The Design of a JSP Framework

Case Study:

The Design of a JSP Framework

By Michael Alford

malford@orbitz.com

March 29, 2001

Acknowledgments

•
Xqsite, Inc.

–
Zak Jacobson

–
John Koszarek

–
Nick.Vujasin

–
Greg Skudlarick

•
Orbitz LLC


Web Development

Forces

•
Quick and easy development

•
Simple to maintain

•
High performance

Quick and Easy Development

•
Just get the damn thing out the door!

•
Engineers vary widely in skill/experience.

–
Make common case very simple.

–
Make difficult stuff easy to plug in.

•
Mismatch: Graphic artists work visually,
while engineers work with code.

•
Reuse previously written code.

Simple to Maintain

•
Low complexity.

•
Easily extensible with use of patterns.

•
Easily configurable.

•
Straightforward to debug & monitor.

–
Judicious logging.

–
Don’t impair app server tools.

High Performance

•
Instantiate as few objects as possible.

•
Cache reusable objects.

•
Avoid Java reflection.

•
Minimize synchronization.

•
Minimize indirection.

•
Minimize dynamic content.

Common Practices

Custom Tags

•
<html:iterate collection=“aList” >


<html:valueOf obj=“anObject” />

•
</html:iterate/>

•
Tag Handler

–
Java code

–
Evaluates custom tags

–
Properties

•
pageContext

•
parent tag


Benefits of Custom Tags

•
Eliminate Java code from HTML.

•
Greater separation of content from
presentation.

•
Promote consistency across site.

•
Reusable within context of a website.


Liabilities of Custom Tags

•
Designers cannot work visually.

–
Inefficient iterative back
-
and
-
forth between
designer and developer.

–
Custom modules for visual tool?

•
Closely tied to HTML, limiting reusability.

•
Lower performance versus embedded code.

•
No industry standard set of tags.


JSP Model 2

(Model
-
View
-
Controller variant)

Benefits of MVC

•
Multiple views of same model.

•
Pluggable views and controllers.

•
Separation of development tasks.

•
Reuse potential.

•
Frameworks potential.

Liabilities of MVC

•
Increased complexity.

•
Close coupling between view & controller.

•
Close coupling of views & controllers to a
model.

•
Difficulty of using MVC with some web
development tools.

Document
-
View

•
Combine View and Controller

•
MVC variant

•
Java Swing’s UIDelegate


Xqsite Page Architecture

Core Classes

Design Goals

•
Fast to develop for the common tasks.

–
Even at the cost of flexibility.

–
Most of a website is specific to a project.

•
Simple.

•
Pluggable services for the hard stuff.

•
JSP 1.0.


Architectural Overview

Page

•
Container of Fields

•
Mediator between View and Business Layer

•
The “use bean” tag in JSP is simple and
easy to use.

Page Benefits

•
Standardizes development team on a common
design.


•
Automates validation and formatting of various
types of data.

•
Automates required field handling.

•
Extensible
-

add new field types or services
without modifying existing architecture.

•
Provides file uploading services.

•
Easily configurable page
-
specific, field
-
specific,
and locale
-
specific error messages; can even use a
properties file.


Page: Init & Control

•
<jsp:useBean id="pageObject"

•

scope="request"

•

class="yourPageSubclass">

•

pageObject.onLoad(pageContext);

•
</jsp:useBean>

•
onLoad

–
Calls initFields

–
Calls business layer depending on whether this
is first view or a submission of form data.


Page: Example OnLoad With
Validation

•
public void onLoad(PageContext pageContext) throws Exception {

•

super.onLoad(pageContext);

•


•

// Check if submit button was pressed

•

if (isButtonSelected("createAccount")) {

•


// Process the state when createAccount is clicked

•


if (isFormValid()) {

•


// call business objects here

•

}

•

}

•

if (isButtonSelected("cancel")) {

•


// Process the state when cancel is clicked

•


redirect("/location/of/maybe/the/previous/page");

•

}

•

else {

•


// User’s initial visit or reload of page

•


// Typically do nothing here

•

}

•
}


Submit form back to JSP

•
<form name=“myForm" action="<%=
request.getServletPath() %>" method="POST">

Page: Buttons

•
Radio buttons

•
<input type="radio" name="salutation" value="mrs" <%=
page.getButtonSelected("salutation", "mrs") %>>

•
Checkboxes

•
<input type="checkbox" name=“factor" value="10" <%=
page.getButtonSelected(“factor", "10") %>>


Page: Text Field and Error
Messages

•
String getErrorMessage(String key, String
fieldName)

•
String getRequiredMessage(String
fieldName)

•
JSP:

•
<input type="text" name=“lastName" size="20" maxlength="40"
value='<%= page.getFieldValue(“lastName") %>'
required="true">

•
<%=page.getErrorMessage(page.MESSAGE_FIELD_MISSING,
“lastName")%>

Field Abstract Class

•
abstract void validate();

•
boolean isRequired();

•
boolean isValid();

•
read/write properties:

–
Name

–
Description

–
Value

–
Invalid and Missing Messages

Field Subclasses

•
ButtonField

•
DateField

•
NumericField

•
SelectField

•
TextField

Xqsite Page Architecture

Minimizing the Designer/Engineer
Conflict

PageGenerator

•
Uses the HTML Parser
sunlabs.brazil.handler.HtmlRewriter

•
For every HTML page, creates a subclass
of Page.

•
For every HTML input field, adds a Field to
the Page subclass.

•
Stubs out the onLoad method of Page
subclass.

•
Creates JSP with most of the necessary
code to access the Page.



HTML Layer Requirements

•
Each input field must be named uniquely.

•
Add a fieldType attribute to those fields that
need to be specified less generically.

–
Will not adversely affect visual HTML editors.

–
Example: HTML Text field may have a
fieldType attribute of DateField.

•
Add a required attribute for those fields that
the user is required to enter.

Xqsite Page Architecture

Service Delegation

Interceptor Pattern

•
“Allows services to be added transparently
to a framework and triggered automatically
when certain events occur.”

•
Services are defined in a config file, and
registered upon app startup.

•
Page class onLoad acts as central
dispatcher, passing a Context object to
services.

Context control

•
Context object provides accessors for
services to retrieve state information.

•
Context object provides mutators so that
services may modify request.

•
Example: Authorization service forwards
user to a login page by setting target on
Context object.

Possible Services

•
File upload

•
Configuration

•
Authorization/Authentication

•
Throttling

•
Logging

•
3
rd
-
Party Integration


Applying Page Concepts to
Current Industry Practices

Adopt Custom Tags

•
All Field subclasses can be made into
custom tag handlers.

•
PageGenerator creates a JSP of custom tags
instead of embedded Java code.

Adopt MVC

•
Eliminate Page in favor of FrontController

•
Make FrontController a Interceptor
dispatcher.

•
If using Struts, PageGenerator can create an
ActionForm.


Other Java
-
based Web
Frameworks

•
Struts

•
WebMacro and Velocity

•
HyperQbs

•
Janx

•
WebWork

•
Cocoon (XSP)

•
Resin (XTP)

•
Brazil



References

Crupi, Malks, Alur,
Core J2EE Patterns
, or
http://developer.java.sun.com/developer/technical
Articles/J2EE/patterns/

Gamma, et al.,(aka Gang of Four),
Design Patterns

Buschmann, et al.,
Pattern
-
Oriented Software
Architecture; A System of Patterns

Schmidt, et al.,
Pattern
-
Oriented Software
Architecture; Patterns for Concurrent and
Networked Objects

Struts Framework, http://jakarta.apache.org/struts