Java Servlets

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Java Servlets

• By
• Abdulmalik S. Al-Gahmi

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Outline

• Overview of Servlet Technology
• The Servlet API
• HttpServlet Class
• HttpServletRequest Interface
• HttpServletResponse Interface
• Handling HTTP GET Requests
• Handling HTTP POST Requests
• Session Tracking
• Hidden variables
• Cookies
• Session Tracking with HttpSession

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Introduction

• Client-server relationship
• Client request action
• Server performs action, responds to client
• This view is foundation of servlets
• Highest-level view of networking
• Servlet extends functionality of server
• Useful for database-intensive applications
• Thin clients - little client-side support needed
• Server controls database access
• Logic code written once, on server

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Overview of Servlet Technology

• HyperText Transfer Protocol (HTTP) Basis for WWW
  and Uses URLs (Uniform/Universal Resource
  Locators) which
• Locates data on internet
• Represent files or directories
• Can represent tasks like database lookups
• Servlets
• Analog to applets
• Execute on server's machine, supported by most
  web servers
• Demonstrate communication via HTTP protocol
• Client sends HTTP request
• Server receives request, servlets process it
• Results returned (HTML document, images, binary
  data)

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The Servlet API(1)

• Servlet interface
• Implemented by all servlets
• Many methods invoked automatically by server
• Similar to applets (paint, init, start, etc.)
• abstract classes that implement Servlet
• GenericServlet (javax.servlet)
• HTTPServlet (javax.servlet.http)
• Methods
• void init( ServletConfig config )
• Automatically called, argument provided

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The Servlet API (2)

• Methods
• ServletConfig getServletConfig()
• Returns reference to object, gives access to
  config info
• void service ( ServletRequest request,
  ServletResponse response )
• Key method in all servlets
• Provide access to input and output streams
• Read from and send to client
• void destroy()
• Cleanup method, called when servlet exiting

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HttpServlet Class

• HttpServlet
• Base class for web-based servlets
• Overrides method service
• Request methods
• GET - retrieve HTML documents or image
• POST - send server data from HTML form
• Methods doGet and doPost respond to GET and POST
• Called by service
• Receive HttpServletRequest and HttpServletResponse
  (return void) objects

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HttpServletRequest Interface

• HttpServletRequest interface
• Object passed to doGet and doPost
• Extends ServletRequest
• Methods
• String getParameter( String name )
• Returns value of parameter name (part of GET or
  POST)
• Enumeration getParameterNames()
• Returns names of parameters (POST)
• String getParameterValues( String name )
• Returns array of strings containing values of a
  parameter
• Cookie getCookies()
• Returns array of Cookie objects, can be used to
  identify client

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HttpServletResponse Interface

• HttpServletResponse
• Object passed to doGet and doPost
• Extends ServletResponse
• Methods
• void addCookie( Cookie cookie )
• Add Cookie to header of response to client
• ServletOutputStream getOutputStream()
• Gets byte-based output stream, send binary data
  to client
• PrintWriter getWriter()
• Gets character-based output stream, send text to
  client
• void setContentType( String type )
• Specify MIME type of the response
• Helps display data

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Handling HTTP GET Requests

• HTTP GET requests
• Usually gets content of specified URL
• Usually HTML document (web page)
• Method doGet
• Responds to GET requests
• Default action BAD_REQUEST error (file not
  found)
• Override for custom GET processing
• Arguments represent client request and server
  response!"

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Handling HTTP POST Requests

• HTTP POST
• Used to post data to server-side form handler
  (i.e. surveys)
• Both GET and POST can supply parameters
• doPost
• Responds to POST requests (default BAD_REQUEST)
• Same arguments as doGet (client request, server
  response)

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Session Tracking

• Web sites
• Many have custom web pages functionality
• Custom home pages - http//my.yahoo.com/
• Shopping carts
• Marketing
• HTTP protocol does not support persistent
  information
• Cannot distinguish clients
• Distinguishing clients
• Hidden variables
• Cookies
• Session Tracking

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Cookies

• Cookies
• Small files that store information on client's
  computer
• Servlet can check previous cookies for
  information
• Header
• In every HTTP client-server interaction
• Contains information about request (GET or POST)
  and cookies stored on client machine
• Response header includes cookies servers wants to
  store
• Age
• Cookies have a lifespan
• Can set maximum age
• Cookies can expire and are deleted

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Session Tracking with HttpSession

• HttpSession is an alternative to cookies. It
  keeps the session data available until browsing
  ends
• Methods
• getSession( createNew )
• Class HttpServletRequest
• Returns client's previous HttpSession object
• putvalue( name, value )
• Adds a name/value pair to object
• getValueNames()
• Returns array of Strings with names
• getValue( name )
• Returns value of name as an Object
• Cast to proper type