Pushlets - Whitepaper

Author: Just van den Broecke
Organization: Just Objects B.V.
Email: just[AT]justobjects.nl

FileID: $Id: whitepaper.xml,v 1.5 2005/02/28 14:31:20 justb Exp $
Date: August 6, 2002

Pushlets are a servlet-based mechanism where data is pushed directly from server-side Java objects to (Dynamic) HTML pages within a client-browser without using Java applets or plug-ins. This allows a web page to be periodically updated by the server.


1. Introduction

Server-side notification to browser-clients is often implemented using applets with RMI, CORBA or custom TCP/IP messaging. These techniques are often complex to implement, have firewall restrictions, and require additional server development/maintenance. Also it is hard to integrate the state of the client-applet with the browser's page-content other than refreshing the page or doing all content-layout within the applet.

Pushlets are a servlet-based mechanism where data is pushed directly from server-side Java objects to (Dynamic) HTML pages within a client-browser without using Java applets or plug-ins. This allows a web page to be periodically updated by the server. The browser client uses JavaScript/Dynamic HTML features available in type 4+ browsers like NS and MSIE. The underlying mechanism uses a servlet HTTP connection over which JavaScript code is pushed to the browser. Through a single generic servlet (the Pushlet), browser clients can subscribe to subjects from which they like to receive events. Whenever the server pushes an event, the clients subscribed to the related subject are notified. Event objects can be sent as either JavaScript (DHTML clients), serialized Java objects (Java clients), or as XML (DHTML or Java Clients). Several application examples are presented such as monitoring (weather, live stock feed, system status) and a multi-user applications (chat).

The mechanism is lightweight in the sense that uses the servlet server's connection management and threading facilities, the javax.servlet APIs and standard Java features such as producer/consumer through Object wait() and notify(). In principle the framework could run within any servlet-supporting server and behind firewalls. When JavaScript/DHTML is used on the client this provides a convenient way to build applications quickly through scripting and to integrate and layout new content with HTML/CSS features.

2. Motivation

Now that more and more Servlets and JavaServer Pages (JSPs) are being deployed on the web, there is often a need to notify and synchronize client web-browsers from state changes in server objects after the browser has loaded a page.

These changes can be caused by users updating an EJB through a servlet or database record or by events in a multi-user application such as chat and shared whiteboard. These type of applications often use a distributed Model View Controller (MVC) pattern where the Model is on the server (with possible caching in clients) and the Control and View (possibly combined) reside within the client.

There are also applications where we would like to subcribe to dynamic content that is continuously pushed from the server. Examples stock feeds, system status, weather conditions or other monitoring applications. This follows an Observer (a.k.a. Publish/Subscribe) pattern where remote clients (Observers) register for updates from server objects (Subjects).

How can we notify browser-clients after their HTML page has loaded ? Or what to do if we would like to selectively update only parts of a page. For example only the stock item in an HTML TABLE whose rate has changed ?

3. Notification Solutions

Let's assume that we have a Java web/application server from which we want to notify client browsers. Solutions we have can be categorized as "polling", "server-side callbacks" and "messaging".

3.1. Polling

The simplest solution is a "timed page refresh". Using HTML META tags in the header of the HTML document, the page is automatically reloaded every N seconds. If in the meantime something has changed on the server we get the new content, otherwise we get the same stuff. But how large should we make the refresh interval ?

3.2. Server-side callbacks

Since we are seasoned Java developers building serious applications, we often use "server-side callbacks". Here a server objects calls back a Java applet client using RMI or CORBA.

3.3. Messaging (MOM)

In this solution an applet is a client from a messaging middleware server that pushes messages over a TCP/IP connection (java.net.Socket) or using connectionless UDP messages (java.net.DatagramSocket), the latter possibly even with multicast (java.net.MulticastSocket). You could use a messaging product such as iBus (SoftWired), MQSeries (IBM) or WebLogic Events (BEA) or develop your own custom messaging with java.io.ObjectStream's over sockets.

3.4. Discussion

Each of the above solutions has its advantages/disadvantages in complexity, security, performance, scalability, browser Java compatibility and restrictions like firewalls. The most optimal solution strongly depends on what your application is supposed to do. For example, when users require a direct interaction with the state such as in a shared whiteboard, server side-callbacks or messaging can be a powerful technique.

But we are still within a browser and unless the applet constitutes the entire client application, it is hard to integrate updates coming from the server with the HTML content. How can we alter this content from within the applet when it gets the callback or message ? One solution is to refresh the page by calling AppletContext.showDocument(URL) within the callback method.

Since HTML is meant for layout, wouldn't it be nice to be able to directly alter parts of the HTML content with incremental data coming from the server ? This would be an ideal scheme for web-applications where content on the server is dynamically changing and the required user-to-server interaction is minimal, e.g. driven by HTML FORMs.

In addition to the above solutions I have developed a technique that is lightweight, thin on the client, requires no applets or plug-ins, directly integrates with scripting/HTML, uses standard HTTP connections and can be deployed (in theory!) in any Java servlet-server. It is certainly not meant to replace the above solutions. My intention is to add another option to your toolbox. You as Java architects/developers should determine the trade-offs and choose what is best for your particular application.

4. Pushlet Basics

So what are these Pushlets and how do they work ? In its basic form a Pushlet is luckily extremely simple. Through a few examples I will show the basics. It is by now also time for some code !

4.1. HTTP Streaming

Pushlets are based on HTTP streaming, a technique that is sometimes used in multimedia viewing applications such as QuickTime. Instead of closing the HTTP connection after fetching an HTML page, the connection is kept open while fresh data is pushed to the client.

4.2. Example 1

Taking the idea of HTTP streaming we could develop a JSP (since that it easier to deploy, but it could be a servlet as well) that continuously sends new HTML content back to the client in a timer loop.

<HTML> <HEAD> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> <META HTTP-EQUIV="Pragma" CONTENT="no-cache"> </HEAD> <BODY BGCOLOR="blue" TEXT="white"> <% int i = 1; try { while (true) { out.print("<h1>"+(i++)+"</h1>"); out.flush(); try { Thread.sleep(3000); } catch (InterruptedException e) { out.print("<h1>"+e+"</h1>"); } } } catch (Exception e) { out.print("<h1>"+e+"</h1>"); } %> </BODY> </HTML>

Click example 1 on the examples/basics page. This is not very useful since the pushed content is continuously appended to the page while we would like to refresh it.

4.3. Example 2

Here we jump right into the Pushlet mechanics. Click on example 2 on the examples/basics page and see that the page is refreshed every 3 seconds. How the ... is this done ?

This example consists of three files: push-js-stream.html, push-js-stream-pusher.jsp and push-js-stream-display.html. push-js-stream.html is the main page which contains each of the two other files in HTML FRAMEs. Let's just follow the route of the events.

The following lists push-js-stream-pusher.jsp. This is a JavaServer page that is executed on the server when requested. The main body of this file is listed below.

7: <% 8: /** Start a line of JavaScript with a function call to parent frame. */ 9: String jsFunPre = "<script language=JavaScript >parent.push('"; 10: 11: /** End the line of JavaScript */ 12: String jsFunPost = "')</script> "; 13: 14: int i = 1; 15: try { 16: 17: // Every three seconds a line of JavaScript is pushed to the client 18: while (true) { 19: 20: // Push a line of JavaScript to the client 21: out.print(jsFunPre+"Page "+(i++)+jsFunPost); 22: out.flush(); 23: 24: // Sleep three secs 25: try { 26: Thread.sleep(3000); 27: } catch (InterruptedException e) { 28: // Let client display exception 29: out.print(jsFunPre+"InterruptedException: "+e+jsFunPost); 30: } 31: } 32: } catch (Exception e) { 33: // Let client display exception 34: out.print(jsFunPre+"Exception: "+e+jsFunPost); 35: } 36: %>

NB there may be a problem with examples 1 and 2 when using JSPs: some servlet engines will "eat" the IOException when a client leaves such that the JSP page will never catch the exception. In that case the loop may run forever. This is one of the reasons that the Pushlet implementation uses a Servlet (where the IOException can be and is caught).

Again we see a timer loop which prints (line 21) some HTML to the browser every three seconds. But wait, it is not pushing HTML but JavaScript ! What does this mean ? Effectively it pushes a line like for example <script language=JavaScript >parent.push('Page 4')</script>. What does this mean for the browser ? The browser has its JavaScript engine running and obediently executes each next line coming in. It is calling a JavaScript function parent.push(). Now the parent is the parent of the FRAME it is in, which is our first file push-js-stream.html. Let's see what happens there.

<script LANGUAGE="JavaScript"> var pageStart="<HTML><HEAD></HEAD><BODY BGCOLOR=blue TEXT=white><H2>Server pushes: <para>"; var pageEnd="</H2></BODY></HTML>"; // Callback function with message from server. // This function is called from within the hidden JSP pushlet frame function push(content) { // Refresh the display frame with the content received window.frames['displayFrame'].document.writeln(pageStart+content+pageEnd); window.frames['displayFrame'].document.close(); } </script> </HEAD> <FRAMESET BORDER=0 COLS="*,0"> <!-- frame to display the content pushed by the pushlet --> <FRAME SRC="push-js-stream-display.html" NAME="displayFrame" BORDER=0 SCROLLING=no> <!-- Hidden frame with the pushlet that pushes lines of JavaScript--> <FRAME SRC="push-js-stream-pusher.jsp" NAME="pushletFrame" BORDER=0 SCROLLING=no> </FRAMESET>

We see the push() function called from within the JSP frame (pushletFrame) is writing whatever it gets passed in its argument 'content' into the displayFrame. This is a piece of Dynamic HTML: you can refresh the content of a frame or window by calling 'writeln' of its 'document' object. So the displayFrame is the real View where the content is displayed. It is initially black and displaying a 'Wait...' text until the first content is pushed from the server.

<HTML> <BODY BGCOLOR=black TEXT=white> <H1>WAIT...</H1> </BODY> </HTML>

This is basically the whole idea of Pushlets: we just stream in lines of JavaScript from a Servlet (or JSP for the example). These lines get interpreted by the browser who may do something interesting. So effectively we have a callback from Java in the server to JavaScript in the browser client ! Phew, that was easy !

This example showed the mechanics, but there are still a couple of issues to be solved and features to be added. For this reason I've built a small server-side Pushlet framework shown in the class diagram, plus some JavaScript libraries for the client. Since the client heavily will rely on more DHTML features such as Layers we will walk through some DHTML first. See examples/dthml.

5. Not just Java - Dynamic HTML

Long gone (in Internet time) are the days when websites could be produced by the local sysop who scanned in some images from the company's brochure and knew a few HTML-tags. The possibilities for manipulating content and user interaction within the browser are expanding through something called "Dynamic HTML" (DHTML). As a Java programmer using servlets and JSPs, DHTML is something that should become part of your toolkit.

DHTML refers to a combination of HTML, Cascading Style Sheets (CSS), JavaScript and the browser's Document Object Model (DOM). Traditionally a page could only be altered by reloading a new page from the server. DHTML allows full control an HTML document within a browser after its page has been loaded. You probably have seen examples on the web such as "image roll-overs", pop-up content and collapsabe menus. DHTML is supported by most version 4 browsers, albeit with some differences in standards (see "Cross-browser DHTML" below).

From a programmer's point of view the entire document in the browser, i.e. its frames, images, paragraphs, tables etc., is represented as an hierarchical object model, the DOM (not to be confused with the XML DOM). Through JavaScript you can manipulate the elements of the DOM and thereby change the content/appearance of the document. Also, you can capture user events from these elements such as mouse moves and form submission, and subsequently process these to modify DOM elements. For example, a mouse moving over an image may produce a "mouse-over" event that is processed by changing the image by a highlighted version or popping up explanatory text. Document elements may even be animated by moving them around. This sounds great doesn't it ? We just need to get familiar with the DHTML standard and off we go ! Hmm, but who defines the DHTML standard ?

This hits an issue that has witheld many developers from embracing DHTML. First of all you need a version 4+ browser. The "official" standards body for DHTML-related specifications is the World Wide Web Consortium (W3). However the Microsoft and Netscape type 4+ browsers each have proprietary DHTML extensions that you must track as well.

Luckily the situation is much better now. Most users by now have type 4 browsers, plus some people (in particular the "Dannymen", Dan Steinman and Danny Goodman, see References section) have done good work on creating cross-browser DHTML libraries that you can reuse. As a Java programmer you may appreciate the fact that you can do reasonably clean object-based or even object-oriented programming in JavaScript. In my basic DHTML demos you will find some examples, but it is worthwhile to check out the DHTML resources. Once you have browser issues hidden behind a facade of cross-browser libraries DHTML programming becomes real fun.

So with Java gaining more market on the server and DHTML having these powerful features on the client, my idea with Pushlets was to directly couple these two great technologies in a very direct way. For this I have implemented a lightweight framework for the server and some DHTML libraries for the client. These are discussed next in the Design section.

6. Design of the Framework

THIS STILL REFLECTS VERSION 1 (REWORKING...).

The Pushlet framework allows clients to subscribe to subjects within a server from which they subsequently receive events. The framework's basic design pattern is Publish-Subscribe also known as Observer and has both server and client components:

6.1. Server-side class design

Below is the UML class diagram of the server-side Java classes [reflects v0.0.4; rework required].

Figure 1. Pushlet Framework Class Diagram.

The key classes are the Pushlet servlet, the Publisher class, Subscriber interface, and the Event class (see class diagram). By invoking the Pushlet servlet through an HTTP request, clients subscribe to receive Events. In the request is indicated:

An example request for receiving AEX stock rates formatted as JavaScript through an HTTP response stream would be:

http://www.fluidiom.com:8080/servlet/pushlet?subject="/stocks/aex"

Subject identifiers are organized as an hierarchical "topic-tree". For example, "/stocks" identifies all Events related to stock rates, while "/stocks/aex" identifies stock rates for the Amsterdam EXchange. Likewise the subject "/" indicates all events.

Currently the only receiver protocol is a client HTTP response stream. In a future extension also the receiver protocol and address can be indicated, e.g. TCP, UDP, RMI, HTTP POSTing, or even SMTP (email).

An Event is merely a set of name/value String pairs (implemented with java.util.Properties). The Publisher has an interface through which classes that generate Events can publish them. The Publisher keeps a list of Subscribers and sends each Event to those Subscribers whose subject matches to the Event's subject. Events may be originating within the server through EventGenerators who may listen to external events such as a stock feed. In addition clients may publish Events through HTTP with the Postlet class. The responsibilities of other classes in class diagram, PushletSubscriber and its contained classes can best be explained through scenario's.

6.1.1. Scenario: Event Subscription

Figure 2. Subscribe Sequence Diagram.

Above is the UML sequence diagram for a browser client subscribing for events from the Publisher. The Pushlet is invoked with the servlet method doGet(). Because multiple clients may invoke the same Pushlet object, it should itself not be a Subscriber. Instead it delegates all subscription (and subsequent Event handling) by creating a new PushletSubscriber object for each doGet() and letting it run until finished with eventLoop(). The PushletSubscriber is a Subscriber object towards the Publisher where it registers with the join() method. To deal with different client formats and protocols it creates a specialized ClientAdapter object, in this case a BrowserPushletAdapter. For browsers supporting Multipart MIME such as Netscape 4+, this would be a MultipartBrowserClientAdapter. The final call in this scenario is a "wait for event loop". Note that deQueue() is a method that suspends the execution of the current thread until an Event becomes available (indicated with half-arrow). This is explained through the next scenario.

6.1.2. Scenario: Sending and Dispatching Events

Figure 3. Publish Sequence Diagram.

Above is the UML sequence diagram for sending an Event. It shows how an Event is generated and dispatched to the browser client. In this scenario an EventGenerator creates an Event and calls Publisher.publish() to have it dispatched. The Publisher walks through its list of Subscribers and asks each if the Event matches its subscription criteria (currently only the subject). If it matches it calls send() on the Subscriber.

Each PushletSubscriber object has a GuardedQueue object in which it queues incoming Events when send() is called. So why isn't it just directly pushing the Event to the BrowserPushletAdapter ? First of all we want to suspend execution of the BrowserPushletAdapter-thread until an Event becomes available, i.e. we don't want to do a "busy-wait" (a.k.a. as polling). The second reason is that a Publisher may notify multiple clients. Having a synchronous send() call a slow client on the other end of the line may block all other clients that are to be notified next. This is actually a design pitfall which I see also in RMI or CORBA callback examples where a list of clients is called back synchronously. Client #13 on a slow connection and 386 processor may spoil it for the rest.

The GuardedQueue is a utility object that allows Objects to be en/dequeued using the readers-writers pattern with guarded suspension using java.lang.Object.wait() and notifyAll(). The thread of a client of GuardedQueue calling deQueue() will be suspended (using wait()) until there is an Object queued. Likewise a client enQueueing an Object will be suspended as long as the queue is full. When clients are fast enough the GuardedQueue is never filling up. After the BrowserPushletSubscriber has dequeued an Event object it will call push() on the BrowserPushletAdapter who will format the Event to a JavaScript element and send it to the browser. For example for a Philips stock rate of 123.45 the JavaScript element looks as follows.

<SCRIPT language=JavaScript >parent.push('subject', '/stocks/aex', 'philips', '123.45') </SCRIPT>

6.2. Client-side framework

By now we have arrived on the client browser side. The Pushlet itself was assigned to a hidden HTML FRAME. The parent of that FRAME is called and has to implement the push() method. Since this is a common task for all browser clients, two reusable files are provided for the client: pushlet.html (see /src/nl/justobjects/pushlet/pushlet.html) and pushlet.js (see /src/nl/justobjects/pushlet/pushlet.js).

pushlet.html is meant to be included in a FRAME within the application-specific client HTML document. It can be parameterized with the subject identifier and a background color (such that it remains invisible). The most important thing it does is implementing the JavaScript push() method as follows:

function push() { // Create a PushletEvent object from the arguments passed in // push.arguments is event data coming from the Server pushletEvent = new PushletEvent(push.arguments) // Show blinking light as data is coming in updateStatusFrame(); // Is parent ready to receive events ? if (!parent.onPush) { return; } // Forward the event to the parent frame who should do application // specific handling of the event parent.onPush(pushletEvent); }

The function push() first creates a JavaScript object from the parameters passed in. Yes you can do object-based programming in JavaScript. Reminiscent of 'varargs' in C/C++, JavaScript functions may have variable number of arguments. A PushletEvent object is created with whatever arguments were passed to push() from the server. PushletEvent is implemented in pushlet.js shown next.

/* Object to represent nl.justobjects.pushlet.Event in JavaScript. Arguments are an array where args[i] is name and args[i+1] is value */ function PushletEvent(args) { // Member variable setup; the Map stores the N/V pairs this.map = new Map(); // Member function setup this.getSubject = PushletEventGetSubject this.put = PushletEventPut this.get = PushletEventGet this.toString = PushletEventToString this.toTable = PushletEventToTable // Put the arguments' name/value pairs in the Map for (var i=0; i < args.length; i++) { this.put(args[i], args[++i] ); } } // Get the subject attribute function PushletEventGetSubject() { return this.map.get('subject') } // Get event attribute function PushletEventGet(name) { return this.map.get(name) } // Put event attribute function PushletEventPut(name, value) { return this.map.put(name, value) } function PushletEventToString() { return this.map.toString(); } // Convert content to HTML TABLE function PushletEventToTable() { return this.map.toTable(); }

pushlet.js in turn uses a Map JavaScript object, a java.util.Hashtable-like object I've added.

Next push() calls updateStatusFrame() to show a blinking light to indicate we are still receiving events and if a parent.onPush() function exists, it calls it with the PushletEvent. parent.onPush() is the application-specific event handling function that in this case may update the 'philips' stock-related Layer in a DHTML page.

This ends the description of the basic framework design.

7. Applications

Pushlets allow many types of web applications to be developed. Since the framework also allows for clients to upload events (through the Postlet), possible applications are not just passive pushing of data. Each application can be categorized according to:

Also since live events are made available to JavaScripting, this allows also scriptable plug-ins to receive live updates. For example you may script your Macromedia Flash or VRML World application.

Below is a range of applications. On the Pushlets website several simple demos are provided.

7.1. Monitoring

Various data sources may be monitored live such as stocks, weather, votes, flight arrivals and systems.

See a real-world FX stocks/news application that I worked on at www.rabotreasuryweb.com (IE only).

I also discovered other real-time stock/news sites that are deploying Pushlets or a variant:

www.marketnews.com.

7.2. Gaming

From a two-user tic-tac-toe up and chess up to more elaborate Risk and Monopoly.

7.3. Distributed Model View Controller (MVC)

This refers to a design pattern often found in user interface frameworks such as Java Swing and Microsoft MFC. In the distributed variant a Model (often the data) resides on a server while clients hold the Views and Controls. Through the Controls the Model is modified. The Model will then notify all attached Views who will subsequently refresh themselves.

Many applications may have a web front-end through which data on the server is updated by multiple users. Examples are reservation systems and bookings. If one client makes an update, the others won't see the update unless they continuously refresh their pages. In some cases this is a simple and workable solution, but there are also cases where users need to be synchronized with the update as it happens. This type of application is easily done with Pushlets pushing a URL as the single event. When a client receives the URL it will refresh the page with it.

One notable example are the much discussed Enterprise JavaBeans (EJBs). Although Java clients are able to talk directly to an EJB (through RMI or CORBA), more often servlets and JSPs are used as a front-end. In this case notification becomes much harder. With Pushlets an EJB could notify its attached web-clients whenever its state changes.

7.4. Web Presentations

Also known as WebTours or like I dubbed once: "Are you being surfed ?". This is actually an application from which I derived Pushlets. I abandoned PowerPoint for making Java course content and developed a content management framework based on XML with presentation (slides etc) in HTML. Since in many situations the classroom had no beamer but all students a networked computer, I developed a simple application (WebPres) that allowed me to change HTML slides with all students automatically following them in their browsers.

7.5. User Assistance

This type of application can also be useful in situations like call centers, banks or help-desks or e-commerce web applications in general. When I phone in with a question the agent may surf me to URLs with solutions, offers or other information.

A real-world application where this example of Pushlet-technology is currently used is a foreign-exchange trading application I helped developing at one of my clients. See the result at www.rabotreasuryweb.com (IE only).

Using a back-end of EJBs and a JSP front-end, clients can buy or sell foreign currencies. An "AutoTrader" object automates the offering process. If somehow the automatic offering fails or the client requests a human dealer, a "Dealer Intervention" takes place, whereby a dealer is notified and enters a new offer which is pushed to the client browser using Pushlet technology.

7.6. Community Tools

Various applications where multiple users can join in a live session. I originally had a prototype of the framework where a multi-user session was realized as a collection of HTTP Session objects. I am planning to extend the Pushlet framework with these capabilities. For now I have a simple web-chat and what I call WCQ (in Dutch this sounds almost like We Seek You), a simple ICQ-like desktop window where you can monitor the presence of friends.

Other applications in this area include live forums and shared document editing.

8. Liabilities

This chapter describes the consequences of using Pushlets

As with any mechanism or design pattern there are obvious advantages and disadvantages with Pushlets as compared to Java-based applet solutions that use messaging or client callbacks with CORBA/RMI.

8.1. Advantages

8.2. Disadvantages

9. Further Work

I am further testing the viability of Pushlets. The Pushlet framework is actually a generic Publish/Subscribe pattern with HTTP-based Pushlet clients as a special case. I am evolving the framework with the following extensions

9.1. Additional client receiver-protocols such as TCP and UDP

One interesting area is the possibility to call an applet that receives UDP messages from within the Pushlet frame on the client. This would mean that we don't keep the HTTP connection. In general clients will subscribe with a HTTP request on which they indicate through which protocol (TCP, UDP, RMI, HTTP-stream, HTTP POST) they want to receive events and in which format (XML, Java serialized objects,...). Where required the address of the receiver is specified. For example to receive XML formatted stock events through UDP on port 5001:

http://www.fluidiom.com:8080/servlet/pushlet?subject="/stocks/aex"&format="XML"&protocol="UDP"&port="5001"

With UDP a lease time may need to be specified (since the server would never know when the client leaves). If the lease is not renewed after this time the server will drop the client.

9.2. additional client sender-protocols

Currently events are generated internally or through the Postlet by using the HTTP POST request. Additional protocols such as RMI, TCP and UDP may be applied.

9.3. subject state

Currently a subject is just an hierarchical string and a client that subscribes gets no history that may have been built up. For example in a chat some discussion may have been going on. In a next version subjects will become first class objects such that when clients subscribe they may implicitly get the current state or may explicitly request the state.

9.4. multi-user

Although some multi-user applications are possible there is no knowledge or management on the server. In earlier versions I have experimented with combining multiple HTTP Sessions into a multi-user HTTP session.

References

[JW1]
An in-depth look at RMI callbacks (4/20/99); www.javaworld.com/javaworld/javaqa/1999-04/05-rmicallback.html
[JW2]
Java Tip 34: POSTing via Java/ Learn how to POST data to Web servers in Java; www.javaworld.com/javaworld/javatips/jw-javatip34.html/
[JW3]
Java Tip 41: POSTing via Java revisited. Learn how to display the HTML document returned by the Web server; www.javaworld.com/javaworld/javatips/jw-javatip41.htm
[JW4]
[JW5]
Networking our whiteboard with servlets.; www.javaworld.com/javaworld/jw-01-1998/jw-01-step.html
[JW6]
[JH]
Jason Hunter's ("Java Servlet Programming", O'Reilly) servlet site.; http://www.servlets.com
[DL]
Doug Lea - Concurrent Programming in Java - 2nd edition.; www.amazon.com
[DD]
The Dynamic Duo; Cross-Browser Dynamic HTML; dansteinman.com/dynduo
[DG]
JavaScript and Dynamic HTML; Danny Goodman ("Dynamic HTML: The Definitive Reference", O'Reilly); www.dannyg.com
[SI]
Scott Isaacs' ("Inside DHTML") community site on DHTML; www.siteExperts.com and www.insideDHTML.com
[WM]