How can I insert and extract variables with "session" with java :?

this is the scenario:

On the one hand I have a .java where I want to put a variable in session doing this:

String [] matriz = new String[5];

matriz[0] = "Hola";

matriz[1] = "que";

matriz[2] = "tal";

matriz[3] = "estas";

request.getSession().setAttribute("Resultado", matriz);

On the other hand I have a .jsp where I want to pick it up and print it using this:

(The code is inserted correctly Code % >

String[] mensaje = (String[]) request.getSession().getAttribute("Resultado"); 

        if(mensaje == null) out.println("No se ha podido recuperar el mensaje de la sesion.");

        else{

            for(int i = 0; i < mensaje.length; i++){

                out.println(mensaje[i]); 
            }
        }

However, when I execute it, it tells me that "message" the attribute where I try to recover the array of the session is "null".

Can you tell me where I'm wrong?

Thanks in advance to all who try.