Why is it so bad to mock classes?

Question

I recently discussed with a colleague about mocking. He said that mocking classes is very bad and should not be done, only in few cases.

He says that only interfaces should be mocked, otherwise it's an architecture fault.

I wonder why this statement (I fully trust him) is so correct? I don't know it and would like to be convinced.

Did I miss the point of mocking (yes, I read Martin Fowler's article)

Answer 1

Edit: Since you have clarified that your colleague meant mock class is bad but mock interface is not, the answer below is outdated. You should refer to this answer.

I am talking about mock and stub as defined by Martin Fowler, and I assume that's what your colleague meant, too.

Mocking is bad because it can lead to overspecification of tests. Use stub if possible and avoid mock.

Here's the diff between mock and stub (from the above article):

We can then use state verification on the stub like this.

class OrderStateTester...
  public void testOrderSendsMailIfUnfilled() {
    Order order = new Order(TALISKER, 51);
    MailServiceStub mailer = new MailServiceStub();
    order.setMailer(mailer);
    order.fill(warehouse);
    assertEquals(1, mailer.numberSent());
  }

Of course this is a very simple test - only that a message has been sent. We've not tested it was send to the right person, or with the right contents, but it will do to illustrate the point.

Using mocks this test would look quite different.

class OrderInteractionTester...
  public void testOrderSendsMailIfUnfilled() {
    Order order = new Order(TALISKER, 51);
    Mock warehouse = mock(Warehouse.class);
    Mock mailer = mock(MailService.class);
    order.setMailer((MailService) mailer.proxy());

    mailer.expects(once()).method("send");
    warehouse.expects(once()).method("hasInventory")
      .withAnyArguments()
      .will(returnValue(false));

    order.fill((Warehouse) warehouse.proxy());
  }
}

In order to use state verification on the stub, I need to make some extra methods on the >stub to help with verification. As a result the stub implements MailService but adds extra >test methods.

Answer 2

Mocking is used for protocol testing - it tests how you'll use an API, and how you'll react when the API reacts accordingly.

Ideally (in many cases at least), that API should be specified as an interface rather than a class - an interface defines a protocol, a class defines at least part of an implementation.

On a practical note, mocking frameworks tend to have limitations around mocking classes.

In my experience, mocking is somewhat overused - often you're not really interested in the exact interaction, you really want a stub... but mocking framework can be used to create stubs, and you fall into the trap of creating brittle tests by mocking instead of stubbing. It's a hard balance to get right though.

Answer 3

Generally you'd want to mock an interface.

While it is possible to mock a regular class, it tends to influence your class design too much for testability. Concerns like accessibility, whether or not a method is virtual, etc. will all be determined by the ability to mock the class, rather than true OO concerns.

There is one faking library called TypeMock Isolator that allows you to get around these limitations (have cake, eat cake) but it's pretty expensive. Better to design for testability.

Answer 4

The answer, like most questions about practices, is "it depends".

Overuse of mocks can lead to tests that don't really test anything. It can also lead to tests which are virtual re-implementations of the code under test, tightly bound to a specific implementation.

On the other hand, judicious use of mocks and stubs can lead to unit tests which are neatly isolated and test one thing and one thing alone - which is a good thing.

It's all about moderation.

Answer 5

IMHO, what your colleague means is that you should program to an interface, not an implementation. If you find yourself mocking classes too often, it's a sign you broke the previous principle when designing your architecture.

Answer 6

Mocking classes (in contrast to mocking interfaces) is bad because the mock still has a real class in the background, it is inherited from, and it is possible that real implementation is executed during the test.

When you mock (or stub or whatever) an interface, there is no risk of having code executed you actually wanted to mock.

Mocking classes also forces you to make everything, that could possibly be mocked, to be virtual, which is very intrusive and could lead to bad class design.

If you want to decouple classes, they should not know each other, this is the reason why it makes sense to mock (or stub or whatever) one of them. So implementing against interfaces is recommended anyway, but this is mentioned here by others enough.

Answer 7

It makes sense to mock classes so tests can be written early in the development lifecycle.

There is a tendency to continue to use mock classes even when concrete implementations become available. There is also the tendency to develop against mock classes (and stubs) necessary early in a project when some parts of the system have not been built.

Once a piece of the system has been built it is necessary to test against it and continue to test against it (for regression). In this case starting with mocks is good but they should be discarded in favour of the implementation as soon as possible. I have seen projects struggle because different teams continue to develop against the behaviour of the mock rather than the implementation (once it is available).

By testing against mocks you are assuming that the mock is characteristic of the system. Often this involves guessing what the mocked component will do. If you have a specification of the system you are mocking then you don't have to guess, but often the 'as-built' system doesn't match the original specification due to practical considerations discovered during construction. Agile development projects assume this will always happen.

You then develop code that works with the mock. When it turns out that the mock does not truly represent the behaviour of the real as-built system (eg. latency issues not seen in the mock, resource and efficiency issues not seen in the mock, concurrency issues, performance issues etc) you then have a bunch of worthless mocking tests you must now maintain.

I consider the use of mocks to be valuable at the start of development but these mocks should not contribute to project coverage. It is best later if the mocks are removed and proper integration tests are created to replace them otherwise your system will not be getting tested for the variety of conditions which your mock did not simulate (or simulates incorrectly relative to the real system).

So, the question is whether or not to use mocks, it is a matter of when to use them and when to remove them.