Using Mock Objects in Java
by Keld H. Hansen.
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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