How to Mock Superclass Method Invocation in a Child Class?

Mocking Superclass Method Invocation in Child Class

In unit testing, it is sometimes necessary to mock a method call within a subclass while leaving the original behavior intact in the superclass. This scenario can arise when testing methods that delegate to parent classes.

Consider the following example:

<code class="java">class BaseService {
    public void save() {...}
}

public Childservice extends BaseService {
    public void save(){
        //some code
        super.save();
    }
}</code>

In this case, a test requires mocking only the call to super.save() in the ChildService method while retaining the original behavior of the save() method in the BaseService class.

One approach to achieve this is by employing the spy functionality provided by Mockito. By spying on the ChildService instance, accessing the BaseService instance is possible, allowing for fine-grained control over the method call:

<code class="java">class BaseService {

    public void validate(){
        fail(" I must not be called");
    }

    public void save(){
        //Save method of super will still be called.
        validate();
    }
}

class ChildService extends BaseService{

    public void load(){}

    public void save(){
        super.save();
        load();
    }
}

@Test
public void testSave() {
    ChildService classToTest = Mockito.spy(new ChildService());

    // Prevent/stub logic in super.save()
    Mockito.doNothing().when((BaseService)classToTest).validate();

    // When
    classToTest.save();

    // Then
    verify(classToTest).load();
}</code>

By mocking the validate() method in the BaseService class, the test ensures that the original logic in super.save() is not executed. Additionally, stubbing the load() method in the ChildService class verifies that it is called as expected. This approach provides fine-grained control over method invocations and allows for the isolation of specific functionality during testing.

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