CAMEL-22760: Bridge spring.kafka.* properties to camel-kafka component

[gnodet]

Summary

When using camel-kafka-starter with Spring Boot, users previously had to duplicate their Kafka configuration under both spring.kafka.* and camel.component.kafka.* properties. This PR adds automatic bridging so that Spring Boot's standard Kafka properties are picked up by the Camel Kafka component.

Properties bridged

Spring Boot (spring.kafka.*)	Camel (camel.component.kafka.*)
bootstrap-servers	brokers
client-id	client-id
security.protocol	security-protocol
consumer.group-id	group-id
ssl.key-store-location	ssl-keystore-location
ssl.key-store-password	ssl-keystore-password
ssl.key-store-type	ssl-keystore-type
ssl.key-password	ssl-key-password
ssl.trust-store-location	ssl-truststore-location
ssl.trust-store-password	ssl-truststore-password
ssl.trust-store-type	ssl-truststore-type
ssl.protocol	ssl-protocol
properties[sasl.mechanism]	sasl-mechanism
properties[sasl.jaas.config]	sasl-jaas-config
properties[sasl.kerberos.service.name]	sasl-kerberos-service-name

Design

• Uses Spring Boot's Binder to detect which camel.component.kafka.* properties the user explicitly set
• Only bridges a Spring Boot value when the user has NOT explicitly set the corresponding Camel property
• Explicit camel.component.kafka.* settings always take precedence
• @AutoConfigureBefore(KafkaComponentAutoConfiguration.class) ensures bridging happens before the generated auto-configuration copies properties to the component
• @ConditionalOnClass(KafkaProperties.class) ensures it only activates when spring-boot-kafka is on the classpath (optional dependency)

Test plan

• 9 unit tests covering all bridged properties, precedence, and edge cases
• All tests pass
• CI build passes
• Integration test with a real Kafka broker (manual)

🤖 Generated with Claude Code

[davsclaus]

this is a good idea, a JIRA would be good so we have it there also. And add a note in the 4.19 migration guide especially if this would default happen so end users would be aware when upgrading

[davsclaus]

I guess if not already exists should be an easy option to set = true or = false to turn this bridge on|off

[gnodet]

Thanks for the review @davsclaus!

We've addressed your feedback and also fixed several issues found during an in-depth code review:

Review fixes (latest commit):

1. @AutoConfiguration ordering — Changed from @Configuration + @AutoConfigureBefore to @AutoConfiguration(after = KafkaAutoConfiguration.class, before = KafkaComponentAutoConfiguration.class). This ensures the bridge runs after Spring Boot creates KafkaProperties and before Camel's component auto-configuration applies the settings.

2. @ConditionalOnBean(KafkaProperties.class) — Added so the bridge gracefully skips when Spring Kafka auto-configuration is excluded (e.g., user excluded KafkaAutoConfiguration), preventing a startup failure from missing constructor dependency.

3. Per-property Binder.bind().isBound() for precedence checks — The original code used Binder.bind("camel.component.kafka", mapOf(...)).containsKey("ssl-keystore-location") to check if a Camel property was explicitly set. However, the Binder does not normalize camelCase keys to kebab-case in map bindings, so containsKey("ssl-keystore-location") would miss sslKeystoreLocation set by the user, causing their explicit Camel property to be silently overwritten. We now use individual binder.bind("camel.component.kafka.ssl-keystore-location", String.class).isBound() calls which properly handle relaxed binding (camelCase, kebab-case, underscore variants).

4. Bootstrap servers default handling — KafkaProperties.getBootstrapServers() defaults to ["localhost:9092"] and is never null/empty. The original null/empty check would always bridge, even when the user didn't set spring.kafka.bootstrap-servers. We now use isSpringPropertyBound("bootstrap-servers") to check if the property was explicitly set.

Regarding your suggestions:

• JIRA ticket: CAMEL-22760 already exists and is linked to this PR.
• On/off toggle: Good point — we could add a camel.component.kafka.bridge-spring-kafka-properties (or similar) property. We can add that in a follow-up if you'd like, or in this PR. What do you prefer?
• Migration guide note: Will add a note to the 4.19 migration guide.

[gnodet]

Added the toggle in the latest commit. The bridge can be disabled with:

camel.component.kafka.bridge-spring-kafka-properties=false

It's enabled by default (matchIfMissing = true). Uses @ConditionalOnProperty so the entire auto-configuration class is skipped when disabled — no beans created, no overhead.

[davidkarlsen]

What about transactional() - https://camel.apache.org/components/4.18.x/eips/transactional-client.html - spring has a KafkaTransactionManager (PlatformTransactionManager) - or would that be too complex?
Perhaps just add a note in https://camel.apache.org/components/4.18.x/kafka-component.html#_kafka_transaction about how it is intended to work with tx.

[davsclaus]

@davidkarlsen can you create a JIRA about Spring Kafka Transaction support for camel-kafka

[davsclaus]

On/off toggle: Good point — we could add a camel.component.kafka.bridge-spring-kafka-properties (or similar) property. We can add that in a follow-up if you'd like, or in this PR. What do you prefer?

Yes this PR

Migration guide note: Will add a note to the 4.19 migration guide.

Add this to the 4.21 migration guide (4.19 and 4.20 has been released)

@gnodet ^^^^

[davsclaus]

@Croway have you seen this PR.

The spring-boot kafka starter is added as optional=true dependency. Which would be needed for this PR. Though the con could be that we have a Camel vs SB kafka client misalignment and a classpath problem. But that can happen anyway today. And I guess if I recall correct optional=true then the SB kafka JARs are not loaded by default by maven, and wont be in the end user app.

[Croway]

@davsclaus exactly, the PR is fine, I was even thinking to remove the optional=true, and always import spring-boot-kafka, but this will add Spring Boot KafkaTemplate and Kafka configurations classes even if the user is not using it. Therefore, this is the right approach.

I just tested the version alignments and I confirm that the Kafka client version is aligned by the Spring Boot BOM. For example, camel 4.18.1 declares kafka 3.9.x. The Spring Boot version used in Camel Spring Boot 4.18.1, declares kafka 3.8.x, that is the version that camel spring boot users uses at runtime.

I'll create an issue to harden the tests and CI so that we might be able to catch potential API changes and misalignments.

[davsclaus]

@gnodet or @Croway can any of you help update the 4.21 upgrade guide (in camel-core) about this - as CSB users that use spring-boot-kafka would be good to be aware of this feature when they are upgrading.

[Croway]

will do, I'll provide an entry in the 4.21 upgrade guide.

EDIT: apache/camel#22786