Controllers implementing interfaces not mapped when the observability module is used

[anguzo]

Description

When using Interface-Driven Controllers in a Spring project configured with spring-web, spring-modulith, and spring-boot-actuator, the controllers do not function as expected. The issue appears specifically when including spring-modulith-observability as a runtime dependency.

For example, the following setup fails to work:

@RestController
class HelloController : HelloResource {
    override fun hello(testText: String): String {
        return testText
    }
}

interface HelloResource {
    @GetMapping("/api/hello")
    fun hello(@RequestParam("testText") testText: String): String
}

Reproducing the Issue

Here is a minimal sample to reproduce the issue:
demo.zip

When adding runtimeOnly("org.springframework.modulith:spring-modulith-observability") in the Gradle configuration, Interface-Driven Controllers fail to function. Removing this dependency allows the controllers to work as expected.

Expected Behavior

The application should process requests to the /api/hello endpoint and return the provided testText parameter as expected.

Observed Behavior

When spring-modulith-observability is included as a dependency, the endpoint does not work.

[anguzo]

I will also include Java/Maven based minimal sample, the problem persists in it as well.
demo_java.zip

[odrotbohm]

This kind of works as designed. We use the default setup of ProxyFactory to create the proxy required to add the observability interceptors. That in turn uses JDK proxies if the object to be proxied implements interfaces. As a result of that, the resulting proxy class does not carry the@RestController annotation of the original class. That annotation missing on the proxy type is the reason that Spring MVC then does not scan that type for mapping annotations.

An easy and recommended workaround is to consistently keep the annotations on the interface, if you decide to implement interfaces with the controller.

I think for 1.4, we should move to pick up the proxying mechanism globally configured for the Spring Boot application (it already seems to default to class-based proxies), but it's too late to change that for 1.3.

[odrotbohm]

We now consider the global AOP proxying settings when applying the observability decoration. Please give the 1.4 snapshots a try!

[anguzo]

Thank you, tried 1.4.0-SNAPSHOT and the issue is indeed resolved.

[odrotbohm]

Thanks for the feedback and happy coding! 🙌