Mockito

Mockito is an open source testing framework for Java released under the MIT License. The framework allows the creation of mock objects in automated unit tests. Other Java mocking frameworks include PowerMock and EasyMock.

SUT

System under test (SUT) refers to a system that is being tested for correct operation.

Mocking

Mocking is a replica or imitation of something. Mocking is used in unit testing. Mock objects are simulated objects that mimic the behavior of real objects in controlled ways. A tested object may have dependencies on other complex objects. To isolate the behavior of the object we want to test we replace the other objects by mocks that simulate the behavior of the real objects. Mock objects never call real methods. Simply put, mocking is creating objects that simulate the behavior of real objects.

Stub

A stub is a method that just returns a simple but valid (though not necessarily correct) result.

During the development of a software system, we do not need every method right now. Instead we create stubs so everything compiles and our IDE's auto-complete knows about the methods we plan to use.

Stubs are using in mocking as well. We supply stub methods instead of the normal methods which return fixed results and then ensure that the code does the right thing with them. This isolates testing to the code we are trying to test. For instance, we do not need to spin up a database or have an online service working just to run those tests.

Spy

A Spy wraps an existing object. Spy delegates method calls to the original object.

When a class is mocked or stubbed, a test double is created and the original code that exists within the mocked or stubbed object is not executed. Spies, on the other hand, will execute the original code from which the Spy was created.

Spies should be used carefully and occasionally, for example when dealing with legacy code.