Test annotated class only has one public method

What is the best way to write an ArchUnit test that tests that classes that are annotated with a specific annotation, e.g. the Spring Framework @Component annotation, only have one public method?

I haven't tried anything yet because I am still learning ArchUnit.

Whenever the predefined fluent API (classes().that().….should().…) does not offer a solution, you can define custom DescribedPredicates and ArchConditionss and use classes().that(…).should(…).

For your issue, I would use this:

import com.tngtech.archunit.junit.ArchTest;
import com.tngtech.archunit.lang.ArchRule;

import static com.tngtech.archunit.base.DescribedPredicate.describe;
import static com.tngtech.archunit.core.domain.JavaModifier.PUBLIC;
import static com.tngtech.archunit.core.domain.properties.HasModifiers.Predicates.modifier;
import static com.tngtech.archunit.lang.conditions.ArchConditions.have;
import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.classes;

public class StackOverflow76272127 {

  @ArchTest
  ArchRule components_should_have_only_one_public_method =
      classes()
          .that().areAnnotatedWith(Component.class)
          .should(have(describe("only one public method", javaClass ->
              javaClass.getMethods().stream().filter(modifier(PUBLIC)).count() == 1
          )));
}