How to login a user with spring 3.2 new mvc testing

This works fine until I have to test a service that needs a logged in user, how do I add user to context :

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration("classpath:applicationContext-test.xml")
@WebAppConfiguration
public class FooTest {
@Autowired
private WebApplicationContext webApplicationContext;

private MockMvc mockMvc;

@Resource(name = "aService")
private AService aService; //uses logged in user

@Before
public void setup() {
    this.mockMvc = webAppContextSetup(this.webApplicationContext).build();
}