Servletsand a little bit ofWeb Services

Russell Beale

Overview In general

Provide remote access to applications

Servlets What are servlets How can we use them

Web Services What are web services…

Objectives Learn about using servlets as one way of providing web based

interfaces to databases and other applications.

Learn how to create and deploy servlets using the NetBeans IDE andTomcat server

Learn about Web Services and their advantages in relation to providingweb based interfaces to databases and other applications

See how to create and deploy Web Services using Java, ApacheTomcat, and Apache Axis

Be aware of other tools for developing, deploying, and consuming webservices

Providing remote access

RMI CORBA

DCOMWeb/HTTP

Application

Access over the WebWeb

Application

Web Server

WebApplication

WebBrowser

HTTP

WebServiceClient

HTTP

Web Pages ApplicationInterface

Servlets and Web Services

Servlets providing generic access to an application,

using a web interface we need to build both client and server

Web Services providing generic access with a defined API allows custom interface at the client we can just build the server

Using servlets A user (1) requests some

information by filling out aform containing a link to aservlet and clicking theSubmit button (2).

The server (3) locates therequested servlet (4).

The servlet then gathers theinformation needed to satisfythe user's request andconstructs a Web page (5)containing the information.

That Web page is thendisplayed on the user'sbrowser (6).

(bit like CGI scripts, bitlike applets)

(from Sun)

Servlets Servlets are server-side resources

Servlets are Java objects that act as compact webservers

Can support all protocols, but are not asflexible/powerful as full servers

Need to run inside a web server that supportsservlets

Take in requests re-directed from the web-server,write HTML back to the client

Advantages of servlets

Based on Java: convenient & powerful,can talk directly to the server

Efficient – lightweight Java processes,servlet code loads only once

Free/very cheap

Typical uses

Processing and/or storing datasubmitted by an HTML form.

Providing dynamic content from, forexample, a database query

Managing state information on top ofHTTP (which is stateless) e.g. an online shopping cart which

manages baskets for many concurrentcustomers and maps every request to theright customer.

Servlets

Servlets are part of J2EE

All servlets implement interfacejavax.servlet.Servlet

We will be usingjavax.servlet.http.HttpServlet

HTTP protocol

8 request methods: GET – retrieve content POST – send data, retrieve content HEAD – retrieve headers only PUT – upload content DELETE – remove content TRACE – echos the request, showing servers etc OPTIONS – returns list of supported methods CONNECT – used with SSL proxy tunnels

Lifecycle

init() set up the servlet

service() respond to requests, after init()

destroy() shutdown the servlet

Using HttpServlet

By extending HttpServlet, we only haveto over-ride the methods we need to

E.g., doGet(), doPost()

HelloWorld servlet

Using NetBeans, we can easily create servletsunder Tomcat

Tomcat is a Java server that supports servlets

Tomcat is bundled with NetBeans IDE

HelloWorld servlet

POST and GET GET and POST allow information to be

sent back to the webserver from abrowser (or other HTTP client for thatmatter)

Imagine that you have a form on aHTML page and clicking the "submit"button sends the data in the form backto the server, as "name=value" pairs.

HTML forms<form action= "PostExample" method=POST>

<input type=text size=20 name=firstname><br>

<input type=text size=20 name=lastname><br><input type=submit></form>

GET… Choosing GET as the "method" will append all

of the data to the URL and it will show up inthe URL bar of your browser.

The amount of information you can send backusing a GET is restricted as URLs can only be1024 characters.

POST… A POST will send the information

through a socket back to the webserverand it won't show up in the URL bar.

It is stored on the request object You can send much more information to

the server this way not restricted to textual data - you can

send files and even binary data such asserialized Java objects

Handling GET requests

GET requests call the doGet() methodon your servlet

Put code in that method to handle GET,or call another method to do it

GET can pass in data through URLencoding

Handling POST requests

POST requests call the doPost() method

Put code in this method, or call another one

Post data is stored on the request object

PostExample.htm

Storing Data

We often want to store some dataabout the user and their requests

We can do this in 2 ways:

Client-side - cookies

Server-side – session data, database etc

What are cookies?

HTTP protocol is stateless Browser contacts server ata URL, requests a page,

provides its capabilities Server sends info to client Connection closed

So to mark one visitor to track visit to site,need to store a piece of information on theclient side

This is the cookie HTTP header that contains text string

Two sorts

Session Temporary, erased when you close

browser Often used by e-commerce sites for

shopping carts Persistent

Written to hard drive Remain until erased or expire Used to store user preferences

Sessions

Live on the server Actually built on top of cookies or URL

rewritin, but you don’t have to bother withthis

HttpSession object Stores all the information for a session Saves you having to access the cookies

yourself

Servlets and JSP

Putting large amounts of HTML intoservlets is a bit cumbersome

JSP pages let you use Java code directlyin a HTML document

The Java code is then executed as aservlet at runtime