacheable("rates")                                   // key = method arguments
	public Rate expensive(String currency) { ... }

	@CacheEvict(value = "rates", allEntries = true)
	public void evict() {}
}

spring.cache.type=caffeine
spring.cache.cache-names=rates
spring.cache.caffeine.spec=maximumSize=500,expireAfterWrite=10m

• Verified on Boot 4.1.1: second call is served from cache, evict() forces a reload. Always set maximumSize and an expiry; unbounded caches are memory leaks.

• Cache immutable DTOs/records, not managed JPA entities. The cache is per JVM: with several instances use a distributed cache (Redis) only if the user asks for it, and expect stale data across nodes with Caffeine.

• @Cacheable is proxy-based: self-invocation and private methods bypass it. Do not cache methods with side effects or null returns you do not intend to cache.

• Enabling caching in @SpringBootApplication breaks slice tests that do not register a CacheManager (verified: @DataJpaTest fails with "No qualifying bean of type CacheManager"), hence the dedicated @Configuration.

• To expose cache statistics in Micrometer call recordStats (Caffeine spec recordStats); otherwise Boot logs a warning that the cache is not recording statistics (observed).

Scheduling

@Configuration(proxyBeanMethods = false)
@EnableScheduling
class SchedulingConfig {}

@Scheduled(fixedDelayString = "${app.tick:PT10S}")   // or cron = "0 0 * * * *", zone = "UTC"
void cleanup() { ... }

Verified: @Scheduled(fixedDelayString = "${...:100ms}") runs on Boot 4.1.1. Prefer fixedDelay over fixedRate to avoid overlapping runs. @Scheduled runs on every instance: with several replicas use a lock (ShedLock) or an external scheduler; add that only after asking. Scheduled methods take no arguments and must not throw unhandled exceptions silently: log and handle.

Async and virtual threads

spring.threads.virtual.enabled=true     # Java 21+, Boot 3.2+

With the property set, Boot runs Tomcat/Jetty request handling, @Async tasks (through the auto-configured task executor) and other Boot-managed executors on virtual threads. Verified on Boot 4.1.1: an @Async method reports Thread.currentThread().isVirtual() == true.

@Configuration(proxyBeanMethods = false)
@EnableAsync
class AsyncConfig {}

@Async
public CompletableFuture<Result> compute() { ... }   // return void or CompletableFuture

• Needs Java 21+; on Boot 3 the property exists from 3.2. Do not enable it on Java 17.

• Virtual threads help blocking I/O workloads (JDBC, HTTP). They do not speed up CPU-bound work. Pool sizes become the bottleneck: size the HikariCP pool deliberately and consider limits on downstream calls. Before JDK 24, synchronized blocks around blocking calls pin the carrier thread; prefer ReentrantLock there.

• @Async is proxy-based like @Transactional: the caller must be another bean, and the transaction/security context does not follow into the async thread unless propagated.

• With @Async returning void, exceptions are lost unless an AsyncUncaughtExceptionHandler is configured.