Saturday, September 29, 2007

Write better tests using a combination of mocks and stubs.

I love using EasyMock. It's just plain, well, easy to use most of the time. Most of the time, it doesn't get in my way. However, there are certain times when I will avoid using a mock object in a test. This is best show by example.

As background, remember that mocks are not stubs and stubs are not mocks. You can use a mocking framework (like easymock) to create stubs, and you can even create mocks by hand (with a good deal of pain). Roy Osherove has done a great job of describing the differences between mocks and stubs. You should check out Roy's explanation. This is a different explanation from what I've seen before. If you want a good, detailed description of the differences, Martin Fowler has provided an in-depth comparison between the two including code samples. You should take a look at Fowler's explanation to understand the terminology.

I won't rehash the explanations. What I will say is that I've found value in doing both interaction-based testing (with mocks) and state-based testing (with stubs). When your classes are well decomposed and decoupled, mock testing makes more sense in most cases. However, when I'm interested in how a particular piece of code affects state on objects that pass through it, I will use a stub.

Here's the promised example. Supposed a I have a class, we'll call Processor that interacts with a dependency that sits behind and interface called EmailService. I'm interested in testing that Processor.emailManagers("we have a problem") can create an instance of Email with the correct properties set and hand that off to the EmailService. Looking at the test first, we have:

public void myTest() {
EmailServiceStub serviceStub = new EmailServiceStub();
Processor processor = new Processor(serviceStub);
processor.emailManagers("This should be working");

email = serviceStub.getLastSentEmail();
assertEquals("This should be working", email.getMessage());
assertEquals("", email.getTo());
assertEquals("An Automated Message from 'The System'", email.getSubject());
assertEquals("", email.getFrom());

Obviously, we'll need something to stand in the place of the EmailService so that we don't actually send an email in the course of running this unit test. The stand-in for this test is done by a stub like this:

public class EmailServiceStub implements EmailService {
Email lastSentEmail = null;

public void send(Email email) {
this.lastSentEmail = email;

public Email getLastSentEmail() { return this.lastSentEmail; }

The actual implementation code for Processor then might look like this:

public class Processor {
EmailService emailService;
public Processor (EmailService emailService) { this.emailService = emailService}

public emailManagers(String message) {
Email email = new Email();
email.setSubject("An Automated Message from 'The System'");


To be fair, this kind of testing is possible with EasyMock as well. You CAN check the properties of the object that are passed to send() using EasyMock. However, I find it incredibly painful to do so. What's worse, it makes the test convoluted to read. EasyMock would have you define your own implementation of IArgumentMatcher as indicated here. (search for "Defining own Argument Matchers"). Blech! We've done this before, and it seems to make the tests much more difficult to grok when I come back to them.

There are plenty of good examples of when you would want to use mocks over stubs. The Fowler post linked to at the top is a good start. I still use mocks for most of my testing, but in this instance, it's easier to use a stub than battle EasyMock.

No comments: