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Real-time data validation is one of the advantages of AJAX technology. By applying this technology, the struts validation framework will enrich the struts MVC and move the Web application closer to the desktop application.

The validation framework is used to validate fields. There are many ways to do validation on a Web application. It falls into two categories: server-side and client-side. A struts validation framework is one of the best frameworks for a Java-based Web application environment. It can configure the application using server-side validation and employ the error message that renders on the validation process invoked during the request processing time, or it can do client-side validation by using the JavaScript rendered on the requested page.

AJAX is a JavaScript technology that can asynchronously call the server and fetch the XML documents that are so popular lately. One of its uses is real-time data validation.

This article is concerned with enriching the existing struts validation framework with AJAX. A few components, such as a controller, have to be developed to select the validation framework and render the specific format message for the client side and a taglib to handle the error message rendering.

Prerequisites
You'll need a Windows system with Eclipse and the Tomcat application server. Make sure that the MSXML 3.0 ActiveX object is registered on your operating system. You'll also need the Struts library (http://struts.apache.org) and the JDOM library (www.jdom.org) for XML development (see Figure 1 and Figure 2).

Server-Side Scenario
StrustsActionServlet
We have to extend the class from org.apache.struts.action.ActionServlet to get the servletMapping variable that stores the information on how the extension will be formatted for action classes into the action path as a browser address. When the code is added, we have to configure web.xml as a Web application descriptor for the application server.

The web.xml configuration:

...
<servlet>
<servlet-name>action</servlet-name>
<servlet-class>net.sf.struts.servlet.StrutsActionServlet</servlet-class>
...
<servlet-mapping>
<servlet-name>action</servlet-name>
<url-pattern>*.do</url-pattern>
</servlet-mapping>
...

The StrutsActionServlet Java code looks like this:

public class StrutsActionServlet extends the ActionServlet.
{
public String getServletMapping() {
return this.servletMapping;
}
}

AjaxValidationRequestProcessor
To support the existing Struts framework in the first step, we have to extend the RequestProcessor from the Struts package. We have to customize the request processor because we have to distinguish how we're going to do the validation - by using the existing Struts framework or the AJAX concept - and because we'll be making a contract between the server and client on how to interpret the message. In the message rendering we'll use the XML format, which is a good media messaging format. The definition of the XML format we'll apply is:

XML Format
<?xml version="1.0" encoding="UTF-8"?>
<message>
<identity name=messageAreaId>
     <description>
   MessageValue
</description>
</identity>
</message>

Description

• Identity is the ID for the client JavaScript to acknowledge where the message will be placed.
• Description is the result after the error message rendering from the server side.

First we need to get the servlet mapping configuration from the Web descriptor before moving onto the process mapping. After invoking the process, the application will prepare the instance of the form that inherits from the AjaxForm class. The processing manages AJAX validation and should check to ensure that the request from the client isn't using the struts validation framework. Other processes to perform during the request are process populate for collecting the information sent by the client into the action form, and process validation by using the method from the existing Struts validation framework that already exists in the parent class of AjaxValidationRequestProcessor (TilesRequestProcessor).

The validation process from the TilesRequestProcessor will invoke all the validation based on the struts validation framework and store the action errors into the request. What we need is to parse the action errors into pieces and generate the XML message validation that will be sent to the client side. Since we want to change the behavior of how the validation will be backed, the validation process should check the indicator of the validation framework being used (see Figure 3).

Generate the XML messages using JDOM as the processing engine. As shown in Figure 4, when the process validation is invoked and the condition of the validation framework is equal with the AJAX validation framework, the process will continue to populate the error message and build the XML message validation.

ErrorMessageHandler
This class handles the functionality of the XML message builder. This Java class will be building the XML message based on the identity and description property. After the caller invokes the buildXMLMessage, it will prepare the document and set the root element of the XML message. This class also has an addNextXMLMessage function to add more validation messages into the XML (see Listing 1).

The process method will set the content type of the response as "text/xml" and send the XML message as a string. The function of the process on the AjaxValidationRequestProcessor code will look like Listing 2.

The processValidation method will populate the action errors and construct the message based on the format contract of the XML for the client. The function of processValidation on AjaxValidationRequestProcessor code will be look like:

...
ActionErrors errors = (ActionErrors) request.getAttribute(Globals.ERROR_KEY);
Locale locale = (Locale) request.getAttribute(Globals.LOCALE_KEY);
generateXMLMessage(errors, identity, locale, sbXMLMessage);
...

Client-Side Scenario
Build the Taglib Component

• AjaxJavaScriptLibraryTag: The taglib component to render the JavaScript function at the client side for basic XMLHTTP controller functions.
• AjaxErrorHtmlRenderTag: The taglib component to render the area of the error message at the JSP page.

Configuring the Taglib Definition
After developing the taglib component, we need to configure the taglib tld file as shown in Listing 3.

Build JSP and Struts Configuration
To simulate the result of the validation processing, we first need to build the presentation layer by incorporating the taglib that we've build. In this case I'm trying to give an example validation by using the validation rules component from Struts, and also the validation that comes from the form itself. Prepare five textboxs under the JSP page. The first to fourth textbox are using the validation rule configuration, and the fifth textbox is using the validate process from the action form. Other than that, we also need a submit button to simulate that after submitting the form, the existing struts validation are still working without AJAX. The user interface will look like Figure 5.