Unit tests verify the behavior of individual units of an application. In a Java EE application, this usually regards single entity, boundary, and control classes.

In order to unit test a single class, no exhaustive test case should be required. Ideally, instantiating the test object and setting up minimum dependencies should be sufficient to be able to invoke and verify its business functionality.

Modern Java EE supports this approach. Java EE components, such as EJBs as well as CDI managed beans are testable in a straightforward way by simply instantiating the classes. As we saw previously, modern enterprise components represent plain Java objects, including annotations, without extending or implementing technically motivated superclasses or interfaces, so-called no-interface views.

This allows tests to instantiate EJB or CDI classes and to wire them up as required. Used delegates such as injected controls that are irrelevant to the test case are mocked away. By doing so, we define the boundaries of the test case, what should be tested, and what is not relevant. Mocked delegates enable to verify the test object interaction.

A mock object simulates behavior of an actual instance of its type. Calling methods on mocks usually only returns dummy or mock values. Test objects are not aware that they communicate with a mock object. The behavior of mocks, as well as the verification of invoked methods, is controlled within the test scenario.