Tutorials : Using Mock Objects in Java :

Using Mock Objects in Java

Introduction

A mock object is, as the name implies, a simulation of some real object. In Java, a mock object would typically be an implementation, with very limited functionality, of an Interface. Mock objects play a significant role in unit testing, for a number of reasons. You'd consider using a mock object for your unit test if the real object has a complex set-up, uses many system resources (like cpu power), or doesn't yet exist! If you're writing an application that uses a database, there's no need to wait for the team that develops the database modules before you start coding the rest of the application. What you need is a mock object that behaves like the database modules. If you have interfaces defined to the database modules you're ready to go. Coding with these interfaces might even bring some problems in the Interface design to the surface before the coding of the database modules has even started!

To be really useful the mock object should contain code which ensures that the user of the mock object follows the rules set up for its usage. To be a "real mock object" it should be able to register how it's going to be used, and then perform "self- validation" during execution.

In a framework like StrutsTestCase (which I covered in two articles here at JavaBoutique), mock objects are used to simulate the Struts servlet environment, making it possible to test a servlet application without having a web server running. Another example is to mock the jdbc classes, making testing without a database possible.

In this article I'll give some simple examples of how to code and use mock objects, and then demonstrate the MockObjects and MockMaker tools, which help you produce the code for the mock objects.

I assume you're familiar with the JUnit testing framework. If not, why not read a good intro to this superb tool?

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