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Similar to the DirectorySearch example, the callback function first checks the request status value, which is the HTTP response code sent from the server A value of 200 here means that everything is okay, and we can read the response from the server Again, the XML message is accessed via the responseXML property of the XMLHttpRequest object The callback function also uses standard JavaScript DOM processing techniques here to extract the important content from the message Once we ve parsed the message, we modify the DOM of the page in the browser by getting access to the innerHTML property for the currentWord and suggestions fields in the table Recall from the rendered markup at the beginning of this section that these fields look like this:
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<tr> <td id="form:textarea:currentWord"> </td> <td id="form:textarea:suggestions"> </td> </tr>
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The suggestions field is populated with all the suggested words sent from the server
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The AJAX UI Component for the SpellCheckTextArea Example
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Now that you have the ability to easily serve up the AJAX script in response to a request to the FacesServlet, and you have an idea of what is in the AJAX script itself, you need to generate the actual <script> element markup and the JavaScript attributes that cause the AJAX requests to be sent based on user interaction This is no different from rendering any other HTML markup as you did earlier in the chapter In the case of SpellCheckTextArea, we add this logic to the encodeEnd( ) method:
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public void encodeEnd(FacesContext context) throws IOException { getStandardTextareaRenderer(context)encodeEnd(context, this); ExternalContext extContext = contextgetExternalContext(); ResponseWriter writer = contextgetResponseWriter(); // Render the main script, only once per page if (!extContextgetRequestMap()containsKey("ajaxflagajaxScript")) { extContextgetRequestMap()put("ajaxflagajaxScript", BooleanTRUE); writerstartElement("script", this);
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Part II:
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Extending JavaServer Faces
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writerwriteAttribute("type", "text/JavaScript", null); writerwriteAttribute("src", SpellcheckPhaseListenerRENDER_SCRIPT_VIEW_ID, null); writerendElement("script"); } // Table rendering code omitted writerstartElement("input", this); writerwriteAttribute("type", "button", "type"); writerwriteAttribute("value", "previous", "type"); writerwriteAttribute("onclick", "getSuggestionsForPreviousWord('" + clientId + "');", "onclick"); writerendElement("input"); // More table rendering code omitted writerstartElement("input", this); writerwriteAttribute("type", "button", "type"); writerwriteAttribute("value", "next", "type"); writerwriteAttribute("onclick", "getSuggestionsForNextWord('" + clientId + "');", "onclick"); writerendElement("input"); // More table rendering code omitted writerstartElement("input", this); writerwriteAttribute("type", "button", "type"); writerwriteAttribute("value", "reset", "type"); writerwriteAttribute("onclick", "resetSpellCheck('" + clientId + "');", "onclick"); writerendElement("input"); // More table rendering code omitted writerstartElement("tr", this); writerstartElement("td", this); writerwriteAttribute("id", clientId + ":currentWord", "id"); writerwrite(" "); writerendElement("td"); writerstartElement("td", this); writerwriteAttribute("id", clientId + ":suggestions", "id"); writerwrite(" "); writerendElement("td"); writerendElement("tr");
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There are four points of interest in the above code The first is the usage of a request scoped attribute to prevent the <script> element from being rendered more than once per page This is desirable because once the script has been loaded into the browser, there is no need to re-load it while the browser is on the same page The second interesting point is the usage of the RENDER_SCRIPT_VIEW_ID constant from the SpellcheckPhaseListener class This is a tight coupling between SpellCheckTextarea and SpellcheckPhaseListener The rendered script element looks like this:
<script type="text/JavaScript" src="SpellCheckTextAreaajaxScript"> </script>
Because the URL from which the JSP page was loaded looks something like
http://localhost:8080/spellcheck/faces/spellCheckjsp
11:
Building AJAX JSF Components
the relative URL in the <script> element causes the browser to fetch to the URL:
http://localhost:8080/spellcheck/faces/SpellCheckTextAreaajaxScript
This runs through the Faces lifecycle, and thus causes our SpellcheckPhaseListener to be invoked, which in turn returns the AJAX script The third interesting point is the liberal sprinkling of clientId as a parameter to the JavaScript function getSuggestionsForPreviousWord( ) This is important because it enables multiple instances of the component to reside in a single page, thanks to the ID uniqueness guarantee provided by the JSF runtime Finally, note that we render empty <td> elements to hold the currentWord and suggestions values from the server, again making sure to use the clientId to distinguish between multiple instances of the spell-check component
PART II
The AJAX Server Component for the SpellCheckTextArea Example
While the SpellCheckTextArea component generates the <script> tag that loads the AJAX browser script, and the standard FacesServlet serves up the script to the browser using the PhaseListener, the remaining portion of this example is the AJAX server component, which responds to the AJAX request issued from the script in the browser Let s now review this in more detail Similar to the DirectorySearch component where the AJAX server code is integrated into the JSF application, the SpellCheckTextArea component example leverages the FacesServlet to allow the JSF lifecycle to guide the processing of AJAX requests As you ve seen in the DirectorySearch example, this is slightly more complex to develop than an independent generic servlet solution, and there may be higher performance costs if the number of AJAX requests is large However, the entire AJAX technology is packaged into a single JSF UI Component, thus ensuring easier access to application data (via EL, ManagedBeans, messages) as well as providing a simpler deployment process The performance cost can also be mitigated by carefully designing your AJAX interactions, and by using a servlet container that can leverage the high-performance javanio package for its network layer, such as the Sun Java System Application Server
Using the FacesServlet and JSF Lifecycle to Host the High-Level AJAX Processing Server Element In order for a request from the browser to get into the JSF component lifecycle, the second URL argument of the open( ) call on the XMLHttpRequest instance must match the servlet-mapping url-pattern defined in the webxml (for example, /faces) There are two ways the JSF request processing lifecycle can be used to orchestrate the AJAX request processing server component: executing within the scope of a view (page) or executing without reference to a specific view The former involves having the AJAX browser script simulate a form submit with the entire view state, which includes adding the necessary request parameters to allow the JSF lifecycle to restore the entire view and perform processing on it This approach has the benefit of allowing your components to have access to the entire view while performing the AJAX processing The latter doesn t ship the component state along with the AJAX request, and therefore the JSF lifecycle cannot access the component state However, it still can access managed beans and other JSF resources Let s examine submitting the AJAX request with the component state first, because omitting the component state is simple once you understand how to include it in the first place Including the JSF Component View State in the AJAX Request In the ajaxjs script, the prepareRequest( ) function is responsible for marshaling the parameters to be sent in
Part II:
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