EBIZ 5535 Lecture 3

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EBIZ 5535Lecture 3

• Handling Cookies

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Outline

• Overview of Cookies
• The Servlet Cookie API
• Reading Incoming Cookies
• Creating Cookies
• Placing Cookies in the Response Header
• Long-Lived Cookies
• Example
• JSP

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Overview of Cookies

• Cookies are small bits of textual information
  that
• a server sends to a browser
• that the browser returns unchanged when visiting
  the same Web site or domain later
• By having the server read information it sent the
  client previously, the site can provide visitors
  with a number of conveniences

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Conveniences from Cookies

• Identifying a user during an e-commerce session
• Avoiding username and password
• Customizing a site
• Focusing advertising

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The Servlet Cookie API

• To send cookies to the client, a servlet would
• Create one or more cookies with the appropriate
  names and values via
• new Cookie(name, value)
• Set any desired optional attributes via
  cookie.setXxx
• Add the cookies to the response headers via
  response.addCookie(cookie)

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Reading Incoming Cookies

• Call request.getCookies(), which returns an array
  of Cookie objects
• Loop down this array until you find the one whose
  name (getName) matches
• Then call getValue on that Cookie to see the
  value associated with that name

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Creating Cookies

• A cookie is created by calling the Cookie
  constructor, which takes two strings
• The cookie name
• The cookie value
• Neither the name nor the value should contain
  whitespace or any of
• ( ) , " / ? _at_

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Reading and Specifying Cookie Attributes

• Before adding the cookie to the outgoing headers,
  you can look up or set attributes of the cookie.
  Here's a summary
• getName/setName
• getValue/setValue
• getComment/setComment
• getDomain/setDomain
• getPath/setPath
• getSecure/setSecure
• getVersion/setVersion

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Placing Cookies in the Response Headers

• The cookie is added to the Set-Cookie response
  header by means of the addCookie method of
  HttpServletResponse
• Cookie userCookie
• new Cookie("user", "uid1234")
• response.addCookie(userCookie)

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Reading Cookies from the Client

• Call getCookies on the HttpServletRequest
• public static String
• getCookieValue(Cookie cookies,
• String cookieName,
• String defaultValue) for(int i0
  iltcookies.length i)
• Cookie cookie cookiesi
• if (cookieName.equals(cookie.getName()))
  return(cookie.getValue()) return(defaultValue)
  

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Long-Lived Cookies

• Cookies that persist between browser sessions
• import javax.servlet.http.
• public class LongLivedCookie extends Cookie
  public static final int
• SECONDS_PER_YEAR 606024365
• public LongLivedCookie(String name,
• String value)
• super(name, value)
• setMaxAge(SECONDS_PER_YEAR)

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Example A Customized Search Engine

• SearchEnginesFrontEnd.java
• CustomizedSearchEngine.java

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Session Tracking

• HTTP is a stateless protocol
• This makes many applications problematic
• Example shopping carts
• Typical solutions
• Cookies
• URL Rewriting
• Hidden Form Fields
• Servlets provide a nice solution
• HttpSession API

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Session Tracking

• Looking up the HttpSession object associated with
  the current request
• HttpSession session request.getSession(true)

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Looking up Information Associated with a Session

• Session objects are automatically associated with
  requesters
• They have a built in data structure that allows
  you to keep any number of key-value pairs with
  the session object

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Example of Looking up Session info

• HttpSession session request.getSession(true)
• ShoppingCart previousItems
• (ShoppingCart)session.getAttribute("previousItems"
  )
• if (previousItems ! null) doSomethingWith(previ
  ousItems)
• else previousItems new ShoppingCart(...)
• doSomethingElseWith(previousItems)

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Associating Information with a Session

• HttpSession session request.getSession(true)
• session.setAttribute("referringPage",
• request.getHeader("Referer"))
• ShoppingCart previousItems (ShoppingCart)session
  .getAttribute("previousItems")
• if (previousItems null)
• previousItems new ShoppingCart(...)
• String itemID request.getParameter("itemID")
• previousItems.addEntry(Catalog.getEntry(itemID))
  
• session.setAttribute("previousItems",
• previousItems)

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Example

• ShowSession.java

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JavaServer Pages

• Lets you separate the dynamic part of your pages
  from the static HTML
• write the regular HTML
• Then enclose the code for the dynamic parts in
  special tags, most of which start with "lt" and
  end with "gt
• Thanks for ordering ltIgtlt request.getParameter("t
  itle") gtlt/Igt

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JSP Files

• .jsp extension
• Store with HTML files
• Jsps are translated to servlets
• Aside from regular HTML, there are three types of
  JSP constructs
• scripting elements
• directives
• actions

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JSP Scripting Elements

• JSP scripting elements let you insert Java code
  into the servlet that will be generated from the
  current JSP page. There are three forms
• Expressions of the form lt expression gt that
  are evaluated and inserted into the output
• Scriptlets of the form lt code gt that are
  inserted into the servlet's service method, and
• Declarations of the form lt! code gt that are
  inserted into the body of the servlet class,
  outside of any existing methods.

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JSP Expressions

• A JSP expression is used to insert Java values
  directly into the output.
• lt Java Expression gt
• The Java expression is evaluated, converted to a
  string, and inserted in the page.
• This evaluation is performed at run-time (when
  the page is requested), and thus has full access
  to information about the request.

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Example of an Expression

• Current time
• lt new java.util.Date() gt
• Predefined variables (to simplify expressions)
• request, the HttpServletRequest
• response, the HttpServletResponse
• session, the HttpSession associated with the
  request (if any) and
• out, the PrintWriter (a buffered version of type
  JspWriter) used to send output to the client

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JSP Scriptlets

• If you want to do something more complex than
  insert a simple expression, use a JSP scriptlet
• lt Java Code gt
• Have access to the same automatically defined
  variables
• lt String queryData request.getQueryString()
  out.println("Attached GET data " queryData)
  gt

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Can Intermix HTML and Scriptlets

• lt if (Math.random() lt 0.5) gt
• Have a ltBgtnicelt/Bgt day!
• lt else gt
• Have a ltBgtlousylt/Bgt day! lt gt

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• if (Math.random() lt 0.5)
• out.println("Have a ltBgtnicelt/Bgt day!")
• else
• out.println("Have a ltBgtlousylt/Bgt day!")

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JSP Declarations

• A JSP declaration lets you define methods or
  fields that get inserted into the main body of
  the servlet class
• outside of the service method processing the
  request
• lt! Java Code gt

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Example

• lt! private int accessCount 0 gt
• Accesses to page since server reboot lt
  accessCount gt

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JSP Directives

• A JSP directive affects the overall structure of
  the servlet class
• lt_at_ directive attribute"value" gt
• Or
• lt_at_ directive attribute1"value1"
• attribute2"value2"
• ...
• attributeN"valueN" gt