Download Servlets and Web Services

Servlets
and a little bit of
Web Services
Russell Beale
Overview
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In general
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Servlets
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Provide remote access to applications
What are servlets
How can we use them
Web Services
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What are web services…
Objectives
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Learn about using servlets as one way of providing web based
interfaces to databases and other applications.
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Learn how to create and deploy servlets using the NetBeans IDE and
Tomcat server
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Learn about Web Services and their advantages in relation to providing
web based interfaces to databases and other applications
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See how to create and deploy Web Services using Java, Apache
Tomcat, and Apache Axis
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Be aware of other tools for developing, deploying, and consuming web
services
Providing remote access
RMI
CORBA
Application
Web/HTTP
DCOM
Access over the Web
Web
Application
Web
Application
Web Server
HTTP
HTTP
Web
Browser
Web
Service
Client
Web Pages
Application
Interface
Servlets and Web Services
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Servlets
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providing generic access to an application,
using a web interface
we need to build both client and server
Web Services
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providing generic access with a defined API
allows custom interface at the client
we can just build the server
Using servlets
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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.
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That Web page is then
displayed on the user's
browser (6).
(bit like CGI scripts, bit
like applets)
(from Sun)
Servlets
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Servlets are server-side resources
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Servlets are Java objects that act as compact web
servers
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Can support all protocols, but are not as
flexible/powerful as full servers
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Need to run inside a web server that supports
servlets
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Take in requests re-directed from the web-server,
write HTML back to the client
Advantages of servlets
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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
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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)
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e.g. an online shopping cart which
manages baskets for many concurrent
customers and maps every request to the
right customer.
Servlets
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Servlets are part of J2EE
All servlets implement interface
javax.servlet.Servlet
We will be using
javax.servlet.http.HttpServlet
HTTP protocol
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8 request methods:
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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
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init()
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service()
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set up the servlet
respond to requests, after init()
destroy()
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shutdown the servlet
Using HttpServlet
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By extending HttpServlet, we only have
to over-ride the methods we need to
E.g., doGet(), doPost()
HelloWorld servlet
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Using NetBeans, we can easily create servlets
under Tomcat
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Tomcat is a Java server that supports servlets
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Tomcat is bundled with NetBeans IDE
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HelloWorld servlet
POST and GET
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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…
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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…
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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
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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
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POST requests call the doPost() method
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Put code in this method, or call another one
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Post data is stored on the request object
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PostExample.htm
Storing Data
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We often want to store some data
about the user and their requests
We can do this in 2 ways:
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Client-side - cookies
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Server-side – session data, database etc
What are cookies?
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HTTP protocol is stateless
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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
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HTTP header that contains text string
Two sorts
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Session
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Temporary, erased when you close browser
Often used by e-commerce sites for
shopping carts
Persistent
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Written to hard drive
Remain until erased or expire
Used to store user preferences
Sessions
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Live on the server
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Actually built on top of cookies or URL
rewritin, but you don’t have to bother with
this
HttpSession object
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Stores all the information for a session
Saves you having to access the cookies
yourself
Servlets and JSP
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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