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.