<jsp:useBean id="user" class="pack.User" scope="session" />
<jsp:setProperty name="user" property="*"/>

<%
  java.util.Map errors = new java.util.HashMap();
  if (request.getParameter("submitted") != null){
    errors = user.validate();
    if (errors.size() == 0){
        // Если все ОК – переходим на следующую страницу
        response.sendRedirect("users.jsp");
        return;
    }
  }%>

<h2>User form</h2>
<form name="myform" method="POST" action="index.jsp">
  <input type="hidden" name="submitted" value="true">
  <%if (errors != null && errors.size() > 0){%>
    <font color="red">
    <%for (java.util.Iterator iter = errors.entrySet().iterator();iter.hasNext();){%>
      <%=((java.util.Map.Entry)iter.next()).getValue()%><br>
    <%}%>
    </font>
  <%}%>

<tr>
      <td>
        <%if (errors.containsKey("name")){%><font color="red"><%}%>
                Name *
        <%if (errors.containsKey("name")){%></font><%}%>
        </td>
      <td>
        <input type="text" name="name" value="<%=user.getName()%>">
      </td>
    </tr>

public Map validate()

    Map errors = new LinkedHashMap();
    if ("".equals(name)){
        errors.put("name", "Input the name, please");
    }
    if ("".equals(pswd)){
        errors.put("pswd", "Input the password, please");
    }
    if ("".equals(mail)){
        errors.put("mail", "Input the mail, please");
    }
    return errors;
}

if ("".equals(name))