Spock unit testing and Inner closures - groovy

Question

I ran into a rather odd closure issue related to spock unit testing and wondered if anyone could explain this.
If we imagine a dao, model, and service as follows:
interface CustomDao {
List<Integer> getIds();
Model getModelById(int id);
}
class CustomModel {
int id;
}
class CustomService {
CustomDao customDao
public List<Object> createOutputSet() {
List<Model> models = new ArrayList<Model>();
List<Integer> ids = customDao.getIds();
for (Integer id in ids) {
models.add(customDao.getModelById(id));
}
return models;
}
}
I would like to unit test the CustomService.createOutputSet. I have created the following specification:
class TestSpec extends Specification {
def 'crazy closures'() {
def mockDao = Mock(CustomDao)
def idSet = [9,10]
given: 'An initialized object'
def customService = new CustomService
customService.customDao = mockDao
when: 'createOutput is called'
def outputSet = customService.createOutputSet()
then: 'the following methods should be called'
1*mockDao.getIds() >> {
return idSet
}
for (int i=0; i<idSet.size(); i++) {
int id = idSet.get(i)
1*mockDao.getModelById(idSet.get(i)) >> {
def tmp = new Model()
int tmpId = id // idSet.get(i)
return tmp
}
}
and: 'each compute package is accurate'
2 == outputSet.size()
9 == outputSet.get(0).getId()
10 == outputSet.get(1).getId()
}
}
Notice that in here I test two things. First, I initialize the dao with my mock, verify that the daos are correctly called and return the proper data, and then I verify that I get the proper output (i.e. "and:").
The tricky part is the for loop, in which I wanted to return models from the mock dao that are related to the method parameter. In the above example, if I use a simple for (__ in idSet), the models only return with id 10: outputSet.get(0).getId() == outputSet.get(1).getId() == 10. If I use the traditional for loop, and set the model with idSet.get(i), I get an IndexOutOfBoundsException . The only way to make this work is by retrieving the value in a local variable (id) and setting with variable, as above.
I know this is related to groovy closures and I suspect that spock captures the mock calls into a set of closures before executing them, which means that the model creation depends on the outer state of the closure. I understand why I would get the IndexOutOfBoundsException, but I don't understand why int id = idSet.get(i) is captured by the closure whereas i is not.
What is the difference?
Note: this is not the live code but rather simplified to demonstrate the crux of my challenge. I would not and do not make two subsequent dao calls on getIds() and getModelById().

Answer 1

While stubbing getModelById by a closure, the arguments to the closure has to match with that of the method. If you try something like below, you would not need the local variable id inside for anymore.
for (int i=0; i<idSet.size(); i++) {
//int id = idSet.get(i)
mockDao.getModelById(idSet.get(i)) >> {int id ->
def tmp = new Model()
tmp.id = id // id is closure param which represents idSet.get(i)
return tmp
}
}
Simplified version would be to use each
idSet.each {
mockDao.getModelById(it) >> {int id ->
def tmp = new Model()
tmp.id = id // id is closure param which represents idSet.get(i)
tmp
}
}
Do we need to worry about how many times method is called if it is being stubbed?

Answer 2

Accessing mutable local variables from a closure whose execution is deferred is a common source of errors not specific to Spock.
I don't understand why int id = idSet.get(i) is captured by the closure whereas i is not.
The former gives rise to a separate hoisted variable per iteration whose value is constant. The latter gives rise to a single hoisted variable whose value changes over time (and before the result generator executes).
Instead of solving the problem by introducing a temporary variable, a better solution (already given by #dmahapatro) is to declare an int id -> closure parameter. If it's deemed good enough to stub the calls without enforcing them, the loop can be omitted altogether. Yet another potential solution is to construct the return values eagerly:
idSet.each { id ->
def model = new Model()
model.id = id
1 * mockDao.getModelById(id) >> model
}