Web component development and JSP technology session 18

Ver. 1.0 Slide 1 of 21

Web Component Development With Servlet and JSP™ TechnologiesObjectives

In this session, you will learn to:Use the asynchronous servlet mechanismUse JavaScript to send an HTTP request from a clientProcess an HTTP response entirely in JavaScriptCombine these techniques to create the effect of server-push


Web Component Development With Servlet and JSP™ Technologies

Java has the option to perform request processing asynchronously.This enables the servlet execution thread to service requests from other clients, and the generation of the response may be performed in another thread.The thread that creates the trigger condition that allows the response to be prepared is an example of asynchronous servlets.

Asynchronous Servlets



The servlet API provides a class called AsyncContext to allow for separation of request from response generation.The AsyncContext object provides access to the original HttpServletRequest and HttpServletResponse objects. This allows processing of the request and generation of the response.

Separating Request Receipt from Response Generation



The following code shows an asynchronous servlet that passes all work immediately to another thread via the handler.addJob(ac) call:protected void processRequest(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException {AsyncContext ac = request.startAsync();handler.addJob(ac);}

Asynchronous Servlet Example



The following code shows the Handler class that processes asynchronous servlet requests:package service;import java.util.ArrayList;import java.util.Iterator;import java.util.List;import javax.servlet.AsyncContext;import javax.servlet.ServletRequest;public class Handler implements Runnable {private static final Handler self;private static final String[] words = {"long", "short", "big", "small", "clever", "foolish", "tidy","disorganized“};

Asynchronous Servlet Example (Contd.)



private Thread myThread;private boolean stop = false;private List<AsyncContext> queue = newArrayList<AsyncContext>(100);private List<AsyncContext> inProgress = newArrayList<AsyncContext>(100);static {self = new Handler();} private Handler() { } public static Handler getHandler() { return self; } public void stop() { stop = true; }




public synchronized void addJob(AsyncContext job) { queue.add(job); System.out.println("Added a job, queue length is "

+queue.size()); if (myThread == null) { System.out.println("Started handler thread"); myThread = new Thread(this); myThread.start(); } } public void run() { while (!stop) { System.out.println("Handler loop running"); try {

Thread.sleep(5000 + ((int) (Math.random() * 5000)));}




catch (InterruptedException ex) { ex.printStackTrace(); } // check the "queue" synchronized (this) { List<AsyncContext> l = inProgress; l.clear(); inProgress = queue; queue = l; } if (!inProgress.isEmpty()) {System.out.println("Queue contains " +inProgress.size() + " elements");// Create the new information




String value = "The word is: " + words[(int) (Math.random() * words.length)] + " and the number

is: " + Math.random()* 1000000; Iterator<AsyncContext> iac = inProgress.iterator(); while (iac.hasNext()) { // generate responses AsyncContext ac = iac.next(); ServletRequest req = ac.getRequest(); req.setAttribute("value", value); System.out.println("Handler dispatching to a jsp"); ac.dispatch("asyncResponse.jsp"); } }} System.out.println("Stopping handler thread"); } }




Asynchronous handlers are permitted to dispatch or forward their processing to other servlets or JSPs.Filters can also be invoked on asynchronous invocations. If the AsyncContext.dispatch call is used on a matching URL, the dispatcher type will by ASYNC.

Forwarding and Filtering



You can add one or more listeners to the AsyncContext object, using the addListener(AsyncListener) method.The AsyncListener interface must be implemented by the listener that will be notified when the following situations arise:

Starting and completion of asynchronous processingError situationsTimeout

Asynchronous Listeners



The AsyncListener interface defines the following listener methods:

onComplete(AsyncEvent)onError(AsyncEvent)onTimeout(AsyncEvent)onStartAsync(AsyncEvent)

Asynchronous Listeners (Contd.)



AJAX enables a single page to use JavaScript to fetch elements of data from the server and update the displayed page without fetching or redrawing the whole page. In AJAX, the request and response is formatted as an XML document.The following code is an example of an HTML page with asynchronous JavaScript:<html><head><title>Async JavaScript Page</title></head><body><h1>Hello World!</h1><form>

Asynchronous JavaScript Clients



<input id="inputField" type="text" onkeyup="doUpdate()"></form><div id="wisdom"></div><script type=”text/javascript”>wisdomTag = document.getElementById("wisdom");inputTag = document.getElementById("inputField");function doUpdate() {var req;if (window.XMLHttpRequest) {req = new XMLHttpRequest();} else if (window.ActiveXObject) {

Asynchronous JavaScript Clients (Contd.)



req = new ActiveXObject("Microsoft.XMLHTTP");} else {alert("AJAX not supported");}req.onreadystatechange = function() {if (req.readyState == 4 && req.status == 200) {wisdomTag.innerHTML = req.responseText;}}text = inputTag.value;req.open("GET", "update.jsp?text=" + text,true);req.send(null);}</script></body></html>

Asynchronous JavaScript Clients (Contd.)



You can send response to a request using:XMLJavaScript Object Notation (JSON)

Server Response Content in an AJAX System



Consider a page containing JavaScript code that makes an asynchronous request for update from the server, and a server-side implementation that uses the asynchronous servlet techniques.Now, if the server does not choose to respond for several minutes, the user is not inconvenienced as the body of the page is operating normally.Similarly, the resource load on the server is minimized because of the use of asynchronous servlet execution.Consequently, the response may be sent at a time convenient to the server, and the effect of server-push is achieved in an architecturally manageable way.

Combining Asynchronous Servlets With Asynchronous JavaScript



Create a servlet that generates delayed responses using the asynchronous servlet API and asynchronous JavaScript techniques.

Demo 11-1: Asynchronous Servlets and Clients



Solution:1. Create a new project and asynchronous servlet.2. Create an asynchronous client.3. Test the application.

Demo 11-1: Asynchronous Servlets and Clients (Contd.)



In this session, you learned that:To allow for separation of request from response generation, the servlet API provides a class called AsyncContext.Asynchronous handlers are permitted to dispatch or forward their processing to other servlets or JSPs.One or more listeners can be added to the AsyncContext object by using the addListener(AsyncListener) method.In pure AJAX, the request and response is formatted as an XML document.

Summary



The next session covers Lab@Home. In this session, you need to perform exercises given in the following table.

Ensure to take the required input files from the faculty for performing these exercises.

What’s Next?

Chapter Number Exercise NumberChapter 6 (Book 2) Exercise 2Chapter 7 (Book 2) Exercise 2

Web component development and JSP technology session 18

Technology

Transcript of Web component development and JSP technology session 18