How to inject mocked dependencies with jmockit

Question

Currently I try to understand how the @Injectable and @Tested annotations are working. I already did some tests and understood the concept but I didn't get how I can use those annotations in real world applications.

Let's say we are developing a language translator class which depends on a web service. The web service methods are encapsulated in a separate class:

// class to test
public class Translator() {
    private TranslatorWebService webService;

    public String translateEnglishToGerman(String word){
        webService = new TranslatorWebService();
        return webService.performTranslation(word);
    }
}

// dependency
public class TranslatorWebService {
    public String performTranslation(String word){
    // perform API calls    
    return "German Translation";
    }
}

To test the Translator class independently, we would like to mock the TranslatorWebService class. According to my understanding, the test class should look like:

public class TranslatorTest {
    @Tested private Translator tested;
    @Injectable private TranslatorWebService transWebServiceDependency;

    @Test public void translateEnglishToGerman() {
        new Expectations() {{
            transWebServiceDependency.performTranslation("House");
            result = "Haus";
        }};

        System.out.println(tested.translateEnglishToGerman("House"));
    }
}

When I executed this test case for the first time, I expected the result "Haus". At second glance I saw that the line

webService = new TranslatorWebService();

will always override the injected mock instance with a real instance. But how can I avoid this behavior without changing the business logic?

Answer 1

Good question. The thing to notice about JMockit's (or any other mocking API) support for dependency injection is that it's meant to be used only when the code under test actually relies on the injection of its dependencies.

The example Translator class does not rely on injection for the TranslatorWebService dependency; instead, it obtains it directly through internal instantiation.

So, in a situation like this you can simply mock the dependency:

public class TranslatorTest {
    @Tested Translator tested;
    @Mocked TranslatorWebService transWebServiceDependency;

    @Test public void translateEnglishToGerman() {
        new Expectations() {{
            transWebServiceDependency.performTranslation("House");
            result = "Haus";
        }};

        String translated = tested.translateEnglishToGerman("House");

        assertEquals("Haus", translated);
    }
}