how to Initialize Servlet Config through mockito

I am writing test case for servlets. In Test I am not able to intiallize Servlet Config variable. I am providing my code and test also

Code is :

   public void service (HttpServletRequest request,
                        HttpServletResponse response)
throws ServletException, IOException
{


    ServletConfig config = getServletConfig();

    msSmtpServer    = getServletConfig().getInitParameter("smptserver");
    msSmtpPort      = getServletConfig().getInitParameter("smtpport");
    msUserName      = getServletConfig().getInitParameter("username");
    msPassword      = getServletConfig().getInitParameter("password");

    String buf = getRequestBuffer(request).toString();

and test is :

public class AddUserServletTest {
@Mock
HttpServletRequest request;
@Mock
HttpServletResponse response;
@Mock
HttpSession session;
@Mock
RequestDispatcher rd;
@Mock
ServletOutputStream  servletOut;
@Mock
ServletConfig sg;
public final static String Seperator = new Character((char)1).toString();
public final static String ContentDelimeter = new Character((char)2).toString();

@BeforeClass
public void setUp() throws Exception 
{
    MockitoAnnotations.initMocks(this);
}

@Test
public void test() throws Exception {
    DataSet ds=new DataSet();

    String buffer=ds.getUserId()+Seperator+ds.getTableID()+Seperator+ds.getMemberID()+Seperator+ds.getNHID();
    ByteArrayOutputStream bos = new ByteArrayOutputStream ();
    ZipOutputStream out = new ZipOutputStream(bos);
    out.putNextEntry(new ZipEntry("request.txt"));
    out.write(buffer.getBytes("UTF-8"));
    out.closeEntry();
    out.close ();
    bos.close();

    String b64String = Base64.encodeBase64String(bos.toByteArray());
    Reader inputString = new StringReader(b64String);
    BufferedReader reqbuffer = new BufferedReader(inputString);

    when(request.getReader()).thenReturn(reqbuffer);
    when(sg.getInitParameter("smptserver")).thenReturn("abc.xyz.com");
    when(sg.getInitParameter("smtpport")).thenReturn("80");
    when(sg.getInitParameter("username")).thenReturn("password@xyz.com");
    when(sg.getInitParameter("password")).thenReturn("abcd");
    when(response.getOutputStream()).thenReturn(servletOut);

    new AddUsers().service(request, response);

    ArgumentCaptor<String> bufferCaptor = ArgumentCaptor.forClass(String.class);
    verify(servletOut).print(bufferCaptor.capture());
    String responseBody = bufferCaptor.getValue();       

    ZipInputStream zipIn = new ZipInputStream(new ByteArrayInputStream(Base64.decodeBase64(responseBody.toString().getBytes())));
    zipIn.getNextEntry();
    StringBuffer sb = new StringBuffer();
    ByteArrayOutputStream out1 = new ByteArrayOutputStream();
    IOUtils.copy(zipIn, out1);
    sb = new StringBuffer();
    sb.append(out1.toString("UTF-8"));
    System.out.println("--------------- :"+sb.toString());
    String[] res=sb.toString().split(Seperator);        
    AssertJUnit.assertEquals("Success",res[0]);
}

}

i tried to intialize but in servlet that values are null not getting values. Can Anyone Suggest me how to initialize ServletConfig variable in test using mockito ?

Thanks in Advance