title: Servlet with java.lang.IllegalStateException: Cannot forward after response has been committed
body: <p>In my servlet I have this code:</p>
  
  <pre><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&lt;Giorno&gt; giorni = new ArrayList&lt;Giorno&gt;();
  
      /*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);
  
      }
  
  }
  </code></pre>
  
  <p>Maybe the problem is in the method CheckSession??</p>
  
  <pre><code>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();
          }
       }
  </code></pre>
  
  <p>but it gives me this exception: </p>
  
  <pre><code>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)
  </code></pre>
  
  <p>the servlet is called with this url</p>
  
  <pre><code>http://localhost:8080/Spinning/TakeDates?op=cal
  </code></pre>
  
  <p>Anyone can help me? thanks in advance!</p>