Abaixo segue um exemplo de código utilizando a classe GenericServlet:

package br.aiec;

import java.io.IOException;

import javax.servlet.GenericServlet;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class GenericoDemoServlet extends GenericServlet {

    @Override
    public void service(ServletRequest request, ServletResponse response)
            throws ServletException, IOException {

        response.getWriter().println(
                "Criando um Servlet a partir da classe GenericServlet");
        response.getWriter().println("IP: " + request.getRemoteAddr());
        response.getWriter().println("PORTA: " + request.getRemotePort());

    }
}
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