Testing Kotlin with Spock Part 3 – Interface default method

Kotlin allows you to put method implementation in an interface. The same mechanism can be found in Java interfaces as default methods (and also Groovy or Scala traits). Let’s see the difference between the Kotlin and Java default methods in interface by testing it with Groovy and Spock.

What do we want to test?

We often have an interface for access object from the database. In domain, they might look similar to this KotlinOrderRepository:

interface KotlinOrderRepository {
    fun save(order: Order)

    fun find(orderId: OrderId): Order?

    fun get(orderId: OrderId): Order =
            find(orderId) ?: throw NotFound()
}

How to fake it with Groovy?

When we want to use such interface in tests, we can, of course, mock it. However, it is far better to fake repositories with a simple, in-memory implementation. Let’s create FakeKotlinOrderRepository in Groovy:

class FakeKotlinOrderRepository implements KotlinOrderRepository {
    private Map<OrderId, Order> data = [:]

    @Override
    void save(Order order) {
        data[order.id] = order
    }

    @Override
    Order find(OrderId orderId) {
        return data[orderId]
    }
}

Unfortunately, this causes a compilation error

/testing-kotlin-in-spock/src/test/groovy/com/github/alien11689/testingkotlinwithspock/defaultmethod/FakeKotlinOrderRepository.groovy: 3: Can't have an abstract method in a non-abstract class. The class 'com.github.alien11689.testingkotlinwithspock.defaultmethod.FakeKotlinOrderRepository' must be declared abstract or the method 'com.github.alien11689.testingkotlinwithspock.defaultmethod.Order get(com.github.alien11689.testingkotlinwithspock.defaultmethod.OrderId)' must be implemented.
 @ line 3, column 1.
   class FakeKotlinOrderRepository implements KotlinOrderRepository {
   ^

1 error

The compiler doesn’t see the implementation of the get method in the Kotlin interface. We have to use some magic to make it work in groovy.

Solution

To solve the problem, let’s look into the generated classes:

$ ls build/classes/main/com/github/alien11689/testingkotlinwithspock/defaultmethod/
JavaOrderRepository.class
KotlinOrderRepository.class
KotlinOrderRepository$DefaultImpls.class
NotFound.class
Order.class
OrderId.class

The KotlinOrderRepository$DefaultImpls class is the one we’re looking for as we can use it in Groovy to implement the missing operation.

class FakeKotlinOrderRepository implements KotlinOrderRepository {

    // ...

    Order get(OrderId orderId) {
        return KotlinOrderRepository.DefaultImpls.get(this, orderId)
    }
}

Now the code compiles and tests pass:

class KotlinRepositoryWithDefaultMethodTest extends Specification {
    OrderId orderId = new OrderId(UUID.randomUUID() as String)
    Order order = new Order(orderId, 'data')
    KotlinOrderRepository kotlinOrderRepository = new FakeKotlinOrderRepository()

    def 'should get order from kotlin repository'() {
        given:
            kotlinOrderRepository.save(order)
        expect:
            kotlinOrderRepository.get(orderId) == order
    }

    def 'should throw NotFound when order does not exist in kotlin repository'() {
        when:
            kotlinOrderRepository.get(orderId)
        then:
            thrown(NotFound)
    }
}

Is there the same problem with Java?

Let’s have a quick look at how this works with Java interfaces. If we write a similar repository in Java:

public interface JavaOrderRepository {
    void save(Order order);

    Optional<Order> find(OrderId orderId);

    default Order get(OrderId orderId) {
        return find(orderId).orElseThrow(NotFound::new);
    }
}

and create a fake implementation in Groovy:

class FakeJavaOrderRepository implements JavaOrderRepository {
    private Map<OrderId, Order> data = [:]

    @Override
    void save(Order order) {
        data[order.id] = order
    }

    @Override
    Optional<Order> find(OrderId orderId) {
        return Optional.ofNullable(data[orderId])
    }
}

there is no compilation error and the tests pass:

class JavaRepositoryWithDefaultMethodTest extends Specification {
    OrderId orderId = new OrderId(UUID.randomUUID() as String)
    Order order = new Order(orderId, 'data')
    JavaOrderRepository javaOrderRepository = new FakeJavaOrderRepository()

    def 'should get order from java repository'() {
        given:
            javaOrderRepository.save(order)
        expect:
            javaOrderRepository.get(orderId) == order
    }

    def 'should throw NotFound when order does not exist in java repository'() {
        when:
            javaOrderRepository.get(orderId)
        then:
            thrown(NotFound)
    }
}

Groovy can implement Java interfaces with the default methods without any problems.

Show me the code

Code is available here.

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Thought static method can’t be easy to mock, stub nor track? Wrong!

No matter why, no matter is it a good idea. Sometimes one just wants to check or it's necessary to be done. Mock a static method, woot? Impossibru!

In pure Java world it is still a struggle. But Groovy allows you to do that really simple. Well, not groovy alone, but with a great support of Spock.

Lets move on straight to the example. To catch some context we have an abstract for the example needs. A marketing project with a set of offers. One to many.

import spock.lang.Specification

class OfferFacadeSpec extends Specification {

    OfferFacade facade = new OfferFacade()

    def setup() {
        GroovyMock(Project, global: true)
    }

    def 'delegates an add offer call to the domain with proper params'() {
        given:
            Map params = [projId: projectId, name: offerName]

        when:
            Offer returnedOffer = facade.add(params)

        then:
            1 * Project.addOffer(projectId, _) >> { projId, offer -> offer }
            returnedOffer.name == params.name

        where:
            projectId | offerName
            1         | 'an Offer'
            15        | 'whasup!?'
            123       | 'doskonała oferta - kup teraz!'
    }
}
So we test a facade responsible for handling "add offer to the project" call triggered  somewhere in a GUI.
We want to ensure that static method Project.addOffer(long, Offer) will receive correct params when java.util.Map with user form input comes to the facade.add(params).
This is unit test, so how Project.addOffer() works is out of scope. Thus we want to stub it.

The most important is a GroovyMock(Project, global: true) statement.
What it does is modifing Project class to behave like a Spock's mock. 
GroovyMock() itself is a method inherited from SpecificationThe global flag is necessary to enable mocking static methods.
However when one comes to the need of mocking static method, author of Spock Framework advice to consider redesigning of implementation. It's not a bad advice, I must say.

Another important thing are assertions at then: block. First one checks an interaction, if the Project.addOffer() method was called exactly once, with a 1st argument equal to the projectId and some other param (we don't have an object instance yet to assert anything about it).
Right shit operator leads us to the stub which replaces original method implementation by such statement.
As a good stub it does nothing. The original method definition has return type Offer. The stub needs to do the same. So an offer passed as the 2nd argument is just returned.
Thanks to this we can assert about name property if it's equal with the value from params. If no return was designed the name could be checked inside the stub Closure, prefixed with an assert keyword.

Worth of  mentioning is that if you want to track interactions of original static method implementation without replacing it, then you should try using GroovySpy instead of GroovyMock.

Unfortunately static methods declared at Java object can't be treated in such ways. Though regular mocks and whole goodness of Spock can be used to test pure Java code, which is awesome anyway :)No matter why, no matter is it a good idea. Sometimes one just wants to check or it's necessary to be done. Mock a static method, woot? Impossibru!

In pure Java world it is still a struggle. But Groovy allows you to do that really simple. Well, not groovy alone, but with a great support of Spock.

Lets move on straight to the example. To catch some context we have an abstract for the example needs. A marketing project with a set of offers. One to many.

import spock.lang.Specification

class OfferFacadeSpec extends Specification {

    OfferFacade facade = new OfferFacade()

    def setup() {
        GroovyMock(Project, global: true)
    }

    def 'delegates an add offer call to the domain with proper params'() {
        given:
            Map params = [projId: projectId, name: offerName]

        when:
            Offer returnedOffer = facade.add(params)

        then:
            1 * Project.addOffer(projectId, _) >> { projId, offer -> offer }
            returnedOffer.name == params.name

        where:
            projectId | offerName
            1         | 'an Offer'
            15        | 'whasup!?'
            123       | 'doskonała oferta - kup teraz!'
    }
}
So we test a facade responsible for handling "add offer to the project" call triggered  somewhere in a GUI.
We want to ensure that static method Project.addOffer(long, Offer) will receive correct params when java.util.Map with user form input comes to the facade.add(params).
This is unit test, so how Project.addOffer() works is out of scope. Thus we want to stub it.

The most important is a GroovyMock(Project, global: true) statement.
What it does is modifing Project class to behave like a Spock's mock. 
GroovyMock() itself is a method inherited from SpecificationThe global flag is necessary to enable mocking static methods.
However when one comes to the need of mocking static method, author of Spock Framework advice to consider redesigning of implementation. It's not a bad advice, I must say.

Another important thing are assertions at then: block. First one checks an interaction, if the Project.addOffer() method was called exactly once, with a 1st argument equal to the projectId and some other param (we don't have an object instance yet to assert anything about it).
Right shit operator leads us to the stub which replaces original method implementation by such statement.
As a good stub it does nothing. The original method definition has return type Offer. The stub needs to do the same. So an offer passed as the 2nd argument is just returned.
Thanks to this we can assert about name property if it's equal with the value from params. If no return was designed the name could be checked inside the stub Closure, prefixed with an assert keyword.

Worth of  mentioning is that if you want to track interactions of original static method implementation without replacing it, then you should try using GroovySpy instead of GroovyMock.

Unfortunately static methods declared at Java object can't be treated in such ways. Though regular mocks and whole goodness of Spock can be used to test pure Java code, which is awesome anyway :)