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Perspective: Servlet and JSP

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Servlet Life Cycle

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Life of a JSP file

http://en.wikipedia.org/wiki/File:JSPLife.png

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Uses of JSP Constructs

Scripting elements calling servlet code directly

Scripting elements calling servlet code indirectly (by means of

utility classes)

Beans

Servlet/JSP combo (MVC)

MVC with JSP expression language

Custom tags

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Servlet vs JSP

JSP is a webpage scripting language that can generate dynamic

content while Servlets are Java programs that are already compiled

which also creates dynamic web content.

Servlets run faster compared to JSP

JSP can be compiled into Java Servlets

Its easier to code in JSP than in Java

JSP and Java Servlets are usually used in conjunction nowadays

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Why Combine Servlets & JSP?

Typical picture: use JSP to make it easier to develop and

maintain the HTML content

For simple dynamic code, call servlet code from scripting elements

For slightly more complex applications, use custom classes called from scriptingelements

For moderately complex applications, use beans and custom tags

But, that's not enough

For complex processing, starting with JSP is awkward Despite the ease of separating the real code into separate classes, beans, and

custom tags, the assumption behind JSP is that a single page gives a single

basic look

P ibili i f H dli

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Possibilities for Handling a

Single Request

Servlet only

Output is a binary type. E.g.: an image

No output. E.g.: you are doing forwarding or redirection as in Search Engine

example.

Format/layout of page is highly variable. E.g.: portal.

JSP only

Output is mostly character data. E.g.: HTML

Format/layout mostly fixed.

Combination

A single request will result in multiple substantially different looking results.

Complicated data processing, but relatively fixed layout.

These apply to a sing le request

You still use both servlets and JSP within your overall application.

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Design PatternsModelViewController

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History

A framework pattern for reusable applications.

Depends on the Observer pattern.

First developed by Xerox PARC for Smalltalk-80.

Used by the Application Kit system in NeXTstep.

Used by the Cocoa APIs for Apples OS X. Recommended structural framework pattern in J2EE.

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10

Design Patterns

The hard problem in O-O programming is deciding what objects to

have, and what their responsibilities are

Design Patternsdescribe the higher-level organization of solutions to

common problems

Design patterns are a major topic in O-O design

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Observable

An Observableis an object that can be observed

An Observeris notified when an object that it is observingannounces a change

When an Observablewants the world toknow about what it has done, it executes:

setChanged(); notifyObservers(); /* or */ notifyObservers(arg);

The ar gcan be any object

The Observabledoesnt know or care who islooking

But you have attach an Observerto theObservablewith: myObservable.addObserver(myObserver);

This is best done in the controller classnot in the mod el class!

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Observerand Observable

java.utilprovides an Observerinterfaceand an Observableclass

An Observableis an object that can be observed

An Observeris notified when an object that it is observingannounces a change

Heres an analogy:

An Observableis like a Button

An Observeris like a Listener

You have to attach a Listenerto a Button

Another analogy:

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Observer

Observeris an interface

An Observerimplementspublic void update(Observable obs, Object arg)

This method is invoked whenever an Observablethat it is listeningto does an addNotify()or addNotify(arg)

The obsargument is a reference to the observable object itself

If theObservabledidaddNotify(), thearg isnull

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Observer Pattern

Defines a one-to-many dependency between objects so that when

one object changes state, all its dependents are notified and

updated automatically.

Used to decouple the subject from the observer, since the subject

needs little information to notify the observer.

Can result in excessive notifications.

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The MVC pattern

MVCstands for Model-View-Controller

The Modelis the actual internal representation

The View(or a View) is a way of looking at or displaying the model

The Controllerprovides for user input and modification

These three components are usually implemented as separate

classes

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MVC

Divides interactive applications into three distinct components:

ModelContains core functionality and data.

ViewDisplays information to user.

ControllerHandles user input.

Includes a change-propagation mechanism.

Ensures consistency between user interface and model.

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Model

Encapsulates data.

Exports procedures that perform specific application processing.

Used by Controller on behalf of user.

Provides functions to access Models data. Used by View, maybe Controller too.

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Model

Manages change-propagation mechanism.

Maintains registry of components dependent on the model.

Views and controllers register their need to be informed about changes.

Changes in state of model trigger notifications.

Responsibilities include:

Providing functional core.

Register dependent views and controllers.

Notify dependent components about changes.

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View

Presents information to user.

Different Views present information in different ways.

Defines an update procedure that is activated by change-

propagation mechanism in Model.

Retrieves current data values to be displayed from Model.

During initialization, associates itself with Model and registers for

change notification.

Corresponds to a single Controller.

Offers functionality that allows Controller to manipulate display.

E.g. scrolling.

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Controller

Accepts user inputs as events.

Translates events into requests for the associated Model or View.

Registers itself with Model if it depends on state of the Model in any

way.

E.g. A certain change in the Model results in a menu item being enabled or

disabled.

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Combining Controller and View

Sometimes the Controller and View are combined, especially in

small programs

Combining the Controller and View is appropriate if they are

very interdependent

The Model should still be independent

Nevermix Model code with GUI code!

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Separation of concerns

As always, you want code independence

The Model should not be contaminated with control code ordisplay code

The View should represent the Model as it really is, not someremembered status

The Controller should talk tothe Model and View, notmanipulatethem

The Controller can set variables that theModel and View can read

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Advantages

Separation between the data layer and the interface is

the key:

The view is easily replaced or expanded.

Model data changes are reflected in all interfaces because all

views are Observers.

Better scalability since UI and application logic are separated.

Distribution over a network is greatly simplified.

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Key points

A Model does the business logic

It should be I/O free

Communication with the Model is via methods

This approach gives maximum flexibility in how the model is used

The Controller organizes the program and provides input

(control) to the Model

The View displays what is going on in the model

It should never display what shouldbe going on in the model

For example, if you ask to save a file, the View shouldnt itself tell

you that the file has been savedit should tell you what the model

reports.

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MVC1 Architecture Page centric

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MVC2 Architecture

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Comparison of Web Frameworks

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MVC Frameworks

Adobe Flex

Grails, together with Groovy

Google Web Toolkit (GWT), together with GWT-Ext

JavaServer Faces (JSF), together with Facelets

Seam

Spring MVC, which is a part of the Spring Framework

Stripes

Struts 2

Tapestry

Wicket

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Pros and Cons

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Spring MVC

Pros:

Lifecyle for overriding binding, validation, etc.

Integrates with many view options seamlessly:

JSP/JSTL, Tiles, Velocity, FreeMarker, Excel, PDF

Inversion of Control makes it easy to test

Cons:

Configuration intensive - lots of XML

Almost too flexible - no common parent

Controller

No built-in Ajax support

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Stripes

Pros:

No XML - Convention over Configuration

Good documentation (easy to learn)

Enthusiastic community

Cons:

Small Community

Not as actively developed as other projects

Hard-coded URLs in ActionBeans

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Struts 2

Pros:

Simple architecture - easy to extend

Tag Library is easy to customize with

FreeMarker or Velocity

Controller-based or page-based navigation

Cons:

Documentation is poorly organized

No feedback for missing properties or invalid

OGNL expressions

Googling results in Struts 1.x documentation