ArchUnit to stop changing the state of instance variables within Spring beans

I have Spring beans this way

@Service
public class HotelsService {

    public List<Hotel> fetchAllHotels() {
        return dummyHotelsList();
    }
}

I should stop my developers from creating an instance variable within this bean and start changing the state within a method (see below snippet which i dont want to happen). Need to do this via archUnit. Didn't want to catch this during code review process

@Service
public class HotelsService {
    private int someInteger;

    public List<Hotel> fetchAllHotels() {
        this.someInteger=1; // just for example
        return dummyHotelsList();
    }
}

I am ok to have instance variables and just read it. But don't want to write on it.

Any straightforward implementation in ArchUnit ?

You may have to implement a custom (but straightforward... 😁) ArchCondition:

ArchRule service_methods_should_not_set_service_fields =
    methods()
        .that().areDeclaredInClassesThat().areAnnotatedWith(Service.class)
        .should(new ArchCondition<JavaMethod>("not set field of @Service class") {
            @Override
            public void check(JavaMethod method, ConditionEvents events) {
                method.getFieldAccesses().stream()
                    .filter(accessType(SET))
                    .filter(target(With.owner(annotatedWith(Service.class))))
                    .forEach(access -> events.add(violated(access, access.getDescription())));
            }
        })
        .because("beans should not have state");

using

import com.tngtech.archunit.core.domain.JavaMethod;
import com.tngtech.archunit.core.domain.properties.HasOwner.Predicates.With;
import com.tngtech.archunit.lang.ArchCondition;
import com.tngtech.archunit.lang.ArchRule;
import com.tngtech.archunit.lang.ConditionEvents;

import static com.tngtech.archunit.core.domain.JavaFieldAccess.AccessType.SET;
import static com.tngtech.archunit.core.domain.JavaFieldAccess.Predicates.accessType;
import static com.tngtech.archunit.core.domain.JavaFieldAccess.Predicates.target;
import static com.tngtech.archunit.core.domain.properties.CanBeAnnotated.Predicates.annotatedWith;
import static com.tngtech.archunit.lang.SimpleConditionEvent.violated;
import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.methods;

Your example fails with:
> Rule 'methods that are declared in classes that are annotated with @Service should not set field of @Service class, because beans should not have state' was violated (1 times):
Method <HotelsService.fetchAllHotels()> sets field <HotelsService.someInteger> in (source code location)

Maybe the problem can be simplified at the level of requirements:
Do you really have to allow for non-constant fields in your services?

A (straightforward! 😁) rule to prevent those would be:

ArchRule services_should_only_have_constant_fields =
    fields()
        .that().areDeclaredInClassesThat().areAnnotatedWith(Service.class)
        .should().beStatic().andShould().beFinal()
        .because("beans should not have state");