Java – How to mock a final class with mockito

javajunitmockito

I have a final class, something like this:

public final class RainOnTrees{

   public void startRain(){

        // some code here
   }
}

I am using this class in some other class like this:

public class Seasons{

   RainOnTrees rain = new RainOnTrees();

   public void findSeasonAndRain(){

        rain.startRain();

    }
}

and in my JUnit test class for Seasons.java I want to mock the RainOnTrees class. How can I do this with Mockito?

Best Answer

Mockito 2 now supports final classes and methods!

But for now that's an "incubating" feature. It requires some steps to activate it which are described in What's New in Mockito 2:

Mocking of final classes and methods is an incubating, opt-in feature. It uses a combination of Java agent instrumentation and subclassing in order to enable mockability of these types. As this works differently to our current mechanism and this one has different limitations and as we want to gather experience and user feedback, this feature had to be explicitly activated to be available ; it can be done via the mockito extension mechanism by creating the file src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker containing a single line:

mock-maker-inline

After you created this file, Mockito will automatically use this new engine and one can do :

 final class FinalClass {
   final String finalMethod() { return "something"; }
 }

 FinalClass concrete = new FinalClass(); 

 FinalClass mock = mock(FinalClass.class);
 given(mock.finalMethod()).willReturn("not anymore");

 assertThat(mock.finalMethod()).isNotEqualTo(concrete.finalMethod());

In subsequent milestones, the team will bring a programmatic way of using this feature. We will identify and provide support for all unmockable scenarios. Stay tuned and please let us know what you think of this feature!

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