ArchUnit: Test a class if it contains an annotated method

I want to write an archunit rule in my J2EE project to test when a class is annotated with @Startup, then it must contain a method that is annotated with @PostConstruct.

I tried this

  @ArchTest
  private val startUpBeansShouldHavePostConstruct: ArchRule = classes().that()
    .areAnnotatedWith(Startup::class.java)
    .shouldHave(methods().that().areAnnotatedWith(PostConstruct::class.java))
    .because("classes with @Startup should contain a @PostConstruct annotated method")

but that doesn't compile in the shouldHave()-line. I am a bit stuck how to define such a rule. My actual goal is to have a rule that says there should be exactly one annotated method.

You can conveniently use ArchConditions.have and DescribedPredicate.describe to define custom conditions and predicates:

With

import com.tngtech.archunit.base.DescribedPredicate.describe
import com.tngtech.archunit.core.domain.JavaClass
import com.tngtech.archunit.junit.ArchTest
import com.tngtech.archunit.lang.conditions.ArchConditions.have
import com.tngtech.archunit.lang.syntax.ArchRuleDefinition.classes

you can use

@ArchTest
private val startUpBeansShouldHavePostConstruct =
    classes()
        .that().areAnnotatedWith(Startup::class.java)
        .should(have(oneMethodAnnotatedWith(PostConstruct::class.java)))

private fun oneMethodAnnotatedWith(annotationClass: Class<out Annotation>) =
    describe("one method annotated @${annotationClass.simpleName}") {
        javaClass: JavaClass ->
            javaClass.methods.count { it.isAnnotatedWith(annotationClass) } == 1
    }