Mock Final Classes and Methods with Mockito

1. Overview

In this short article, we’ll focus on how to mock final classes and methods – using Mockito.

As with other articles focused on the Mockito framework (like Mockito Verify, Mockito When/Then, and Mockito’s Mock Methods) we’ll use the MyList class shown below as the collaborator in test cases.

We’ll add a new method for this tutorial:

public final class MyList extends AbstractList {
    final public int finalMethod() {
        return 0;
    }
}

And we’ll also extend it with a final subclass:

public final class FinalList extends MyList {

    @Override
    public int size() {
        return 1;
    }
}

2. Configure Mockito for Final Methods and Classes

Before Mockito can be used for mocking final classes and methods, it needs to be configured.

We need to add a text file to the project’s src/test/resources/mockito/extensions directory named org.mockito.plugins.MockMaker and add a single line of text:

mock-maker-inline

Mockito checks the extensions directory for configuration files when it is loaded. This file enables the mocking of final methods and classes.

3. Mock a Final Method

Once Mockito is properly configured, a final method can be mocked like any other:

@Test
public void whenMockFinalMethodMockWorks() {

    MyList myList = new MyList();

    MyList mock = mock(MyList.class);
    when(mock.finalMethod()).thenReturn(1);

    assertNotEquals(mock.finalMethod(), myList.finalMethod());
}

By creating a concrete instance and a mock instance of MyList, we can compare the values returned by both versions of finalMethod() and verify that the mock is called.

4. Mock a Final Class

Mocking a final class is just as easy as mocking any other class:

@Test
public void whenMockFinalClassMockWorks() {

    FinalList finalList = new FinalList();

    FinalList mock = mock(FinalList.class);
    when(mock.size()).thenReturn(2);

    assertNotEquals(mock.size(), finalList.size());
}

Similar to the test above, we create a concrete instance and a mock instance of our final class, mock a method and verify that the mocked instance behaves differently.

5. Conclusion

In this quick tutorial, we covered how to mock final classes and methods with Mockito by using a Mockito extension.

The full examples, as always, can be found over on GitHub.