spring-messaging-demo/kafka-basics/README.md

kafka-basics — producer, consumer and serialisation, from nothing

Companion project for Spring Boot 4.1 and Apache Kafka: Producer, Consumer and Serialisation from Scratch on ankurm.com.

Eight tests against a real Kafka broker, started in-process in KRaft mode. No Docker daemon, no local Kafka install, no ZooKeeper. ./scripts/run-all.sh regenerates everything under docs/output/.

Versions

	Version	Notes
JDK	25 (Temurin 25.0.4.1+1)	current LTS
Spring Boot	4.1.1	GA of the 4.1 line was 10 June 2026
Spring Kafka	4.1.1	Boot-managed
kafka-clients	4.2.1	Boot-managed. Central has 4.3.1; let the BOM decide
Jackson	3.1.5 (tools.jackson)	why JacksonJsonSerializer and not JsonSerializer
Testcontainers	2.0.5	artifact is testcontainers-kafka, not kafka

Versions were read from repo1.maven.org/.../maven-metadata.xml and from Boot's own spring-boot-dependencies POM, not from release announcements.

Quickstart

./scripts/run-all.sh     # runs every test and regenerates docs/output/
mvn test                 # the same thing without the capture

Documentation

1. The on-ramp, and the dependency that is not the one you remember
2. Producing, and the return value everybody throws away
3. Serialisation: two Jackson families
4. Keys and partitions: the ordering guarantee in disguise
5. Consuming
6. Acknowledgement, and a property that is not where you look for it
7. Testing without installing Kafka

Captured output

File	Produced by
effective-config.txt	client defaults vs Boot's overrides vs effective
key-to-partition.txt	real placement against a real broker
serialised-payload.txt	the Jackson 3 wire format
tests.txt	8 tests

Four things this module exists to prove

1. spring-kafka alone gives you no auto-configuration under Boot 4. You need spring-boot-starter-kafka; the symptom is a missing KafkaTemplate bean.
2. JsonSerializer is the Jackson 2 one. It cannot write a java.time.Instant with its default mapper. JacksonJsonSerializer is the Jackson 3 one and can.
3. The default partitioner is murmur2 & 0x7fffffff, not Math.abs(murmur2). They disagree on two of the six keys in the committed transcript.
4. ConsumerFactory.isAutoCommit() answers true while no consumer auto-commits. The container overrides the property per-consumer and never tells the factory.