Best Practices for Testing Abstract Classes with Mockito

Introduction

Testing abstract classes is a common challenge in unit testing. Traditional approaches require manually creating subclasses to implement abstract methods, which not only increases code volume but can also introduce unnecessary complexity. Mockito, as a powerful Java mocking framework, offers a more elegant solution. This article delves into how to use Mockito's CALLS_REAL_METHODS feature to test abstract classes by creating partial mock objects that directly invoke concrete methods without implementing abstract ones.

Core Concept: Using CALLS_REAL_METHODS

The Answers.CALLS_REAL_METHODS parameter in Mockito allows the invocation of real methods when creating mock objects. For abstract classes, this enables testing of concrete methods while abstract methods are handled automatically by Mockito. For example, consider an abstract class My with a concrete method methodUnderTest and an abstract method methodIDontCareAbout. The following code can be used to test the concrete method:

public abstract class My {
    public Result methodUnderTest() {
        // Concrete method implementation
        return someResult;
    }
    protected abstract void methodIDontCareAbout();
}

public class MyTest {
    @Test
    public void shouldFailOnNullIdentifiers() {
        My my = Mockito.mock(My.class, Answers.CALLS_REAL_METHODS);
        Assert.assertSomething(my.methodUnderTest());
    }
}

In this example, Mockito.mock creates a mock object with CALLS_REAL_METHODS specified, allowing the methodUnderTest method to be called directly, while the abstract method methodIDontCareAbout does not need implementation. Mockito handles calls to abstract methods, preventing runtime errors.

Advantages Analysis

The primary advantage of using CALLS_REAL_METHODS lies in code simplicity and maintainability. Compared to manually creating subclasses, this method reduces boilerplate code, allowing tests to focus more on business logic. Moreover, it avoids potential errors that could arise from implementing abstract methods, enhancing test reliability. In most cases, this is more efficient than using Spy, as Spy requires an instantiable subclass, which abstract classes cannot provide directly.

Supplementary Methods

While CALLS_REAL_METHODS is effective in many scenarios, issues may arise if concrete methods invoke abstract methods. In such cases, Mockito's doCallRealMethod or thenCallRealMethod can be used to explicitly call concrete methods. For instance, for non-void methods, use thenCallRealMethod:

when(myInstance.myNonVoidMethod(someArgument)).thenCallRealMethod();

This approach is suitable for scenarios requiring control over method invocation order or handling dependency injection, but CALLS_REAL_METHODS is generally the more straightforward choice.

Conclusion

By leveraging Mockito's CALLS_REAL_METHODS feature, testing abstract classes becomes simple and efficient. It allows developers to concentrate on testing concrete logic without worrying about abstract method implementations. In practical projects, selecting the appropriate method based on specific needs can significantly improve the quality and efficiency of unit testing. It is recommended to prioritize this method when testing abstract classes and refer to other Mockito features as supplements in complex scenarios.