Grails / Spock: How to mock single method within class where method is called from within the class itself?

Question

Given the following, how do I mock processMessage() using Spock, so that I can check that processBulkMessage() calls processMessage() n times, where n is the number of messages within a BulkMessage?

class BulkMessage {
    List messages
}

class MyService {

    def processBulkMessage(BulkMessage msg) {
        msg.messages.each {subMsg->
            processMessage(subMsg)
        }
    }

    def processMessage(Message message) {

    }
}

Answer 1

You can use spies and partial mocks (requires Spock 0.7 or newer).

After creating a spy, you can listen in on the conversation between the caller and the real object underlying the spy:

def subscriber = Spy(SubscriberImpl, constructorArgs: ["Fred"])
subscriber.receive(_) >> "ok"

Sometimes, it is desirable to both execute some code and delegate to the real method:

subscriber.receive(_) >> { String message -> callRealMethod(); message.size() > 3 ? "ok" : "fail" }

Answer 2

In my opinion this is not a well designed solution. Tests and design walk hand in hand - I recommend this talk to investigate it better. If there's a need to check if other method was invoked on an object being under test it seems it should be moved to other object with different responsibility.

Here's how I would do it. I know how visibility works in groovy so mind the comments.

@Grab('org.spockframework:spock-core:0.7-groovy-2.0')
@Grab('cglib:cglib-nodep:3.1')

import spock.lang.*

class MessageServiceSpec extends Specification {

    def 'test'() {
        given:
        def service = new MessageService()
        def sender = GroovyMock(MessageSender)

        and:
        service.sender = sender

        when:
        service.sendMessages(['1','2','3'])

        then:
        3 * sender.sendMessage(_)
    }
}
class MessageSender { //package access - low level   
   def sendMessage(String message) {
      //whatever
   }
}

class MessageService {

   MessageSender sender //package access - low level

   def sendMessages(Iterable<String> messages) {
      messages.each { m -> sender.sendMessage(m) }
   }
}