meljun_cortes_jedi slides-web programming-chapter01-introduction to the course

8/3/2019 MELJUN_CORTES_JEDI Slides-Web Programming-Chapter01-Introduction to the Course

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Introduction to WebProgramming

Web Programming


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Why the Web?

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Technology-Neutral Environment

Communication is done through popular protocols (HTML/HTTP)

Users only need a web browser


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Why the Web?

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Ease of Distribution/Updates

Distribution

No need to give away programs through CDs.

No need to go through a possibly lengthy installation sequence.

The users only need the location of the application in the Internet.

Updates

No need to periodically check for newer versions of the program.

Problem of how to actually get the program updates is eliminated.

Users need not even be informed of an update; The codebase in the web serveronly needs to be updated and all users automatically will enjoy the benefits of theupdates.


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Client-Server Architecture

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Client-server architecture

Used by web applications.

A client program connects to a server for information that it needs tocomplete the tasks that the user has set it to do.

Thick and Thin clients

Thin clients

Contain only a minimum of what is required for the user experience, mostly onlyan interface.

Thick clients Aside from an interface, also contain some, if not many, of the processing logic

required for user-specified tasks.


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5Web Programming

Client-server architecture from a web perspective

Web applications use thin clients.

The client program, a browser in this case, is only an interface thatthe user makes use of to perform tasks.

Everything else resides on the server.


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6Web Programming


Web server

The server takes in requests from web browser clients and returns a response

Server response (containsthe document requested bythe user or an error code ifthe item does not exist)

Client request (containsthe name and addressof the item the client islooking for)

Machine running aWeb Server

Machine runninga Web Browser

Serverprocessesclient requestby looking forthe resourcerequested bythe client


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Web client

The browser provides the user with an interface that helps the user to issuerequests to the server and to view the server's response.


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HTML

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Hypertext Markup Language

A set of instructions for the web browser on how to presentcontent to the user.

An open standard updated by the World Wide WebConsortium (W3C).

All browsers know what to do when it encounters HTML,although some older browsers might have problems inrendering some pages that were written using newer

versions of HTML that were updated after theirdevelopment.


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HTTP

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Hypertext TransferProtocol

A network protocol with Web-specific features that runs ontop of two other protocol layers, TCP and IP, to make surethat requests and responses are delivered completelybetween each end of the communication.

TCP makes sure that a file sent from one end of a network isdelivered completely and successfully at its destination.

IP routes file pieces from one host to another on their way to itsdestination. HTTP uses these two protocols.


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HTTP

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Uses a Request/Response sequence

HTTP client opens a connection and sends a request message to anHTTP server.

Server returns a response message and closes the connection.

Request and response messages

Similar and English-oriented

Consist of:

an initial line

zero or more header lines

a blank line and an optional message body


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HTTP Requests

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Requests from the client to the server contain:

The information about the kind of data the user is requesting

A method name, which tells the server the kind of request beingmade, as well as how the rest of the message from the client isformatted

GET

POST


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HTTP Requests: GET

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Simplest HTTP method

Used mainly to request a particular resource from theserver, whether it be a web page, a graphic image file, adocument, etc.

Can also be used to send data over to the server

Limitations

Total amount of characters that can be encapsulated is limited.

Data sent is simply appended to the URL sent to the server.

Advantage URL requesting can be bookmarked by the browser.


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GET: Sample URL

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http://jedi-master.dev.java.net/servlets/NewsItemView?newsItemID=2359&filter=true

All of the items before the question mark (?) is the originalURL of the request.

http://jedi-master.dev.java.net/servlets/NewsItemView

Everything after that are the parameters or data that yousend along to the server.

Parameters are encoded as name and value pairs.

Format: name=value

If there are more than one set of parameters, they are separatedusing the ampersand symbol (&).

newsItemID=2359&filter=true


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HTTP Requests: POST

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Designed such that the browser can make:

Complex requests to the server

Simple requests that require uploading of files to the server

Encapsulates or hides the data inside of the message bodyit sends to the server.


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HTTP Response

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Also contains both headers and a message body

Headers

Different from that of HTTP requests

Contain: Information about the version of the HTTP protocol that the server is using

Content type that is encapsulated within the message body

MIME-type

Value for the content type

Tells the browser if the message contains HTML, a picture, or someother type of content


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Dynamic over Static Pages

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Content

Static content does not change

e.g. archived newspaper articles, family pictures from an online photo gallery

Dynamic content changes according to user input

Dynamic pages have a lot more flexibility and have moreutility than static pages.


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Dynamic over Static Pages

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Some scenarios where dynamic content is the only thingthat will fit the bill:

The Web page is based on data submitted by the user.

e.g. results pages from search engines, orders for e-commerce sites

The data changes frequently.

e.g. weather report or news headlines page built dunamically

The Web page uses information from corporate databases or othersuch sources.


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Java 2 Enterprise Edition

(J2EE)

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A platform introduced for the development of enterpriseapplications in a component-based manner

Uses a distributed multi-tier application model Distributed: most applications designed and developed with this

platform can have their different components installed in differentmachines

Multi-tier: applications are designed with multiple degrees of

separation with regards to the various major components of theapplication


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J2EE: Multi-Tiered

Application Example

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A web application has:

The presentation layer (the client browser)

The business logic layer (the program that resides on the web server) The storage layer (the database which will handle the application data)

The layers of a web application are distinctly separated, butare all needed to create one application for the user


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J2EE Web Tier Overview

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Web Tier

One of the tiers in the J2EE platform

The layer which interacts with browsers in order to create dynamiccontent

Two technologies within this layer:

Servlets

JavaServerPages (JSP)


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J2EE Web Tier Overview

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Web Tier: Servlets

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Java's primary answer for adding additional functionality toservers that use a request-response model

Have the ability to read data contained in the requestspassed to the server and generate a dynamic responsebased on that data

Not necessarily limited to HTTP-based situations, but arecurrently their primary use, so Java has provided an HTTP-specific version that implements HTTP-specific features


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Servlets Disadvantage

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Servlets

Since servlets are simply Java language classes, they produceoutput the way other Java programs would: through printingcharacters as Strings into the output stream, in this case the HTTP-

response. Problems:

HTML can be quite complex and could be very hard to encode through the use ofString literals.

Engaging the services of a dedicated graphics and web page designer to help inthe static parts of the pages is hard if not impossible.


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Web Tier: JavaServerPages

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Looks just like HTML, only it has access to all the dynamiccapabilities of servlets through the use of scripts andexpression languages

Designers can concentrate on simple HTML design andsimply leave placeholders for developers to fill with dynamiccontent


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Containers

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All J2EE components rely on the existence of a container;without the appropriate container, they would not run.


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Containers

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Features

Communications support

Lifecycle management

Multi-threading support Declarative security

JSP Support


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Basic Structure of a Java

Web Application

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For a container to recognize your application as a valid web application,it must conform to a specific directory structure:

Contains HTML, images, other static content, plus JSPs

Contains meta-information about your application (optional)

All contents of this folder cannot be seen from the web browser

Contains class files of Java classes created for this application (optional)

Contains JAR files of any third-party libraries used by your app (optional)

XML file storing the configuration entries for your application


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Web Application

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There are some points regarding this structure:

One: The top-level folder (the one containing your application) doesNOT need to be named Document Root. It can be, in fact, named

any way that you like, though it is highly recommended that the top-level folder name be the same name as your application. It is onlynamed Document Root in the figure to indicate that it serves as theroot folder of the files or documents in your application.


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Web Application

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Two: Any other folder can be created within this directory structure.For example, for developers wishing to organize their content, they

can create an images folder from within the document root to hold alltheir graphics files, or maybe a configdirectory inside the WEB-INFfolder to hold additional configuration information. As long as theprescribed structure as shown above is followed, the container willallow additions.


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Web Application

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Three: The capitalization on the WEB-INF folder is intentional. Thelowercaps on classes and lib are intentional as well. Not following

the capitalization on any of these folders will result in yourapplication not being able to see the contents of these folders.


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Web Application

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Four: All contents of the WEB-INF folder cannot be seen from thebrowser. The container automatically manages things such that, for

the browser, this folder does not exist. This mechanism protectsyour sensitive resources such as Java class files, applicationconfiguration, etc. The contents of this folder can only be accessedby your application.


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Web Application

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Five: There MUST be a file named web.xml inside the WEB-INFfolder. Even if, for example, your web application contains only static

content and does not make use of Java classes or library files, thecontainer will still require your application to have these two items.

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