请教关于session的问题.

现有一个过滤器和两个webservice, 一个webservice用于登录，另一个用于取得图书信息。当登录成功后，往session中写入用户名，
这样当访问图书信息的webservice时，如果在过滤器中发现用户名为空时,则提示还没登录.

现在的问题是, 登录时往session中写了用户名, 但当访问图书信息webserice时,在过滤器中发现用户名为空,而且sessionID也是新的.

具体如下:
过滤器:
public class bookFilter implements Filter 
{
public void init(FilterConfig filterConfig) throws ServletException 
{
}

public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException 
{
HttpServletRequest req = (HttpServletRequest) request;
HttpServletResponse res = (HttpServletResponse) response;
HttpSession session = req.getSession();

String userName = (String)session.getServletContext().getAttribute("userName");

if(req.getRequestURI().contains("booksdir/books") && userName == null)
{
System.out.println("请先登录");
return;
}

//...
chain.doFilter(request, response);
}

public void destroy() 
{
}
}

有下面两个webservice:
1.登录的webservice
@Path("login")
public class LoginService {

@GET
@Path("login")
public boolean login(@Context HttpServletResponse response, @Context HttpServletRequest request) throws Exception 
{
String username = request.getParameter("username");
String password = request.getParameter("password");
//...

HttpSession session = request.getSession();

session.setAttribute("userName", username);

//...

return true;
}
}

2. 获取图书信息的webservice:
@Produces("application/xml")
@Path("books")
public class Books 
{
@GET
public List<book> getbooks(@Context HttpServletRequest request) {
List<book> books = new ArrayList<book>();

//...

return books;
}
}


现在用如下的方式调用LoginService，即登录 (LoginService会把参数username存到session中，见如上代码):
http://localhost:8080/booksdir/login/login?username=liming&password=88888

接着访问如下的URL：
http://localhost:8080/booksdir/books

但发现过滤器中的userName为空:
String userName = (String)session.getServletContext().getAttribute("userName");

请问是怎么回事?

String userName = (String)session.getServletContext().getAttribute("userName");
 你这个的session 是HttpSession么？ 信息放在session里了为什么还要session.getServletContext() 直接session.getAttribute("userName");