how to write unit test case for controller class using mockito

It's definitely possible to write pure unit tests for Spring MVC controllers by mocking their dependencies with Mockito (or JMock) as jherricks showed above. The challenge that remains is that with annotated POJO controllers there is a lot that remains untested -- essentially everything that's expressed in annotations and done by the framework when the controller is invoked.

Support for testing Spring MVC controllers is under way (see the spring-test-mvc project). While the project will still undergo changes it's usable in its present form. If you're sensitive to change however you should not depend on it. Either way I felt it was worth pointing out if you want to track it or participate in its development. There is a nightly snapshot and there will be a milestone release this month if you want to lock into a specific version.

The real question is:
How to set up an integration testing environment of your application which is using Spring?
The answer to this question is not simple, it really depends on how your web application works.

You should first focus on how to JUnit a Java web application, then on how to use Mockito.

There are a couple things you seem to have crossed up in your test. There are integration tests and unit tests. Integration tests will test everything (or almost everything) all hooked up - so you use Spring configuration files very close to the real ones and real examples of objects get injected to your class under test. That's mostly what I use @ContextConfiguration but I use that in conjunction with @RunWith(SpringJUnit4ClassRunner.class)

If you are using Mockito (or any mocking framework), it is usually because you want to isolate the class you are testing from real implementations of other classes. So instead of, for example, having to contrive a way to get your RegistrationService to throw a NumberFormatException to test that code path, you just tell the mock RegistrationService to do it. There are lots of other examples where it is more convenient to use mocks than to use real class instances.

So, that mini-lesson finished. Here is how I would re-write your test class (with an extra example and commented along the way).

@RunWith(MockitoJUnitRunner.class)
public class RegistrationControllerTest {

    // Create an instance of what you are going to test.
    // When using the @InjectMocks annotation, you must create the instance in
    // the constructor or in the field declaration.
    @InjectMocks
    private RegistrationController controllerUT = new RegistrationController();

    // The @Mock annotation creates the mock instance of the class and
    // automatically injects into the object annotated with @InjectMocks (if
    // possible).
    @Mock
    private RegistrationService registrationService;
    // This @Mock annotation simply creates a mock instance. There is nowhere to
    // inject it. Depending on the particular circumstance, it may be better or
    // clearer to instantiate the mock explicitly in the test itself, but we're
    // doing it here for illustration. Also, I don't know what your real class
    // is like, but it may be more appropriate to just instantiate a real one
    // than a mock one.
    @Mock
    private ModelMap model;
    // Same as above
    @Mock
    private BulkRegistration bulkRegistration;
    // Same as above
    @Mock
    private FileData fileData;

    @Before
    public void setUp() {
        // We want to make sure that when we call getFileData(), it returns
        // something non-null, so we return the mock of fileData.
        when(bulkRegistration.getFileData()).thenReturn(fileData);
    }

    /**
     * This test very narrowly tests the correct next page. That is why there is
     * so little expectation setting on the mocks. If you want to test other
     * things, such as behavior when you get an exception or having the expected
     * filename, you would write other tests.
     */
    @Test
    public void testCreate() throws Exception {
        final String target = "bulkRegistration";
        // Here we create a default instance of BindingResult. You don't need to
        // mock everything.
        BindingResult result = new BindingResult();

        String nextPage = null;
        // Perform the action
        nextPage = controllerUT.create(bulkRegistration, result, model);
        // Assert the result. This test fails, but it's for the right reason -
        // you expect "bulkRegistration", but you get "registration".
        assertEquals("Controller is not requesting the correct form", nextPage,
                target);

    }

    /**
     * Here is a simple example to simulate an exception being thrown by one of
     * the collaborators.
     * 
     * @throws Exception
     */
    @Test(expected = NumberFormatException.class)
    public void testCreateWithNumberFormatException() throws Exception {
        doThrow(new NumberFormatException()).when(registrationService)
                .processFile(any(File.class), anyList());
        BindingResult result = new BindingResult();
        // Perform the action
        controllerUT.create(bulkRegistration, result, model);
    }
}