AJAX is the latest revolution in web development circles, allowing rich dynamic interfaces deployed within a normal web browser. Struts has been one of the de facto standards for Java-Web development for a number of years, with a large number of applications already deployed. This article will show you how to combine the richness of an AJAX user interface with your existing Struts applications.

This article shows a simple and elegant way to do this by including a couple of lines of JavaScript on your JavaServer Pages (JSPs). While we show how to reuse existing Struts actions, the techniques are equally applicable to the Java-Web framework of your choice. The method proposed will also allow a move to the next version of Struts (Shale) or JavaServer Faces (JSF) in the future.

What is AJAX?

AJAX stands for "Asynchronous JavaScript and XML." It is a technique, rather than a framework (such as Struts). The reason for the buzz around it is that it allows web pages to behave less like flat documents and more like dynamic GUI apps that users might expect from their desktop environments. AJAX techniques can be used for all recent browsers (including Internet Explorer and Netscape/Mozilla). It is already used by (among others) Microsoft (for its Outlook web client) and Google (for Google Maps and Gmail).

Life Before AJAX

Most current Struts applications follow the standard "web page as a flat document" structure. If you wanted to mimic the behavior of GUI desktop apps (such as those built using Java Swing, Visual Basic, or Delphi) you had two choices: you could either send all of the information that might possibly required as part the web page with (a lot of) JavaScript to handle the dynamic behavior (a slow and not very enterprise-Java way to do things), or you could do constant form submits back to the server (an effective, if somewhat clunky, method). AJAX gives you the best of both worlds: dynamic web pages, but with most of the application running in Java on your web server.

AJAX 101

AJAX is similar to existing Dynamic HTML techniques, with the addition of a "background" call to the server to get new/updated information as required. The mechanics of AJAX have already been covered in detail elsewhere--take a look at the Resources section at the end of this article for some good examples. The minimum you need to know is:

1. The XMLHttpRequest (or Microsoft.XMLHTTP ActiveX object if you are using Internet Explorer). These objects can be called from the JavaScript on your web page. They allow you to request content from your web server as a background call (i.e., the screen does not "go blank" as usually happens during a form submit).
2. The content that the XMLHttpRequest and Microsoft.XMLHTTP objects return can be treated as either XML or plain text. JavaScript (on your web page) can then update the page with this new content as required.
3. The whole process can be triggered by the usual JavaScript events: onclick, onchange, onblur, etc.

Using AJAX in Your Struts Application

The chances are that if you are reading this article, then you are interested in AJAX's ability to create dynamic web interfaces and would like to know how to add it to a Struts application. What are your options if you want to do this?

• Wait until the next version of Struts (Shale) incorporates AJAX techniques. For Struts developers starting a new application this is probably the best option. The downside is that this will probably require moving to JavaServer Faces--not a bad thing in itself, but this may entail fundamental changes if you have an existing application.
• You could move to a new approach, like Direct Web Remoting (DWR) or Ruby on Rails, which are specifically built for AJAX applications. While these are both very impressive frameworks, and are worth taking a look at if you wish to consider web development without Struts, this option would mean rewriting your entire application.
• Add AJAX to your existing Struts application. Since AJAX is a technique, not a framework, it is straightforward to add it to Struts. For existing applications where stability (e.g., keeping existing libraries) is important, this option is recommended and is the one we explore in more detail.

Some other advantages of our preferred option are:

1. It should not require any new libraries or server side code; only the Struts libraries and actions already in the application need be used.
2. All of the parts of the solution--JavaScript, XML, Java and Struts--are already widely understood.
3. The application can be migrated to AJAX piece by piece; we can identify those parts which will most benefit users, and then choose to upgrade them to dynamic AJAX behavior first.

Implementing the Solution

How do we actually implement our chosen solution? We start by reminding ourselves how a "standard" (non-AJAX) Struts application works. In this application, the normal flow of events is as follows:

1. The user requests a screen by clicking a hyperlink or form submit button.
2. The web server runs the requested Struts Action, generating the web page.
3. The browser displays the page.
4. When the user presses Save, the information is posted to the server, where it's converted by the Struts framework to an ActionForm class containing the posted data.
5. The Struts framework then calls the Struts Action that then processes the request (e.g., saves the data to a database).
6. The page is rendered as per item 2, and the process starts again.

Existing Struts Application

A simple Struts application demonstrating this flow of events can be downloaded here: struts-non-ajax.zip. This application, based on the sample applications provided with Struts, either hides or displays blue and green tables depending on the values entered by the user. Figure 1 shows the screen on initial page load. Figure 2 shows the screen after the user has entered values and pressed Submit. Although simple, it is enough to demonstrate a Struts application at work.

Figure 1. Non-AJAX sample: Initial screen

Figure 2. Non-AJAX sample: Values entered, Submit pressed

The server-side code is as you would expect: a Struts Action that forwards to the (same) JSP using the values defined in struts-config.xml. Some other points to note in this code sample are:

• The struts-config.xml file is set up to redirect all requests to http://localhost:8080/struts-non-ajax/ (or the equivalent in your web server) to index.jsp.
• index.jsp contains a Struts form with two text boxes (showBlue and showGreen). The page also contains <logic:equal> tags, but as the values for these text boxes are initially blank, the content within them is not displayed.
• The user enters values (true or false) and presses the Submit button, passing control (via the Struts Framework, reading struts-config.xml) to the SampleAction class.
• SampleAction logs the values, and then forwards back to index.jsp. A more sophisticated Struts application would do more, such as saving to or retrieving from a database.
• index.jsp now evaluates the request; if showBlue or showGreen are true, the tables will be displayed.

There is nothing "wrong" with this application. After all, similar Struts projects have been deployed for years. But how do we to add dynamic behavior to this application, without adding complex JavaScript or continual form submits?

Our First Struts AJAX Application

Take a look at the Figures 3 and 4 below. At first glance, they seem similar to our previous ones. The difference is that after then screen loads (Figure 3) and the values in the textboxes are changed, the screen automatically updates without the screen going blank, giving the result as per Figure 4. The normal Submit button is also still there, should you choose to use it.

Figure 3. AJAX sample after page load

Figure 4. AJAX sample after AJAX call

Adding this AJAX behavior is surprisingly easy. The server-side code is the same as usual: a Struts ActionForm to hold the data, and a Struts Action that performs the tasks required (e.g., database access) and then forwards to the appropriate JSP to display the result.

Don't Just Sit There

If you wish to stop reading here (and skip the explanation of how this works) then here is all you need to do to convert your Struts application to a Struts-AJAX application in a similar manner:

1. Include the Ajax.js JavaScript file on your web page (this file is part of the struts-ajax.zip sample file). Ajax.js contains all of the JavaScript functions necessary to send and receive AJAX calls.
2. Ensure the parts of the web page that you wish to update during AJAX calls are surrounded by <span> tags, giving each an id.
3. When something happens that should update the page (e.g., the onchange() method of a textbox), call the retrieveURL() function, passing in the URL to the Struts Action that will do the necessary server-side processing.
4. For page display/update, the easiest option is for the Struts Action to forward back to the same JSP. In our sample, we trigger the AJAX call in the onchange() method of the showGreen/showBlue textboxes.

The JavaScript function retrieveURL() calls Struts on the server (via the URL provided), takes the JSP response, and updates the web page being displayed, where the <span> tags on the existing web page match those on the newly returned JSP. Simple!

The AJAX Solution in More Detail

When we converted the previous sample into an AJAX-Struts application we made three changes:

1. Added a JavaScript function to do the "behind the scenes" AJAX call to the server.
2. Added JavaScript code to receive the server response and update the web page.
3. Added <span> tags to the JSP page, which mark sections that will be updated during AJAX calls.

We will look at each of these in more detail.

Making the AJAX Call to the Server

There are two functions (listed below) that are used to call the server.

• The retrieveURL() function takes a parameter of the URL of the server and the name of the Struts form. The URL will be called using AJAX and the values of the form passed to the server.
• getFormAsString() is a function that converts the values on the form named in retrieveURL() into a query string suitable for posting to Struts on the server.

To use, simply add the retrieveURL() function to the onclick()/onChange() method of the event you wish to trigger the screen update.

There are some interesting items going on in both methods.

Within the retrieveURL() method, the line req.onreadystatechange = processStateChange (note: no brackets) tells the browser to call the processStateChange() method (which we talk through later in this article) once the server sends back its response. This method (now standard in AJAX) also determines whether it should use the Internet Explorer (ActiveX) or Netscape/Mozilla (XmlHttpRequest) object to ensure cross-browser compatibility.

The getFormAsString() method converts the HTML form into a string to be appended to the URL (which allows us to do a HTTP GET request). This string is escaped (spaces are converted to %20, etc.) and is in a format that Struts can use to populate an ActionForm (without Struts being aware of the special AJAX nature of the request). Note that while we use a HTTP GET in this sample, it would be equally possible to use a HTTP POST by looping in a similar manner and adding the form fields to the request.

function retrieveURL(url,nameOfFormToPost) {
    
  //convert the url to a string
  url=url+getFormAsString(nameOfFormToPost);
    
  //Do the AJAX call
  if (window.XMLHttpRequest) { 
     
    // Non-IE browsers
    req = new XMLHttpRequest();
    req.onreadystatechange = processStateChange;
    try {
          req.open("GET", url, true); 
    } catch (e) {
      alert("Server Communication Problem\n"+e);
    }
    req.send(null);
  } else if (window.ActiveXObject) {
    // IE
   
    req = new ActiveXObject("Microsoft.XMLHTTP");
    if (req) {
      req.onreadystatechange=processStateChange;
      req.open("GET", url, true);
      req.send();
    }
  }
}

getFormAsString() is a "private" method used by the retrieveURL() method.

function getFormAsString(formName){
        
  //Setup the return String
  returnString ="";
        
  //Get the form values
  formElements=document.forms[formName].elements;
        
  //loop through the array, building up the url
  //in the format '/strutsaction.do&name=value'
 
  for(var i=formElements.length-1;i>=0; --i ){
        //we escape (encode) each value
        returnString+="&" 
        +escape(formElements[i].name)+"=" 
        +escape(formElements[i].value);
 }
        
 //return the values
 return returnString; 
}
  

Updating the Web Page with the AJAX Response

So far, we have looked at the JavaScript to do the AJAX call (listed above) and the Struts Action, ActionForm, and JSP (mostly the same, with the addition of <span> tags). To complete our understanding of the Struts-AJAX project, we need to look at the three JavaScript functions that are responsible for updating the current web page when the results from the server are received.

• processStateChange(): The method name that we set before making the AJAX call. This method is called by the XMLHttpRequest/Microsoft.XMLHTTP object once the server has completed sending back its response.
• splitTextIntoSpan(): Loops through the response, picking out an array of <span id="someName">NewContent</span> elements.
• replaceExistingWithNewHtml(): Loops through this array of span elements, searching for <span> elements in the existing page with 'someName' and replacing them with the new content from the server. Note that we get the returned content via req.responseText for robustness (since it allows us to manipulate any text response), rather than req.responseXml (which is more powerful, but requires that you return valid XHTML or XML).

function processStateChange() {
  
  if (req.readyState == 4) { // Complete
    if (req.status == 200) { // OK response
        
    //Split the text response into Span elements
    spanElements = 
        splitTextIntoSpan(req.responseText);
    
    //Use these span elements to update the page
    replaceExistingWithNewHtml(spanElements);
    
    } else {
      alert("Problem with server response:\n " 
        + req.statusText);
    }
  }
}

replaceExistingWithNewHtml() is a "private" method used by the processStateChange() method.

  
function replaceExistingWithNewHtml 
        (newTextElements){
 
  //loop through newTextElements
  for(var i=newTextElements.length-1;i>=0;--i){
  
    //check that this begins with <span
    if(newTextElements[i]. 
        indexOf("<span")>-1){
                        
          //get the span name - sits
      // between the 1st and 2nd quote mark
      //Make sure your spans are in the format
      //<span id="someName">NewContent</span>
          startNamePos=newTextElements[i]. 
              indexOf('"')+1;
      endNamePos=newTextElements[i]. 
              indexOf('"',startNamePos);
      name=newTextElements[i]. 
              substring(startNamePos,endNamePos);
                        
      //get the content - everything 
      // after the first > mark
      startContentPos=newTextElements[i]. 
               indexOf('>')+1; 
      content=newTextElements[i].
               substring(startContentPos);
                        
     //Now update the existing Document 
     // with this element, checking that 
     // this element exists in the document
     if(document.getElementById(name)){
                document.getElementById(name). 
                innerHTML = content;
     }
  }
}

splitTextIntoSpan() is a "private" method used by the processStateChange() method.

function splitTextIntoSpan(textToSplit){
 
  //Split the document
  returnElements=textToSplit. 
            split("</span>")
        
  //Process each of the elements        
  for(var i=returnElements.length-1;i>=0;--i){
                
    //Remove everything before the 1st span
    spanPos = returnElements[i]. 
             indexOf("<span");               
                
    //if we find a match, take out 
    //everything before the span
    if(spanPos>0){
          subString=returnElements[i].
              substring(spanPos);
          returnElements[i]=subString;
    } 
  }
  return returnElements;
}

New Flow of Control

By adding the above JavaScript code to our application, the following steps now happen on the server and on the browser.

1. The page loads as per a normal Struts application.
2. The user changes a textbox value, triggering an onChange() event, which calls the retrieveURL() JavaScript function.
3. This JavaScript function makes a (background) call to the Struts Action on the server, passing in all of the form variables in a way that Struts will understand.
4. This JavaScript function also sets the name of a second JavaScript function, which will be called when the server response is finished. In this case, it is set to the processStateChange() method.
5. As expected, when the server response is finished, the processStateChange() method is called.
6. The JavaScript loops through all of the <span>elements in the (new) server response. Where it finds a <span> in the existing page with the same name, it updates it with the new content.

Designing AJAX into Your Application

The JavaScript outlined above can cope with the way Struts is used in most applications, including those that are much more complex than our simple example. However, you may find that following the points below makes it easier to write and use your code:

• To avoid duplicated code, it can often be better to use the same Struts Action and JSP for the initial request (i.e., show full page) and the AJAX (update part of page) requests.
• Within the common Action (controller) class, decide which sections of the JSP page (all of the JSP or only part of it) need to be sent to the browser. By setting flags in either the web server session or ActionForm, the JSP page knows which sections need to be rendered.
• Within the JSP, use Struts <logic:equal> or JSTL tags to decide if we need to render a section of HTML or not.

An updated version of this project, with AJAX enabled, can be downloaded here: struts-Ajax.zip

Conclusion

AJAX techniques promise to completely revolutionize how we build and use web applications. This article showed a simple technique to add AJAX behavior to existing Struts applications. It allows us to reuse our existing investment, not only in code but also in developer skills. As a nice by-product, it also allows us to write cleaner, more reusable, Java Struts applications.

Resources

• Sample code for this article
• Definition of AJAX
• "Using AJAX to Catch JavaScript Errors" (XML.com article)
• "Developing AJAX Applications the Easy Way" (java.net article)
• "An Introduction to AJAX" (dev2dev article)
IBM article on using AJAX with web services -->
• "AJAX on Rails" (ONLamp.com article)
• DWR: Direct Web Remoting Project
• Struts Framework Project
• JSF: Java Server Faces Project

Paul Browne has been consulting in enterprise Java with FirstPartners.net for almost seven years.