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12.1 Applets and Objects
Applets
represent a shift in the basic model of web communications. In most
other web applications, servers perform most of the computational
work, client browsers being not much more than glorified terminals.
With applets, web technology shifts to the client, distributing some
or all of the computational load from the server to the client
computer and its browser.
Applets also represent a way of extending a
browser's features without forcing users to acquire
new browsers, as is the case when developers implement new tag and
attribute extensions to HTML. Nor do users have to acquire and
install a special application, as is required for helper or plug-in
applications.[1] This means that once users have a browser that supports
applets, you can deliver applets directly to the browser, including
display and multimedia innovations.
[1] Actually, Internet Explorer 6 users
must download and install Java support. Read on for details.
12.1.1 The Object Model
Java-based applets web
page-referenced programs retrieved from a network server and executed
on the user's client computer are an example
of what the HTML 4 and XHTML standards call
inclusions.
As with images, the browser first loads the HTML document, then
examines it for inclusions additional, separate, and discrete
content that is to be handled by the client browser. A GIF image is
one type of inclusion. A .wav sound file, an
MPEG movie, and a Java-based clock program are other types.
The HTML 4 and XHTML standards generally call the inclusion contents
objects. In fact, in your document you may
identify and load nearly any object file over the network through a
universal <object> tag, which we discuss in
detail in Section 12.2.1.
Once it has been downloaded, the standards dictate that the browser
somehow render the object, by internal or external mechanisms.
Otherwise, plug-ins and other helper applications may provide the
necessary rendering mechanism. Internet Explorer, for example, has
its internal resources play an AVI movie, whereas Netscape relies
upon some third-party software, such as RealPlayer, to reproduce the
movie.
12.1.1.1 The applet model
With Java applets, the browser sets aside a portion of the document
display space. You may control the size and position of this display
area; the applet controls what is presented inside.
The applet is software, an executable program. Accordingly, besides
providing a display space, the browser, in tandem with the client
computer environment and resources, provides the applet with a
runtime environment called a virtual machine.
During execution, Java applets have access to a restricted
environment within the user's computer. For
instance, applets have access to the mouse and keyboard and may
receive input from the user. Depending on the security policy in
place, they may initiate network connections and retrieve data from
other servers on the Internet. In sum, applets are full-fledged
programs, complete with a variety of input and output mechanisms,
along with a full suite of network services.
Several applets may be placed in a single document; they all execute
in parallel and may communicate with each other. While the browser
may limit their access to its computer system, applets have complete
control of their virtual environment within the browser.
12.1.1.2 The applet advantage
There are several advantages of applets, not the least of which is
providing more compelling user interfaces within a web page. For
instance, an applet might create a unique set of menus, choices, text
fields, and similar user-input tools different from those available
through the browser. When the user clicks a button within the
applet's interaction/display region, the applet
might respond by displaying results within the region, signaling
another applet, or even by loading a completely new page into the
browser.
We don't mean to imply that the only use of applets
is to enhance the user interface. An applet is a full-fledged program
that can perform any number of computational and user-interactive
tasks on the client computer. An applet might implement a real-time
video display, perform circuit simulation, engage the user in a game,
provide a chat interface, and so on.
12.1.1.3 Using applets correctly
An
applet is nothing more than another tool you can use to produce
compelling and useful web pages. Keep in mind that an applet uses
computational resources on the client to run and therefore places a
load on the user's computer. It can degrade system
performance.
Similarly, if an applet uses a lot of network bandwidth to accomplish
its task (a real-time video feed, for example), it may make other
network communication unbearably slow. While such applications are
fun, they do little more than annoy your target audience.
To use an applet correctly, balance the load between the browser and
the server. For each page, decide which tasks are best left to the
server (forms processing, index searches, and the like) and which
tasks are better suited for local processing (user-interface
enhancements, real-time data presentation, small animations, input
validation, and so on). Divide the processing accordingly. Remember
that many users have slower network connections and computers than
you do, and design your applets to satisfy the majority of your
audience.
Used the right way, applets seamlessly enhance your pages and provide
a satisfying experience for your audience. Used improperly, applets
are just another annoying bandwidth waster, alienating your users and
hurting your pages.
12.1.1.4 Writing applets
Creating Java applets is a programming task, not usually a job for
the HTML or XHTML author. For details, we recommend that you consult
any of the many Java programming texts, including those from
O'Reilly.
Developed by Sun Microsystems, Inc. of Mountain View, CA, Java
supports an object-oriented programming style wherein classes of
applets can be used and reused to build complex applications. One
would think that applets written in the same language should run in
any browser that supports Java. As is so often the case, reality is
more complex. Until Netscape 6 and Internet Explorer 6, browsers
included their own Java Virtual Machines ( JVMs), and their
implementations, especially Microsoft's, could be
quirky. Certain Microsoft implementation decisions in Internet
Explorer 4 and earlier caused some valid Java applets to fail when
running. Microsoft fixed these problems with Internet Explorer
Version 5 but, because of its lawsuit with Sun, chose not to include
a JVM in Internet Explorer 6.[2] Although this may sound like
bad news for applets, in fact Internet Explorer 6 prompts you to
download Microsoft's JVM. Running a Java applet for
the first time is not transparent in Internet Explorer 6, but after
the initial installation, Java support is available. Netscape 6
includes Sun's Java Plug-in, which provides the only
way to get support for the latest version of Java ( Java 2 Standard
Edition 1.4). Users of either browser can install the Java Plug-in to
get state-of-the-art Java support. As an HTML author collaborating
with Java developers, you should assume that most browsers,
unfortunately, have outdated Java support.
[2] As we wrote this, even
this situation may change, with Microsoft reversing itself and
deciding to include a JVM in a service pack for Windows XP. There is
still no sign of default inclusion of a JVM in Internet Explorer 6
downloads, however.
We should take this opportunity to also mention ActiveX, an alternative
executable content technology available from Microsoft. ActiveX is
proprietary, closely coupled with various versions of Microsoft
Windows, and works only when used with Internet Explorer. ActiveX controls (as
they are called) run on versions of Internet Explorer targeted to
various versions of Windows, but a single ActiveX control will not
run on these different versions without recompilation. This is in
contrast with Java applets; a single Java applet can be written and
compiled once and immediately run on a broad range of browsers and
operating systems.
ActiveX also presents an unacceptably high
security risk to any user whose browser
supports ActiveX technology.[3] It is ridiculously easy to penetrate and damage a
computer running a browser that allows ActiveX applets to be
executed. For this reason, we cannot recommend ActiveX as a viable
applet implementation technology and we go so far as to recommend
that users disable ActiveX capability within their
browsers specifically, Internet Explorer.
[3] A good description of
the risks can be found at http://www.digicrime.com/activex/.
