5 minute Mockito

Some might find EasyMock a little frustrating and perhaps even too convoluted - here's a look at Mockito, a similar framework but with less complex use leading to quicker and simpler mock testing.

What is Mockito?

Apart from having a catchy name, Mockito attempts to think about mocking in a different way. Stated as an easy and lightweight alternative to EasyMock, Mockito is what you might call a Test Spy, and it seems to live up to all three of those ideologies. From its feature list, you can see it has a K.I.S.S approach (which I always like);

 Some other features:

• Mocks concrete classes as well as interfaces
• Little annotation syntax sugar - @Mock
• Verification errors are clean - click on stack trace to see failed verification in test; click on exception's cause to navigate to actual interaction in code. Stack trace is always clean.
• Allows flexible verification in order (e.g: verify in order what you want, not every single interaction)
• Supports exact-number-of-times and at-least-once verification
• Flexible verification or stubbing using argument matchers (anyObject(), anyString() or refEq() for reflection-based equality matching)
• Allows creating custom argument matchers or using existing hamcrest matchers

One of my pet hates about EasyMock is its rather convoluted stacktraces when your mocks have failed or have been setup incorrectly. With Mockito, you are supposed to be able to get what you need from a failure - answers! Of course, there's only so much a framework can do; your tests need to be written properly in the first place, but we all do that, don't we?! 

A scenario

To demonstrate Mockito in a real-world example, we'll be mocking one interface. Below is DocumentRepository, whose FileBaseDocumentRepository implementation depends on a FileService instance (injected and the subject of our mock):

// package & import removed for brevity
public interface DocumentRepository {

    void addDocument(Document doc);

    Document getDocument(DocumentID docId);
}

And here's FileService, the class we'll be mocking (haha!) in our example:

// again, no packages or imports for brevity
public interface FileService {
    File createFile(String name);

    File getFile(String name);

    boolean fileExists(String filename);

    boolean fileExists(File file);
}

Brewing Mockito

In our FileBasedDocumentRepository implementation of DocumentRepository, we use the FileService to check if a file exists - via the fileExists(File f) method - and to get a file if so - via the getFile(File f) method. So, we need Mockito to do it's thing, and as you might expect we'd need to create our mocked instance by doing the following:

FileService fileService = mock(FileService.class);

Pretty easy stuff so far.

Next we'll need to tell the framework what to do when our two calls to FileService - fileExists and getFile - are made. To do so we use the when method:

when(fileService.fileExists(anyObject()));

Now we'll use our DocumentRepository and make an assertion using standard JUnit:

assertNull("Doc with non-existent name doesn't exist", docRepo.getDocument(doc.getID()));

 and finally we verify our mock calls:

verify(fileService);

So there you have it - a simple 5 minute introduction to Mockito. I recommend you read the Mockito wiki as it has a comparison of EasyMock amongst plenty of other things. Also, head on over to its feature list and FAQ pages or just download it.