Mocking a class vs. mocking its interface

问题:

For a unit test, I need to mock several dependencies. One of the dependencies is a class which implements an interface:

public class DataAccessImpl implements DataAccess {
    ...
}

I need to set up a mock object of this class which returns some specified values when provided with some specified parameters.

Now, what I'm not sure of, is if it's better to mock the interface or the class, i.e.

DataAccess client = mock(DataAccess.class);

vs.

DataAccess client = mock(DataAccessImpl.class);

Does it make any difference in regard to testing? What would be the preferred approach?

回答1:

It may not make much difference in your case but the preferred approach is to mock interface, as normally if you follow TDD (Test Driven Development) then you could write your unit tests even before you write your implementation classes. Thus even if you did not have concrete class DataAccessImpl, you could still write unit tests using your interface DataAccess.

Moreover mocking frameworks have limitations in mocking classes, and some frameworks only mock interfaces by default.

回答2:

It depends. If your code depends on the class and not on the interface you must mock the class to write a valid unit test.

回答3:

In most cases technically there is no difference and you may mock as class so an interface. Conceptually it is better to use interfaces because of better abstraction.

回答4:

You should mock the interface since it will help ensure you are adhering to Liskov Substitution Principal (https://stackoverflow.com/a/56904/3571100).

回答5:

If you only use it through interface and it's not a partial mock, there is no difference other than your inner feeling. Mocking the class will also mock non-used public method if the class has them, but that is not a big deal to consider.