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 and Tomcat server

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

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

Be aware of other tools for developing, deploying, and consuming web services

Providing remote access

RMI CORBA

DCOMWeb/HTTP

Application

Access over the WebWeb

Application

Web Server

WebApplication

WebBrowser

HTTP

WebServiceClient

HTTP

Web PagesApplicationInterface

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 a form containing a link to a servlet and clicking the Submit button (2).

The server (3) locates the requested servlet (4).

The servlet then gathers the information needed to satisfy the user's request and constructs a Web page (5) containing the information.

That Web page is then displayed on the user's browser (6).

(bit like CGI scripts, bit like applets)

(from Sun)

Servlets Servlets are server-side resources

Servlets are Java objects that act as compact web servers

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

Need to run inside a web server that supports servlets

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 data submitted by an HTML form.

Providing dynamic content from, for example, a database query

Managing state information on top of HTTP (which is stateless) e.g. an online shopping cart which manages baskets for many concurrent customers and maps every request to the right customer.

Servlets

Servlets are part of J2EE

All servlets implement interface javax.servlet.Servlet

We will be using javax.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 have to over-ride the methods we need to

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

HelloWorld servlet Using NetBeans, we can easily create servlets under 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 a browser (or other HTTP client for that matter)

Imagine that you have a form on a HTML page and clicking the "submit" button sends the data in the form back to 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 in the URL bar of your browser.

The amount of information you can send back using a GET is restricted as URLs can only be 1024 characters.

POST… A POST will send the information through a socket back to the webserver and 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 as serialized Java objects

Handling GET requests GET requests call the doGet() method on your servlet

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

GET can pass in data through URL encoding

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 data about 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 the client 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 with this

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 into servlets is a bit cumbersome

JSP pages let you use Java code directly in a HTML document

The Java code is then executed as a servlet at runtime