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Netdesign Manual

Part 1

Java XML Programmers Reference

Chapter 11: XML Tools for Information Appliances

September 17, 2001


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Check It Out!

Here is part 1 of a 4-part chapter excerpt from the book Java XML Programmers Reference by Mohammad Akif, Steven Brodhead, Andrei Cioroianu, James Hart, Eric Jung and Dave Writz; ISBN 1861005202; published July 2001, 750 pages.

Our series takes a look at Chapter 11: XML Tools for Information Appliances.

This chapter focuses on Java, XML, and XSLT technologies for lightweight clients. Lightweight clients are defined as those with more limited resources than traditional clients. The term information appliance is used interchangeably with lightweight client. The obvious devices that fit within this category are personal digital assistants (PDAs), mobile phones, and pagers. However, many other embedded devices and consumer electronics may fit into this category: television set-top boxes, global positioning system (GPS) receivers, thermostats, watches, digital cameras, even Internet appliances such as kitchen stoves, refrigerators, and radios. Non-consumer-oriented devices also can fit in this category, such as industrial automation and control sensors.

However, this definition is not limited to non-PC devices. Any environment that requires the following should qualify:

  • A small memory footprint
  • Limited CPU overhead or availability
  • Restricted network bandwidth
Applets, in a typical browser, also fit this lightweight category. As we shall see, XML generation, parsing and transformation are just as important for these types of clients as they are for thin clients (browsers) and servers.

In this chapter, we will address three key Java XML technologies for lightweight clients:

  • Lightweight parsers and document generators – Reference material for three parsers and document generators. Through examples, we will demonstrate the usage of three lightweight XML parsers, two of which can also generate documents by enabling you to create a DOM-style node tree.

  • XSLT compiler – a Java tool that creates fast and lightweight Java class files for transforming XML given an XSL stylesheet

  • CLDC (Connected Limited Device Configuration) and the Java KVM (Kilobyte Virtual Machine) – a Java specification for limited devices, which includes a reference implementation written by Sun. The Java KVM, a virtual machine redesigned for the constraints of limited devices, is part of that implementation.
We can do the same essential processing, parsing, and transforming tasks with these tools that we have used elsewhere in the book. Examples in the chapter will show how to work around some of the limitations of these tools, and how we can leverage their small size to get them to run on devices that would not support their heavyweight counterparts.

There are a number of acronyms and terms you'll encounter in the following sections, so let's briefly cover some terminology before we continue:

Term Definition
CDC Connected Device Configuration – defines a base set of I/O, connectivity, and other classes for "heavy" lightweight clients such as set-top boxes and audio/visual equipment
CLDC Connected Limited Device Configuration – defines a base set of I/O, connectivity, and other classes for lightweight clients such as pagers
J2ME Java2 Platform, Micro Edition – the Java2 platform for information appliances (lightweight clients)
Java KVM Java Kilobyte Virtual Machine – a Java virtual machine designed to minimize its memory footprint instead of maximizing its speed. Currently ported and compiled for Linux, Solaris, Windows, and Palm OS
PDA Personal Digital Assistant – a digital organizer, consisting of applications such as an address book, date book, and notepad

This chapter starts by discussing why we should consider using XML on lightweight clients. We introduce the Java 2 Platform, Micro Edition (J2ME) and its architecture. Then, we cover three lightweight XML parsers and the XSLT Compiler. Finally, we conclude with a Palm OS application that beams address book entries in XML format from one Palm device to another using the Java KVM.

Any discussion of using XML on lightweight clients typically leads to solutions that do not adhere well to standards. W3C XML-related recommendations and standards usually don't have lightweight clients in mind, as they are written independently of any platform or operating system. The implementation of these recommendations often involves resource-intensive processing not possible on lightweight clients. Therefore, many W3C recommendations, for instance namespaces and DOM, are not supported, in order to keep library sizes down.

Instead of looking at the XML components covered in this chapter as non-standard, I encourage you to view them as you might have viewed tools of the early World Wide Web: useful, but non-standard and non-standardized. This will change with time, as we can see by the recent conglomeration of multiple lightweight Java initiatives into the far-reaching J2ME.


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