How to use mocks in Scala to mock a singleton object

Question

I recently started programming in Scala. I have a project with a hierarchy of classes that call each other. Eventually, they last one calls a singleton DAL (Data Access Layer) object that calls a stored procedure in MySQL.

I have a DAL object with the following signature:

def callStoredProcedure(procName: String, params: Array[String]): Boolean

I'd like to write a test that calls a function of the top level class, and checks out what procName was passed to the function.

How do I go about creating a mock for the DAL object? How can I inject it into the process pipeline, or is there a better/recommended way to replace the singleton with a mock that just returns the procedure name rather than calling it?

We're currently using Mockito, but I'm open to anything.

score 3 · Accepted Answer · answered Apr 05 '16 at 00:31

Don't use singletons directly, that's not a good idea. You know why? Because you can't mock them for unit testing, duh. Make it a parameter to your class instead:

trait DAL {
  def callStoredProcedure(procName: String, params: Array[String]): Boolean       
}

object DALImpl extends DAL {
    def callStoredProcedure(procName: String, params: Array[String]): Boolean = doStuff
}

class Foo(dal: DAL = DALImpl) 

val testMe = new Foo(mock[DAL])

or

class Foo {
    def dal: DAL = DALImpl
}

val testMe = new Foo {
  override def dal = mock[DAL]
}

Joan · Answer 2 · 2016-04-05T13:09:32.753

0

You can do:

class Foo(dal: DAL)

val testMe = new Foo(dal = mock[DAL.type])

Cheers

edited Apr 05 '16 at 13:09

answered Apr 05 '16 at 00:35

Joan

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How does that help, if I may ask? I want to grab the stored proc's name, without it being actually called. Spying won't do that. Mocking would still require an injection through all layers. Correct me if I'm wrong? – Traveling Tech Guy Apr 05 '16 at 01:06
Yes indeed `mock[DAL.type]` will require injection just like Dima showed you but without having to bother with a trait. `spy(DAL)` is actually not relevant for you as it will actually compute the function. I edit my answer. – Joan Apr 05 '16 at 13:01

How to use mocks in Scala to mock a singleton object

2 Answers2