老师，我这个没映射，原来我没Servlet的，是新创建的Servlet

public class HelloServlet extends javax.servlet.http.HttpServlet {
   protected void doPost(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, java.io.IOException {

   }

   protected void doGet(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, java.io.IOException {
    response.getWriter().println("Hello World");
   }
}