将请求和响应从jsp发送到servlet

我正在为注册表单编写验证servlet.表单位于.jsp文件中,并具有以下布局:

        

            
Register
            

                

                    User name: 

                    
 First Name: 

                    
 Last Name: 

                    
 Email Address: 

                    
 Password: 

                    
 Retype password:
                    

                    
 
                
            

            
        

验证代码应检查每个字段是否满足一组约束,如果不满足,则写回jsp错误消息.

ValidationServlet.java

protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        // get request parameters for username and password
        String user = request.getParameter("username");
        String firstName = request.getParameter("first_name");
        String lastName = request.getParameter("last_name");
        String email = request.getParameter("email");   
        String pwd = request.getParameter("password");
        String pwd2 = request.getParameter("password2");
        // Validate parameters and forward them to the ejb
        Map messages = new HashMap();

        if(user != null) {
            // Validate user name
            if(user.trim().isEmpty()) {
                messages.put("error", "Please enter username");
            } else if(!user.matches("^[a-zA-Z0-9]$")) {
                messages.put("error", "Invalid username.");
            } else if(user.length() > MAX_LEN){
                messages.put("error", "Invalid username");
            } else {
                // Username is valid
                messages.put("success", "good");
            }
        }
        if(firstName != null) {
            // Validate first name
            if(firstName.trim().isEmpty()) {
                messages.put("error", "Please enter your first name");
            } else if(!firstName.matches("^[a-zA-Z]$")) {
                messages.put("error", "Invalid name. Use alpha values only.");
            } else if(firstName.length() > MAX_LEN){
                messages.put("error", "Invalid name. Name should be at most " + MAX_LEN + " characters long");
            } else {
                // First name is valid
                messages.put("success", "good");
            }
        }
        if(lastName != null) {
            // Validate last name
            if(lastName.trim().isEmpty()) {
                messages.put("error", "Please enter your last name");
            } else if(!lastName.matches("^[a-zA-Z]$")) {
                messages.put("error", "Invalid name. Use alpha values only.");
            } else if(lastName.length() > MAX_LEN){
                messages.put("error", "Invalid name. Name should be at most " + MAX_LEN + " characters long");
            } else {
                // Last name is valid
                messages.put("success", "good");
            }
        }
        if (email != null) {
            // Email is specified as request parameter, do the business logic here.
            if (email.trim().isEmpty()) {
                messages.put("error", "Please enter email");
            } else if (!email.matches("([^.@]+)(\\.[^.@]+)*@([^.@]+\\.)+([^.@]+)")) {
                messages.put("error", "Invalid email, please try again.");
            } else {
                // Email is valid
                messages.put("success", "good");
            }
        }
        if(pwd != null) {
            // Validate password
            if(pwd.trim().isEmpty()) {
                messages.put("error", "Please enter password");
            } else if(pwd.length() > MAX_LEN){
                messages.put("error", "Invalid password");
            } else {
                // Password is valid
                messages.put("success", "good");
            }
        }
        if(pwd2 != null) {
            // Validate password
            if(pwd2.trim().isEmpty()) {
                messages.put("error", "Please enter password");
            } else if(pwd2.length() > MAX_LEN){
                messages.put("error", "Invalid password");
            } else if(!pwd2.equals(pwd)) {
                messages.put("error", "Passwords do not match");
            } else {
                // Password is valid
                messages.put("success", "good");
            }
        }
        // Put messages in request scope so that it's accessible in EL by
        // ${messages}.
        request.setAttribute("messages", messages);

        // Forward request to JSP for display.
        request.getRequestDispatcher("login.jsp").forward(request,
                response);
    }

我的问题是如何从哈希映射中访问每个消息并在jsp中的相应输入字段之前相应地显示它们？