Investigate why code sample in reference doc fails checkArchitectureMain task

[ChildrenGreens]

Description

When building the spring-boot-docs module in Spring Boot 4.0.0 using JDK 25, the task:

:documentation:spring-boot-docs:checkArchitectureMain

fails due to an architecture rule violation triggered by one of the documentation sample classes.

The build output shows:

Execution failed for task ':documentation:spring-boot-docs:checkArchitectureMain'.
> Architecture check failed. See '.../spring-boot/documentation/spring-boot-docs/build/checkArchitectureMain/failure-report.txt' for details.

The generated failure-report.txt contains:

Architecture Violation [Priority: MEDIUM] - Rule 'members that aren't default constructors and aren't constants and don't override public methods should not be public' was violated (1 times):
Method <org.springframework.boot.docs.features.developingautoconfiguration.conditionannotations.classconditions.MyAutoConfiguration$SomeServiceConfiguration.someService()> has modifier PUBLIC in (MyAutoConfiguration.java:38)

This means the method someService() inside the documentation sample class MyAutoConfiguration$SomeServiceConfiguration is declared public, which violates the Spring Boot architecture rule disallowing public members that are not constructors, constants, or overriding methods.

Steps to Reproduce

1. Clone Spring Boot:

   git clone https://github.com/spring-projects/spring-boot
   cd spring-boot

2. Build the documentation module:

   ./gradlew :documentation:spring-boot-docs:build

   or run the architecture check directly:

   ./gradlew :documentation:spring-boot-docs:checkArchitectureMain

3. The task fails with the architecture violation described above.

Expected Behavior

The spring-boot-docs module should build successfully.
Documentation sample code should comply with Spring Boot’s architecture rules, meaning methods that:

• are not default constructors,
• are not constants,
• and do not override public methods

should not be declared public.

Actual Behavior

The architecture check fails because someService() in:

org.springframework.boot.docs.features.developingautoconfiguration.conditionannotations.classconditions.MyAutoConfiguration$SomeServiceConfiguration

is declared as public, violating the architecture rule.

Environment

Component	Value
Spring Boot version	4.0.0
Module	documentation/spring-boot-docs
JDK	25
OS	macOS

[snicoll]

Thanks for the report. This check is for our own code as we prefer those methods not to be public. I am not sure we should push this team's decision in the official documentation. If we do, then that would address the concern.

However, this doesn't fail for me. @bclozel had the same issue a few times and it went away. We'll have to investigate to understand what's going on.

[wilkinsona]

There are a few things going on here.

ArchUnit is being confused by spring-boot-docs having Java and Kotlin versions of the same class. Which one wins seems to be somewhat variable, although in my case it's (almost?) always the Kotlin version. When the Kotlin version wins, the problem does not occur as the Kotlin versions of both ….beanconditions.MyAutoConfiguration and ….classconditions.MyAutoConfiguration are not annotated with @AutoConfiguration. When the Java version wins, the problem occurs because the Java versions of those two MyAutoConfiguration classes are annotated with @AutoConfiguration and the rule added in #47715 is then applicable. With @AutoConfiguration added to the Kotlin versions of those classes, the problem occurs reliably.