Saving Client State

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Saving Client State

• Session Tracking Maintain state about series of
  requests from same client over time
• Using Cookies Clients hold small amount of
  their state information. Servlets use
  information in cookie

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Session Tracking

• Bookstore uses to keep track of books ordered
• Obtain session object for a user (HttpSession)
  (boolean for creation)
• Store or get data from object
• Invalidate session
• Session properties (session identifier)
• Key-value pairs for application data

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Steps

• Obtain Session for a user
• Store or get data from session object
• Invalidate the session (manual or automatic)
• Bookstore uses session to keep track of books a
  user orders
• Shared by all servlets in application

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Handling All Browsers

• Session Tracking uses cookies by default to
  associate session id with user
• If browser doesnt support cookies, must use URL
  rewriting (not supported by servletrunner)
• Session Id included in links session id sent as
  part of URL

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public class CatalogServlet extends HttpServlet
public void doGet (HttpServletRequest
request,
HttpServletResponse response) throws
ServletException, IOException
// Get the user's session and shopping cart,
the Writer, etc. ... // then
write the data of the response
out.println("lthtmlgt" ...) ...
// Get the catalog and send it, nicely
formatted BookDetails books
database.getBooksSortedByTitle()
... for(int i0 i lt numBooks i)
... //Print out
info on each book in its own two rows
out.println("lttrgt" ...
"lta href\""
response.encodeUrl("/servlet/bookdetails?bookId"

bookId) "\"gt ltstronggt"
booksi.getTitle() "
lt/stronggtlt/agtlt/tdgt" ...
"lta href\""
response.encodeUrl("/servlet/catalog?Buy"
bookId) "\"gt Add to
Cart lt/agtlt/tdgtlt/trgt"

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Using Cookies

• Key-value pair
• Way for server to store information on client
• Server appends to HTTP response headers
• Client appends to HTTP request headers
• Cookies are single-valued

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Procedure

• To send Cookie
• Instantiate Cookie Object
• Set attributes
• send the cookie
• Get information from Cookie
• Retrieve all cookies from the users request
• Find cookie with name interested in
• Get values from cookies

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Cookie Drawbacks

• Can only be strings
• take up client disk space
• browsers limit their number and size

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public void doGet (HttpServletRequest request,
HttpServletResponse
response) throws ServletException, IOException
BookDBServlet database
(BookDBServlet) getServletConfig().get
ServletContext().getServlet("bookdb")
// Check for pending adds to the shopping cart
String bookId request.getParameter("Buy")
//If the user wants to add a book,
remember it by adding a cookie if (bookId
! null) Cookie getBook new
Cookie("Buy", bookId) ...
// set content-type header before accessing
the Writer response.setContentType("text/h
tml") // now get the writer and write
the data of the response PrintWriter out
response.getWriter() out.println("lthtmlgt"
"ltheadgtlttitlegt Book
Catalog lt/titlegtlt/headgt" ...) ...
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public void doGet (HttpServletRequest request,
HttpServletResponse
response) throws ServletException, IOException
... //If the user wants to
add a book, remember it by adding a cookie
if (values ! null) bookId
values0 Cookie getBook new
Cookie("Buy", bookId)
getBook.setComment("User wants to buy this book "
"from the
bookstore.") ...
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public void doGet (HttpServletRequest request,
HttpServletResponse
response) throws ServletException, IOException
... / Handle any pending deletes
from the shopping cart / String bookId
request.getParameter("Remove") ...
if (bookId ! null) //
Find the cookie that pertains to the book to
remove ... //
Delete the cookie by setting its maximum age to
zero thisCookie.setMaxAge(0)
... // also
set content type header before accessing the
Writer response.setContentType("text/html"
) PrintWriter out response.getWriter()
//Print out the response
out.println("lthtmlgt ltheadgt"
"lttitlegtYour Shopping Cartlt/titlegt" ...)
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public void doGet (HttpServletRequest request,
HttpServletResponse
response) throws ServletException, IOException
... //If the user wants to
add a book, remember it by adding a cookie
if (values ! null) bookId
values0 Cookie getBook new
Cookie("Buy", bookId)
getBook.setComment("User has indicated a desire "
"to buy this
book from the bookstore.")
response.addCookie(getBook)
...
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public void doGet (HttpServletRequest request,
HttpServletResponse
response) throws ServletException, IOException
... / Handle any pending deletes
from the shopping cart / String bookId
request.getParameter("Remove") ...
if (bookId ! null)
// Find the cookie that pertains to the
book to remove Cookie cookies
request.getCookies() ...
// Delete the book's cookie by setting its
maximum age to zero
thisCookie.setMaxAge(0)
// also set content type header before
accessing the Writer response.setContentTy
pe("text/html") PrintWriter out
response.getWriter() //Print out the
response out.println("lthtmlgt ltheadgt"
"lttitlegtYour Shopping
Cartlt/titlegt" ...)
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public void doGet (HttpServletRequest request,
HttpServletResponse
response) throws ServletException, IOException
... / Handle any pending
deletes from the shopping cart / String
bookId request.getParameter("Remove")
... if (bookId ! null)
// Find the cookie that pertains to that
book Cookie cookies
request.getCookies() for(i0 i lt
cookies.length i) Cookie
thisCookie cookiei if
(thisCookie.getName().equals("Buy")
thisCookie.getValue().equals(bookId))
// Delete the cookie by
setting its maximum age to zero
thisCookie.setMaxAge(0)
// also set
content type header before accessing the Writer
response.setContentType("text/html")
PrintWriter out response.getWriter()
//Print out the response
out.println("lthtmlgt ltheadgt"
"lttitlegtYour Shopping Cartlt/titlegt" ...)
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HTTP 1.0 Tokens

• Tokens dont contain special characters reserved
  by RFC2068
• Alphanumeric OK
• URLEncoder Converts string to MIME format
  x-www-form-urlencoded
• Converts spaces to
• Other characters to 3 character hex number xy

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Applets and Servlets

• Applet programming problem not servlet
• Let applet use http to communicate to servlet
• servlet can communicate with applet in text,
  binary, or html

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ltHTMLgt ltHEADgt ltTITLEgtElizalt/TITLEgt lt/HEADgt ltBODYgt
ltAPPLET NAME"Eliza" CODE"ElizaApplet" WIDTH400
HEIGHT60gt ltPARAM NAME"server" VALUE"http//192.
168.0.1728080/servlet/eliza"gt ltPARAM
NAME"method"VALUE"GET"gtlt/APPLETgt lt/BODYgt lt/HTMLgt