Servlets and a little bit of Web Services

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Servletsand a little bit ofWeb Services

• Russell Beale

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Overview

• In general
• Provide remote access to applications
• Servlets
• What are servlets
• How can we use them
• Web Services
• What are web services

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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

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Providing remote access
RMI
CORBA
Application
DCOM
Web/HTTP
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Access over the Web
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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

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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)

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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

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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

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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.

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Servlets

• Servlets are part of J2EE
• All servlets implement interface
  javax.servlet.Servlet
• We will be using javax.servlet.http.HttpServlet

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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

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Lifecycle

• init()
• set up the servlet
• service()
• respond to requests, after init()
• destroy()
• shutdown the servlet

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Using HttpServlet

• By extending HttpServlet, we only have to
  over-ride the methods we need to
• E.g., doGet(), doPost()

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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

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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 "namevalue"
  pairs.

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HTML forms

• ltform action "PostExample" methodPOSTgt
• ltinput typetext size20 namefirstnamegt
• ltbrgt
• ltinput typetext size20 namelastnamegt
• ltbrgt
• ltinput typesubmitgt
• lt/formgt

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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.

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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

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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

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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

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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

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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

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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

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Sessions

• Live on the server
• Actually built on top of cookies or URL rewritin,
  but you dont have to bother with this
• HttpSession object
• Stores all the information for a session
• Saves you having to access the cookies yourself

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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