MockHttpRequest | A mock implementation of XMLHttpRequest for unit testing | Mock library

The mocks need to be initialised. You can use MockitoAnnotations.initMocks(this) in the setup() method to initialise the @Mock annotated mocks. Also you have to use the http servlet request mock and not a servlet request mock as it cannot be downcasted

Your current test method makes no sense for me.

If you are testing your RestClient class, which implements IRestClient, you don't need to mock IRestClient itself. You need to mock all external dependencies instead - you've already created mock for IHttpWebRequestFactory, now you need to inject it into tested object. I don't see rest of your class, but I assume that your WebRequest object is of type IHttpWebRequestFactory - you need to assign your factory mock to it.

Now you need to define your test cases. I can quickly see following (but you can have more of cause):

1. StatusCode is not OK.
2. StatusCode is OK, but responseStream is null.
3. StatusCode is OK, responseStream is not null, method executes successfully.
4. Exception is thrown in try block.

Now for each test case you need to prepare proper setup and validation. For example, for 1st you need your mocked factory to return mocked request which will return mocked response with not OK result code. Now you need to call your actual object. As a validation you need to check that ApplicationException is thrown and all your mocks were actually called.

Ok, this is partial setup for 2nd test case. It will prepare mocked factory which returns mocked request which returns mocked response with OK code: