Oracle Application Server 10g Essentials [Electronic resources] نسخه متنی

8.1 Developing Web Applications

Trying
to understand what technologies
should be applied when developing a web-based application is a common
dilemma. Should you use HTML, Java, JavaScript, applets, servlets,
EJBs, and/or XML? In some organizations there is no dilemma because
someone else, someplace else in your organization, has already made
the choice for you. If you are given the choice, however, what
approach should you use?

Your first step is to understand how Java and related technologies
can be used for web-based applications. This section examines the use
of Java and related technologies for web-based application
development in three layers: the presentation layer, the data and
business rules layer, and the control layer, as depicted in Figure 8-1.

Figure 8-1. Web application architecture

8.1.1 Presentation Layer

The presentation layer represents the
interface to the application that will be used by a human being. In a
presentation (or view) layer of a web-based application, it
isn't uncommon to see a combination of HTML,
JavaScript, Java applets, and XML employed to provide
a graphical user
interface for an end user. The following are some examples:

HTML can create a data-entry form, a menu, and so on.

JavaScript can provide client-side (web-browser) based validation and
enhanced functionality that is beyond the capabilities of a simple
HTML form.

A Java
applet
can be used in an HTML form to extend the capabilities of JavaScript
or to replace HTML form tags altogether, by providing a rich GUI
using Java.

8.1.1.1 HTML

An HTML
form, created
using the <form> related tags in HTML,
provides a simple user interface. It doesn't allow
you to do any fancy client-side
validation or dynamic population
of drop-down list boxes. It's like using a
block-mode terminal from the good old days, where everything in the
form must be transmitted to the server to be validated. If a
validation error occurs, the server must rerender the entire HTML
form and send it back to the end user's browser.

This type of validation can be both time-consuming and
resource-intensive. It also creates a poor experience for the user of
the application. However, it is extremely lightweight, efficient,
portable, and well supported.

8.1.1.2 JavaScript

By adding
JavaScript routines to an HTML page you can perform client-side
validation, dynamic loading of drop-down list box values, custom
error windows, and so on. Using JavaScript can improve the GUI
capabilities of an HTML form so that the end user's
experience is closer to that of using a client-server application. Of
course, this enhanced end-user experience comes with a price, which
is that JavaScript isn't equally supported on all
browsers. That,
in turn, means that you will have compatibility issues. Regardless,
the use of JavaScript as an extension to HTML forms is very popular.

8.1.1.3 Java applets

If JavaScript
can't
extend an HTML form's capabilities to match your
requirements, you can extend JavaScript's
capabilities with a Java applet or use an applet to provide some
additional GUI functionality on part of a web page. You can also
replace the combination of an HTML form and JavaScript altogether by
using a Java applet as a rich-content GUI.

Using a Java applet as a rich-content GUI allows you to provide a
client-server application look-and-feel inside a browser. That is how
Oracle's Forms applications, for example, are used
on the Web, by employing an applet run inside your browser. The one
drawback with applets used in this fashion is that they are large,
and consequently take longer than HTML pages to download and display
in a browser.

8.1.1.4 JSP and Java servlets

JavaServer
Pages are nothing but
Java
servlets alternatively
created by coding HTML with scriptlets and other JSP syntax, instead
of by coding Java that outputs HTML, as is done in a servlet. Because
JSP is nothing but a servlet, the discussion in this section covers
both technologies.

Java servlets can generate the HTML pages that are then presented in
an end user's browser. Essentially, you use a
servlet to create HTML forms, possibly with embedded JavaScript
routines and applet tags, to load an applet. Servlets (or JSPs) allow
you to retrieve values from a database, create a form, and send it to
the browser. When the user submits the form back to the server, it
calls another servlet (or possibly the same one, depending on design)
to process the data values and update the database.

You can see from the discussion in the previous sections that HTML,
JavaScript, and applets are probably used together, with the HTML
generated by a servlet. But what about the data and business rules?
How do you retrieve and save an application's data?
Where are the business rules kept? That is where JDBC and EJBs are
used:

JDBC allows your program to access a relational database.

EJBs allow you to create modular, reusable software components that
encapsulate business logic and data persistence.

The following sections describe these capabilities.

8.1.2 Data and Business Rules Layer

In the data and business
rules layer of a web-based application, it isn't
uncommon to see a combination of JDBC, JavaBeans, Java Data Objects
(JDO), EJBs, and XML manipulating data and executing business rules.
Several approaches can be used to manipulate data used in a web
application:

JDBC used directly in a servlet

JDBC used in JavaBeans

OracleAS Toplink, OracleAS Business Components for Java, or another
proprietary object-relational mapping tool that implements its
solution as JavaBeans or EJBs

JDO

EJB

Several of these persistence technologies may also be combined and
then used in a servlet. As you go through the options listed here,
the complexity, programming required, and infrastructure required all
increase.

8.1.2.1 JDBC used in servlets

You can use JDBC directly in your servlets.
JDBC allows you to perform
SQL
Data Manipulation Language (DML), such
as inserts, updates, deletes, and selects, along with stored
procedure calls and Data Definition Language (DDL).

If you do use JDBC directly in your servlets, make sure to use
prepared statements to
prevent SQL
injection.

With SQL injection, values entered into fields of your HTML form are actually additional SQL syntax. This maliciously added SQL syntax allows the perpetrator to select all values from a table or damage the validity of data in a table by erroneously inserting, updating, or deleting rows. SQL injection is possible if you use a JDBC Statement which allows you to perform dynamic SQL, but not if you use a PreparedStatement.

8.1.2.2 JDBC used in JavaBeans

If the size of your application requires multiple servlets to access
the same information, you should consider consolidating your JDBC
code in a JavaBean that represents
a table in the database. Alternatively, use one of the other more
complex solutions covered in the following sections.

If you create a JavaBean that represents a table in the database, you
can add routines to select, insert, update, and delete entries as
well as accessors and mutators. Accessors

and mutators are
methods that allow you to get and set column values. This level of
abstraction is well suited for JSPs because you can then use JSP bean
declarations instead of large embedded scriptlets in your JSPs to
retrieve and save data.

8.1.2.3 Object-relational mapping tools

Object-relational (OR) mapping tools also allow you to use simple
JavaBeans for data abstraction. These tools provide a framework that
glues JavaBeans to a persistence layer such as a database. Oracle
Application Server provides two OR mapping tools: OracleAS TopLink
and OracleAS Business Components for Java. Object-relational mapping
tools add value because they significantly reduce the amount of time
it takes to create code for persisting data in a relational database.

8.1.2.4 Java Data Objects

Java Data Objects is a more recent
public specification and standard for
object-relational mapping.
It can also be used in the same way as custom JavaBeans. JDO
simplifies a lot of the coding required to make your own beans.
However, using JDO comes with a price: you have to license a JDO
implementation, learn how to use yet another technology, and do some
special coding just for JDO.

8.1.2.5 Enterprise JavaBeans

If your application is one with high transaction volumes or a large
user base, or one in which many servlets will need to share the
information and business processes, Enterprise Java Beans are
your best bet. EJBs provide a business process and data abstraction
that treats each EJB as a distributed component that can be called by
any program. EJBs provide services that ease management of
transactions and deployment of an application.

EJBs allow you to more easily separate data from the business rules
that may manipulate the data. This, in turn, leads to a more
adaptable and longer-lived application. Once again, this comes at a
price. As with JDO, you'll have to learn how to use
yet another technology and do some special coding just for EJBs.

8.1.3 Control Layer

The control layer acts as a traffic cop
between the presentation layer and the data and business rules layer.
One important aspect of a web-based application is how to direct the
response of an HTML
form to another servlet (or other
entity) in the presentation layer. One common approach is to hardcode
the URL of the servlet that will process a form in the
action
attribute of the HTML <form> tag. This
approach is referred to as model 1
programming. This works just fine, but it makes
your application's modules programmatically
dependent on each other.

An improvement on this approach is model 2
programming with which the same controller
servlet is specified in the action attribute of
every form. The controller servlet then forwards the HTML
form's response on to the desired servlet. The
desired servlet's URL is stored in a configuration
file, as shown in Figure 8-2.

Figure 8-2. A model 2 controller

Struts is a configurable
implementation of a model 2,
Model-View-Controller (MVC) framework. You can
find additional information about Struts at http://jakarta.apache.org/struts/.

8.1.4 Other Considerations for Web-Based Applications

You might also consider making sure that the output of your servlets
is XHTML instead of HTML. XHTML is an XML implementation of HTML, one
in which HTML documents are well-formed and valid XML documents.
Using XML from the start gives you an advantage if you need to
provide a presentation layer of your application to another device,
such as a cell phone. In this case, your servlet's
output can be forwarded to a filter servlet that transforms HTML
(XHTML) to Wireless Markup Language for the handheld device. This
process is possible because your HTML is actually XML. Chapter 10 describes XML in more detail.

After reading this overview of Java web-based application
development, you may have realized that the most formidable part of
web-based application development is the number of technologies you
must master to build a user-friendly application. There are many good
Java books out there that can help you. O'Reilly,
for example, has books on each of these topics. You can find very
well-written books on servlets, EJBs, Struts, and JavaScript at http://www.oreilly.com. Choosing a
development tool that aids you in these technologies can also help
tremendously.