Mockito is a mocking framework. In his article Mocks aren’t Stubs, Martin Fowler mentions that Mocks allow for behavior verification, rather than state verification, which verifies values after method calls to confirm the proper behavior occurred. Behavior verification instead makes sure the proper behavior happened, by confirming the proper methods themselves were called.
So mocking is a means of setting expectations of method calls on a mock object, and then verifying that those method calls actually happen. But how does this apply to Mockito?
Mockito is a bit more comprehensive. It has mocks, stubs and spies. It has a lot of methods, and a few different philosophies on how it should be used. Should everything be mocked except the system under test? Should you mock types that you didn’t create? When is partial mocking ok? These are questions that are not simply answered and will likely have answers that evolve over time. But the takeaway is that there is a lot to Mockito.
The best way to learn how Mockito can be used is to look at examples. Mockito’s documentation is a good resource and describes many of the concepts. However, the examples are a bit simple and isolated. A Vogella tutorial on Mockito has many better examples that shows how you can use Mockito in actual production code.
Complicated behavior is where mocking comes in handy. Stubs are great for making sure simple methods are called within objects. But in complicated systems, there is a lot more going on that may affect the behavior of the object under test. A mock object can trick external classes into believing that the Mock object is doing real work, by providing expected return values when a method is called. Furthermore, you cannot always check the state of an object without modifying the class itself, making behavior verification the only means of testing.
Mockito is commonly used in Android mobile app testing, especially when Android was officially using Java. In a mobile environment, you are building an app on top of a complicated framework, and as such you may encounter problems, or at least complications, when trying to test. A common example is the Context object in Android. It provides necessary information about the environment in which code is being run. Android Fragments and Activities might need this context internally, but the behavior of the Fragments and Activities should not depend on the actual Context object. Another issue arises: how do you set up application context in a test environment? With Mockito, these complications are resolved by simply creating a Mock Context object in the test cases. If you rely on return values within a mock object, you can even tell Mockito what it needs to return when methods are called.
Mockito can be used in combination with other test methods, using as many or as few of its features as desired. Mocking is quite possibly a necessity when testing complicated systems, and at the very least powerful and convenient.