Web Development with JavaServer Pages
ColdFusion
The primary differences among template systems, then, lie in their
scripting languages,
and the capabilities provided therein. ColdFusion, from Allaire,
provides a
set of HTML-like tags which were initially targeted at embedding
database queries
into web pages, but it has since been extended to support a wide
variety of data
sources for dynamic content generation. The adoption of HTML-like
tags has the advantage that there is a single, consistent style of syntax
throughout the page; the
ColdFusion tags are comfortable to web designers because they look
just like the
other tags present in the document. ColdFusion supports both UNIX
and
Microsoft Windows platforms.
Active Server Pages
Microsoft's Active Server Pages, often abbreviated ASP, support
multiple scripting
languages, including PerlScript, Jscript, and VBScript. PerlScript is
based on Perl,
and Jscript is based on JavaScript but the default scripting language
for ASP is
VBScript, a subset of Microsoft's popular Visual Basic programming
language.
VBScript includes support for accessing ActiveX components, which are
compiled
code objects that can encapsulate virtually any functionality,
including database
access and file manipulation. A large body of commercial
off-the-shelf ActiveX components
is available, and Microsoft provides tools and documentation for
writing
your own, as well. The major limitation of ASP, however, is that it
is available only
with Microsoft's Internet Information Server (IIS), running under the
Windows
NT operating system.
NOTE As a result of its popularity, a number of vendors
have developed tools for deploying
ASP on other platforms. Chili! Soft, for example, has an ASP product
for the Apache HTTP server running on the UNIX platform, which even
supports
interoperability between Active Server Pages and server-side Java
(i. e., servlets and JSPs). Unfortunately, ASP derives much of its
power from its sup-port
for ActiveX components, which, at least at the time of this writing,
are not widely available on non-Microsoft platforms.
Server-Side JavaScript
As you might expect, Server-Side JavaScript (SSJS) uses JavaScript as
its scripting
language. JavaScript is an object-oriented language (based on
prototypes rather
than classes) with a C-like syntax, but, although it has a similar
name, it is not Java. SSJS
is an extension of the core JavaScript language, which is also the
basis for the
popular client-side JavaScript language used for scripting web
browsers. SSJS adds
built-in features for database and email support, session management,
and interoperability
with server-side Java classes using Netscape's LiveWire technology.
In a
departure from the other dynamic content systems described here, SSJS
is a compiled
language. A collection of web pages containing SSJS is compiled into
a web
application that is executed whenever the corresponding URLs are
requested. Like compiled Java code, compiled SSJS is not platform-specific with
respect to hard-ware
or operating system. SSJS is, however, specific to Netscape's HTTP
servers
(i. e., Netscape Enterprise Server and Netscape Application
Server).
PHP
A fourth dynamic content system that is growing in popularity is PHP.
PHP was
originally an acronym for Personal Home Page tools. As its scope and
functionality
have grown over the years, that moniker is no longer adequate and the
software is
now referred to only as PHP. Like JavaScript, PHP employs a C-like
syntax, and provides
strong support for pattern matching and database access. Extensions
for communicating
with other network resources, such as mail and directory servers,
are
also available. Unlike most of the other dynamic content systems now
available,
however, PHP is an Open Source product. As with other Open Source
products,
such as the Linux operating system and the Apache HTTP server, PHP is
not a commercial
product. It is instead the result of contributions from a community
of interested
developers, freely contributing to and supporting its code base.
One
important result of its Open Source nature is that PHP is now
available on a large
number of platforms. PHP is compatible with Windows NT and several
UNIX operating
systems, and with a number of HTTP servers, such as Apache,
Microsoft's IIS,
and Netscape Enterprise Server.
Java servlets
In light of the importance of dynamic content generation to web
development
then, it was natural for Sun to propose extensions to Java in this
domain. In much
the same way that Sun introduced applets as small Java-based
applications for adding
interactive functionality to web browsers, in 1996 Sun introduced
servlets as
small Java-based applications for adding dynamic functionality to web
servers. Java
servlets have a programming model similar to CGI scripts, insofar as
they are given
an HTTP request from a web browser as input, and are expected to
locate and/ or
construct the appropriate content for the server's response.
Unlike traditional CGI programs that require spawning a new process
to handle
each new request, all of the servlets associated with a web server
run inside a single
process. This process runs a Java Virtual Machine (JVM), which is the
platform-specific
program for running (cross-platform) compiled Java programs. As
illustrated
in figure 1.2, instead of creating a process for each request, the
JVM creates a
Java thread to handle each servlet request. Java threads have much
less overhead
than full-blown processes, and execute within the processor memory
already allocated
by the JVM, making servlet execution considerably more efficient than
CGI processing. Since the JVM persists beyond the life of a single
request, servlets can
also avoid many time-consuming operations, such as connecting to a
database, by
sharing them among all requests. At the same time, because servlets
are written in
Java, they enjoy all the benefits of the core Java platform: an
object-oriented programming
model, automatic memory management, cross-platform portability,
and
access to the rich collection of Java APIs now available for
accessing databases,
directory servers, network resources, and so forth.
When reduced to their essence,
servlets provide a Java-based methodology
for mapping HTTP requests
into HTTP responses. Generating
dynamic web content using servlets
then, is accomplished by means of
Java code that outputs the HTML (or
other data) representing that content.
In the case of HTML data, one
approach is for the Java code to construct
strings containing the appropriate
markup text and then print those
strings on the output stream associated
with the HTTP response. This is
often referred to as the out. println
approach, because a significant portion
of the resulting code consists of
lines that start with this (or a very similar)
sequence of characters.
Another option is to take a more
object-oriented approach to modeling
the response data, by first constructing
a model of the page being constructed as a collection of Java
objects. Many web
pages, for example, can be modeled as a hierarchy of textual
elements, including a
title, various levels of headings, and paragraphs of text providing
the content for each
section, subsection, and so on. Java classes could be defined which
represent each of
these textual elements, as well as the page itself. The classes for
the textual elements
would provide methods for accessing and modifying their content. The
page class
would provide methods for adding and removing textual elements. When
generating
dynamic content, then, an instance of the page class is created, to
which instances of
the appropriate title, heading, and paragraph classes are then added.
Once the
complete model of the page was constructed, other methods could be
called to render
these objects as strings of HTML to be sent back as the servlet's
response.
One popular library of Java classes that enables this style of
dynamic content
generation is the Element Construction Set (ECS), which, like PHP,
Apache, and
Linux, is an Open Source software effort. One of the key advantages
of this object-oriented
approach to dynamic content generation is that it lends itself to
supporting
multiple forms of document output. ECS, for example, supports output
in both
HTML and XML, and can be extended to support additional formats, as
needed.
In a similar manner, the Swinglets toolkit from Javelin Software
allows developers
to construct servlet output using a set of components modeled after
those of Java's
Swing library of user interface components. The Swinglets library
provides several
classes for data presentation, including tables, links, buttons, and
radio buttons.
Data is added to these interface components, which may then be
rendered in various
formats, including HTML, Dynamic HTML, and WML (Wireless Markup
Language,
an HTML-like language geared toward wireless devices such as mobile
phones).
A potential disadvantage of this approach, however, is that all
document con-tents,
both static and dynamic, reside in program source code. As a result,
any
change to such a document requires intervention by a programmer. An
HTML
page designer cannot change the layout of the page unless the
associated source
code is changed. Indeed, any change to the static elements of the
document, such
as changing the URL of a link or image, requires corresponding
changes to the
source code. This dilemma also applies to the out. println
approach.
New on the Java Boutique:
New Review:
Time Management Made Easy with the Quartz Enterprise Job Scheduler
Why not just use the Java timer API? This open source scheduling
API boasts simplicity, ease-of-integration, a well-rounded feature
set, and it's free!
New Applet:
Reverse Complement
Reverse Complement is a simple applet that converts DNA or RNA
sequences into three useful formats.
Elsewhere on internet.com:
WebDeveloper Java
Lots of Java information on webdeveloper.com
WDVL Java
Thorough Java resource at the Web Developer's Virtual Library.
ScriptSearch Java
Hundreds of free Java code files to download.
jGuru: Your View of the Java Universe
Customizable portal with online training, FAQs, regular news updates, and tutorials.
|
