class double in directories ?

Hi,

I have the following code on my apache-tomcat server (see below).
(debian, java 1.5)

it should call HelloWorldServlet, which is in the same directory.

but I get again and again the following error-message (see below).

I tried to find an answer in the web, but only could find that may be there
is a class double in different directories with different versions.
for compilation and for running there would be used different
java-classpathes.

I do compilation on that server of that HelloWorldServlet2-java-class
without problems.

could there be another problem ?
how could I find out, which class I have double on my server ?

thanks a lot for help in advance.

regards
Alex

--- code ---------------

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class HelloWorldServlet2 extends HttpServlet {

                public void doGet(HttpServletRequest request,
                        HttpServletResponse response)
                        throws ServletException, IOException {
                PrintWriter out = response.getWriter();
                String url="./servlet/HelloWorldServlet";
                request.getRequestDispatcher(url).forward(request,response);
                }
        }

--- error-message------------

java.lang.LinkageError: Class javax/servlet/RequestDispatcher violates
loader constraints
        java.lang.ClassLoader.defineClass1(Native Method)
        java.lang.ClassLoader.defineClass(ClassLoader.java:620)
        java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)

org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLoader.java:1815)

org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.java:869)

org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1322)

org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1201)
        java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
        HelloWorldServlet2.doGet(HelloWorldServlet2.java:16)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:802)