将ArrayList从Servlet传递给JSP时出错

Question

I have an important question but firs sorry for my english, I only know the basic. Well my problem is that I have an error passing an ArrayList from a servlet to jsp page:

<% ArrayList<Usuario> u= (ArrayList<Usuario>)session.getAttribute("listado");%>
<table align="left" cellpadding="0" cellspacing="1">
    <tr bgcolor="blue">
        <td>Usuario</td><td>Nombre</td>
        <td>Apellido</td><td>Clave</td> 
    </tr>
<% for(int i=0;i<u.size();i++){ %>
<% Usuario usuario = u.get(i); %>
<tr>
<td> <%= usuario.getUsuario() %></td>
<td> <%= usuario.getNombre() %></td>
<td> <%= usuario.getApellido() %></td>
<td> <%= usuario.getClave() %></td>
</tr>
<%} %>


</table>

That's how I'm doing this but I receive an error in:

<% for(int i=0;i<u.size();i++){ %>

What I'm doing wrong? also my servlet Method is like this:

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    RequestDispatcher rd;

    try {
        Connection cn = MySQLConnection.obtenerConexion();
        String sql = "select * from tb_usuario";
        PreparedStatement ps = cn.prepareStatement(sql);
        ResultSet rs = ps.executeQuery();
        ArrayList<Usuario> listado = new ArrayList<Usuario>();
        while (rs.next()){
            Usuario usu = new Usuario(rs.getString(1),rs.getString(2),rs.getString(3),rs.getString(4));
            listado.add(usu);
        }
        request.setAttribute("listado", listado);
        request.getRequestDispatcher("/listado.jsp");
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

I hope you could help me!

Answer 1

You should not use scriptlets in your JSP. You should use EL and tags in your JSP.

eg

${listado}

Answer 2

You are setting the variable in request object while retrieving from session , which is not present hence the issue.

You are setting the attribute in doPost as below":

 request.setAttribute("listado", listado);

You are retrieving the attribute in your JSP as below":

 <% ArrayList<Usuario> u= (ArrayList<Usuario>)session.getAttribute("listado");%>

Please use the same scope session or request in both places.

Answer 3

scriptlets are discouraged to use in ajsp page, use JSTL tags instead. use c-foreach tag to iterate over your arrayList in your jsp page. and you are setting an attribute in a request scope and trying to get it in session scope in your jsp.

heres the link which explains c-foreach tag

Answer 4

you are setting the value in to request scope

  request.setAttribute("listado", listado);

but then trying to access it in session scope.

 session.getAttribute("listado");

due to this u might get a null pointer exception in

 u.size()...

try to access it in request scope

 request.getAttribute("xxxxxx") 

try to avoid adding java code inside JSP whihc is a bad practice. use EL and JSTL instead. you can to the casting part inside the code too..