Introduction to jMockit

Mockobjects plays a vital role in unit testing and TDD. They are not only used for unit testing but also helpful in integration testing.

There are various tools available for creating mockobjects. For Java EasyMock and jMock are mostly used. Both are based on java.lang.reflect.Proxy. Because of that EasyMock and jMock have their own constraints.

They are actually pattern specific. Each of these tools requires an interface to be implemented and if there is no interface is present, create the same. This way developer can refactor their design. But most of the time developers are not allowed to change the design of project or they are working on Legacy Code (like me).

These tools have rich APIs to express expectations that has to be verified when objects are called. But most of the time, checks written as EasyMock/jMock expectation instead of using JUnit's already available assertion methods.

No static method for which a mock implementation might be needed can be used.

All mocked methods need to be public for mock classes that implement an interface, or at least protected for mock sub classes.

To solve these problems jMockit comes into picture. It provides a facility to create mockobject without any design restriction. jMockit is based on java.lang.instrument package. This package is only available with Java 5 or more.

jMockit can be applicable for any design. Developer can test their code without worrying about design. Test writer can also keep using testing framework assertion rather then tool specific assertion.