some problem about GWT

I code a servlet in GWT:
Line 8: public class AddServvlet extends HttpServlet{
private static final long serialV = 1L;
protected void doPost(HttpServletRequest req,HttpServletResponse res)
{
int a = Integer.parseInt(req.getParameter("a"));
int b = Integer.parseInt(req.getParameter("b"));
try {
res.getWriter().print(a+b);
} catch (IOException e) {
// TODO: handle exception
}
}
}
when I run the project in IE,it say: [ERROR] [hellogwt] - Line 8: No
source code is available for type javax.servlet.http.HttpServlet; did
you forget to inherit a required module?
[ERROR] [hellogwt] - Line 10: No source code is available for type
javax.servlet.http.HttpServletRequest; did you forget to inherit a
required module?
[ERROR] [hellogwt] - Line 10: No source code is available for type
javax.servlet.http.HttpServletResponse; did you forget to inherit a
required module?
thank you!

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