Mocking httpsession for servlet in junit

I have servlet which is written below. i need to test the servlet in junit. inside the servlet i use http session but when i use my test class , give an error how can i mock httpsession ? HttpSession session = request.getSession(true);

        String Product_id = request.getParameter("productID") ;

        DB_Con DB = new DB_Con();
        Statement smt = DB.getStatement();

        if ( session.getAttribute("Username") != null && session.getAttribute("BasketID") != null )
        {
             ....
        }

and my unit test is

@Test
public void testStart()  throws IOException, ServletException
{
    AddtoBasket AP = new AddtoBasket();

    HttpServletRequest request = mock(HttpServletRequest.class);       
    HttpServletResponse response = mock(HttpServletResponse.class);    
    //HttpSession session = mock(HttpSession.class);
    session.setAttribute("Username", "payam");
    session.setAttribute("BasketID", null);

    when( request.getParameter("productID") ).thenReturn("300");
    //when( request.getSession(true) ).thenReturn(session);


    when( session.getAttribute("Username") ).thenReturn("payam");
    when( session.getAttribute("BasketID") ).thenReturn(null);




    PrintWriter writer = new PrintWriter("somefile2.txt");
    when(response.getWriter()).thenReturn(writer);

    AP.doPost(request,response);
    verify(request, atLeast(1)).getParameter("P_code");

    writer.flush(); // it may not have been flushed yet...
    assertTrue(FileUtils.readFileToString(new File("somefile2.txt"), "UTF-8")
               .contains("Product Added to basket!"));



}