J2ee_Session2

Session 2.1 J2EE Architecture

Session 2.2 Http Basics & Servlet

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Objectives:

What is J2EE?

Why J2EE?

J2EE Tire Architecture.

MVC Architecture.

Http Basics

Servlet

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What is J2EE?

Open and standard based platform for

developing, deploying and managing. n-tier, Web-enabled, server-centric, and component-

based enterprise applications.

It includes Java Standard Edition(J2SE) plus most of the other Java technologies including JavaMail, API for XML Binding, Servlets, JSP (Java Server Pages), JMS (Java Messaging Service), EJB (Enterprise Java Beans)……….

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Why J2EE?

We can use all the properties of Java Technology….Platform Independence, Secure Code, Portability etc……

Simplifies the complexity of a building n-tier application.

Standardizes an API between components and application server container.

J2EE Application Server and Containers provide the framework services

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J2EE ARCHITECTURE

Single Tier (Mainframe-based) Dumb terminals are directly

connected to mainframe

Centralized model (as opposed distributed model)

Presentation, business logic, and data access are intertwined in one application

Eg. FOXPRO

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J2EE ARCHITECTURE (Cont.)

Single-Tier : Pros & Cons

Pros: – No client side management is required – Data consistency is easy to achieve

Cons: – Functionality (presentation, data model, business logic)

intertwined, difficult for updates and maintenance and code reuse

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J2EE ARCHITECTURE (Cont.)

Two-Tier Clients talking to back end

database

– SQL queries sent, raw data

returned

Presentation, Business logic

and Data Model processing logic

in client application

•Client – Server systems

•Ex:- Unix, mainframes

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J2EE ARCHITECTURE (Cont.)Two-Tier : Pros & Cons Pro:

– DB product independence (compared to single-tier model)

Cons:

– Presentation, data model, business logic are intertwined (at client side),

difficult for updates and maintenance

– Data Model is “tightly coupled” to every client: If DB Schema changes, all

clients break

– Updates have to be deployed to all clients making System maintenance

nightmare

– DB connection for every client, thus difficult to scale

– Raw data transferred to client for processing causes high network traffic

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J2EE ARCHITECTURE (Cont.)

Three-Tier (RPC based) Thinner client: business & data

model separated from presentation

– Business logic and data access

logic reside in middle tier server

while client handles presentation

Middle tier server is now required to

handle system services

– Concurrency control, threading,

transaction, security, persistence,

multiplexing, performance, etc.

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J2EE ARCHITECTURE (Cont.)

Three-Tier (RPC based) : Pros & Cons

Pro:– – Business logic can change more flexibly than 2-tier model

• Most business logic reside in the middle-tier server Cons:

– – Complexity is introduced in the middle-tier server– – Client and middle-tier server is more tightly coupled (than the three-tier

object based model)– – Code is not really reusable (compared to object model based)

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J2EE ARCHITECTURE (Cont.)N-Tier

• N-Tiered architecture: Various components that make up the application are logically separated or distributed across network.

• Client Server Server Database• Eg.. ATM Application

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J2EE ARCHITECTURE (Cont.)

Multi - Tier Architecture in J2EE

• Front end (Client):

• Viewed and manipulated by the users.

• It can live in a Web browser or a standalone application.

• Presents customized information to clients requirements.

• Servlets and JSP is used as Front end development.

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J2EE ARCHITECTURE (Cont.)

Multi - Tier Architecture in J2EE

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J2EE ARCHITECTURE (Cont.)

Multi - Tier Architecture in J2EE

• Middle:

• Contains business logic Ex: Discounts.

• It may contain two sub-tiers:

• Web Tier – It handles communication to client.

• EJB Tier – It manages business logic and access to corporate data.

• Backend (EIS):

• Provides access to various corporate data stores (Databases, E-Mail system,

Legacy systems…)

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MVC ARCHITECTURE

Need for MVC

• J2EE designers were faced with a question of where to place the code in a

distributed environment.

• Reasonable solution: To divide the code and put them closer to their respective

tier.

• This approach reminds a popular, proven 20 year old application design pattern

called MVC.

• MVC is a design pattern for building maintainable applications, introduced by

Xerox researchers.

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MVC ARCHITECTURE(Cont.) What is MVC?

• Technique aimed to make large applications flexible/simple during development and

maintenance.

• It is a design pattern for partitioning of labor into three layers.

• The "model" is the internal workings of the program (the algorithms), the "view" is

how the user sees the state of the model and the "controller" is how the user changes

the state or provides input.

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MVC ARCHITECTURE(Cont.)

What is MVC? (Cont.)

• In this pattern, application flow is mediated by a central controller that

delegates requests to an appropriate handler.

• Controller is the means by which users interacts with web application.

Controller is responsible for input to the model.

• Ex: A pure GUI controller accepts input from the user and instructs the model to

create a view based on that input. If an invalid input is sent to the controller

from the view, model informs controller to direct the view that error occurred

and to tell it to try again.

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MVC Example

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Servlet & HTTP Basics

Servers

• A server is a computer that responds to requests from a client– Typical requests: provide a web page, upload or download a file, send

email

• A server is also the software that responds to these requests; a client could be the browser or other software making these requests

• Typically, your little computer is the client, and someone else’s big computer is the server– However, any computer can be a server– It is not unusual to have server software and client software running

on the same computer

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Apache• Apache is a very popular server

– 66% of the web sites on the Internet use Apache• Apache is:

– Full-featured and extensible– Efficient– Robust– Secure (at least, more secure than other servers)– Up to date with current standards– Open source– Free

• Why use anything else?

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Ports

• A port is a connection between a server and a client– Ports are identified by positive integers– A port is a software notion, not a hardware notion, so there may be

very many of them• A service is associated with a specific port

– Typical port numbers:• 21—FTP, File Transfer Protocol• 22—SSH, Secure Shell• 25—SMTP, Simple Mail Transfer Protocol• 53—DNS, Domain Name Service• 80—HTTP, Hypertext Transfer Protocol• 8080—HTTP (used for testing HTTP)• 7648, 7649—CU-SeeMe• 27960—Quake III

These are the ports of most interest to us

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Common Gateway Interface (CGI)

• CGI stands for Common Gateway Interface• CGI is a standard programming interface to Web

servers that allows building dynamic and interactive Web sites

• CGI is not a programming language. – It is just a set of standards (protocols)– The standards specify how Web-applications can be

executed on the server-side

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Dynamic Pages

Run CGI program………print $result

Return dynamically generated HTML file

Request service

<HEADER><BODY

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A servlet is a small Java program that runs within a Web server. Servlets receive and respond to requests from Web clients, usually across HTTP, the Hyper Text Transfer Protocol. Servlet is an opposite of applet as a server-side applet. Applet is an application running on client while servlet is running on server.

Client Server

Request

Response

Servlet

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What is java servlet ?

Servlets vs. CGI

• Servlet – Java-based CGI– Executed by servlets container

• Golden goals: "performance, flexibility, portability, simplicity and security"

• Faster and thinner– No fork-process execution like Perl– No need to initialize for each request– Only lightweight thread context switching– Built-in multithreading

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Servlets vs. CGI

• Multi-threaded execution allows to:– share data across successive requests– share data between concurrent requests– use hidden fields, cookies, or sessions

• Java supports “write once, run anywhere” paradigm– Easier than unportable Perl

• Java provides enhanced security• Supports all HTTP request methods

– GET, POST, PUT, DELETE, and others

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• Request-response model.

request

responseHTTP

HTML

HTTP Request

<html><head> <body>

…<html><head> <body>

…

Client

Server

Introduction – request-response model

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HTTP Request HTTP Response

Key elements of a “request” stream:

HTTP method (action to be performed).

The page to access (a URL).

Form parameters.

Key elements of a “response” stream:

A status code (for whether the request was successful).

Content-type (text, picture, html, etc…).

The content ( the actual content).

Introduction – what is a request and response

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Where does Servlet come into the picture?

Web ServerApplication

Helper Application

Web Server machine

I can serve only static HTML

pages

Not a problem. I can handle

dynamic requests.

“The Helper Application is nothing but a SERVLET”

Introduction – What is a Servlet

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• What is a Web Container?

GET.…..

Web Server

ServletWeb Container

GET.…..

GET.…..

request

Client

Servlet Architecture -Web Container

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• How does the Container handle a request?

Web Container

Servlet

Thread

Service()

doGet()

<Html> <Body> …….

</Body> </Html>

request

response

response

Web Server

Http request

Client

Servlet Architecture – Web Container

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What is the role of Web Container ?

• Communication Support

• Lifecycle Management

• Multi-threading support

• Security

• JSP Support

The CONTAINER

S1

S3

S4

S2

JSP1

The container can contain multiple Servlets & JSPs within it

Servlet Architecture – Web Container

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• How does the Container know which Servlet the client has requested for?

A Servlet can have 3 names

Client known URL name

Deployer known secret internal name

Actual file name

<web-app> ……… <servlet> <servlet-name>LoginServ</servlet-name> <servlet-class>com.Login</servlet-class> </servlet>

<servlet-mapping> <servlet-name>LoginServ</servlet-name> <url-pattern>/Logon</url-pattern> </servlet-mapping> ……….. ………..</web-app>

Web.xml

Servlet Architecture – Deployment Descriptor

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• The Servlet lifecycle is simple, there is only one main state – “Initialized”.

Initialized

Does not exist

constructor()

init()destroy()

Service()

Servlet Lifecycle

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GenericServlet

HttpServlet

Your Servlet

Servlet

Interface

Abstract class

Abstract class

Concrete class

If not overridden, implements init() method from the ‘Servlet’ interface,

If not overridden, implements service()method.

We implement the HTTP methods here.

Servlet Lifecycle - Hierarchy

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Understanding Servlet API

Packages in the Java Servlet API 2.1

http://java.sun.com/products/servlet/2.1/servlet-2.1.pdfIETE

Understanding Servlet API (Contd.)

Packages in the Java Servlet API 2.1

http://java.sun.com/products/servlet/2.1/servlet-2.1.pdfIETE

When is it called

What it’s for Do you override it

init() The container calls the init() before the servlet can service any client requests.

To initialize your servlet before handling any client requests.

Possibly

service() When a new request for that servlet comes in.

To determine which HTTP method should be called.

No. Very unlikely

doGet() or doPost()

The service() method invokes it based on the HTTP method from the request.

To handle the business logic.

Always

Servlet Lifecycle – 3 big moments

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Using the PrintWriter

• From here on, it’s just a matter of using our PrintWriter, named out, to produce the Web page

• First we create a header string:String docType = "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " + "Transitional//EN\">\n";– This line is technically required by the HTML spec– Browsers mostly don’t care, but HTML validators do care

• Then use the println method of out one or more times out.println(docType + "<HTML>\n" + "<HEAD> ... </BODY></HTML>");

• And we’re done!

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A “Hello World” servlet

public class HelloServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); String docType = "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " + "Transitional//EN\">\n"; out.println(docType + "<HTML>\n" + "<HEAD><TITLE>Hello</TITLE></HEAD>\n" + "<BODY BGCOLOR=\"#FDF5E6\">\n" + "<H1>Hello World</H1>\n" + "</BODY></HTML>"); }} Don’t worry, we’ll take this a little at a time!

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• The HTTP request method determines whether doGet() or doPost() runs.

GET (doGet()) POST (doPost())

HTTP Request

The request contains only the request line and HTTP header.

Along with request line and header it also contains HTTP body.

Parameter passing

The form elements are passed to the server by appending at the end of the URL.

The form elements are passed in the body of the HTTP request.

Size The parameter data is limited (the limit depends on the container)

Can send huge amount of data to the server.

Idempotency

GET is Idempotent POST is not idempotent

Usage Generally used to fetch some information from the host.

Generally used to process the sent data.

Request and Response – GET v/s POST

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Request

Can the Servlet Serve the request?

Send resource

Yes

Does the Servlet knowWho can serve?

Error page

Send Redirect

Request Dispatcher

No

Yes

No

Request and Response – The response

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Servlet 2Servlet 1 Servlet 3 JSP 1

Servlet Context

Servlet Config Servlet ConfigServlet Config Servlet Config

Being a Web Container – Servlet Config and Context

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• What are init parameters?• Difference between Servlet Context and Config Init

parameters

Context Init Parameters Servlet Init Parameters

Scope Scope is Web Container Specific to Servlet or JSP

Servlet code

getServletContext() getServletConfig()

Deployment Descriptor

Within the <web-app> element but not within a specific <servlet> element

Within the <servlet> element for each specific servlet

Being a Web Container – init parameters

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• What exactly, is an attribute?• Difference between Attributes and parameters

Attributes Parameters

TypesContextRequestSession

ContextRequest

Servlet Init

Method to set

setAttribute(String, Object)We cannot set Init parameters.

Return type Object String

Method to get

getAttribute(String)getInitParameter

(String)

Being a Web Container - Attributes

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Q & A

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