Diese Frage hat hier bereits eine Antwort:

In meinem Servlet habe ich diesen Code:

protected void doGet(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub

    Utils.CheckSession(request,response);

    String op = request.getParameter("op");
    int DaysToAdd = request.getParameter("daysToAdd") != null ? Integer.valueOf(request.getParameter("daysToAdd")) :  0;

    ArrayList<Giorno> giorni = new ArrayList<Giorno>();

    /*HERE I FILL ArrayList giorni and calculate other variables*/


    if ("cal".equals(op))
    {
        request.setAttribute("giorni", giorni);
        request.setAttribute("daysToAdd", DaysToAdd);
        request.getRequestDispatcher("GestioneCalendario.jsp").forward(request, response);

    }
    else if("utente".equals(op))
    {
        // ricavare abbonamento dell'utente
        String idu = (String) request.getAttribute("idu");
        Abbonamento a = null;
        int iDa = Utils.getIdAbbonamentoAttivoUtente(idu);
        a = Utils.getAbbonamentoFromId(iDa);
        request.setAttribute("abbonamento", a);
        request.getRequestDispatcher("Lezioni.jsp").forward(request, response);

    }
    else
    {
        request.setAttribute("giorni", giorni);
        request.setAttribute("daysToAdd", DaysToAdd);       
        request.getRequestDispatcher("GestioneLezioniNuovoLayout.jsp").forward(request, response);

    }

}

Vielleicht liegt das Problem in der Methode CheckSession ??

public static void CheckSession(HttpServletRequest request,
        HttpServletResponse response) {
    // TODO Auto-generated method stub
     HttpSession session = request.getSession(true);
     String logged = null;
     if (session!=null)
          logged = (String) session.getAttribute("logined");
     if(logged == null)
     {
         request.setAttribute("errore", "Devi loggarti!");
        try {
            request.getRequestDispatcher("Admin.jsp")
            .forward(request, response);
            return;
        } catch (ServletException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
     }

aber es gibt mir diese Ausnahme:

GRAVE: Servlet.service() for servlet [TakeDates] in context with path [/Spinning] threw     exception
java.lang.IllegalStateException: Cannot forward after response has been committed
at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:349)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:339)
at servlet.TakeDates.doGet(TakeDates.java:368)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:936)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1004)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
at java.lang.Thread.run(Thread.java:695)

Das Servlet wird mit dieser URL aufgerufen

http://localhost:8080/Spinning/TakeDates?op=cal

Kann mir jemand helfen? Danke im Voraus!