EBIZ 5535 Lecture 2

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EBIZ 5535Lecture 2

• Form Data and Headers

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Outline

• Handling Form Data
• Reading HTTP request headers
• Setting HTTP status codes
• Specifying HTTP response headers

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Handling Form Data

• URLs like http//host/path?userJeffVBoriginbwi
  destlax
• The part after the ? is form data attached to a
  get request (or appearing on lines after the
  first in a post request
• Unlike CGI, Java servlets handle the parsing of
  form data for you
• You simply call the getParameter method of the
  HttpServletRequest, supplying the parameter name
  as an argument
• String parm1 request.getParameter(user)

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Getting Parameters

• Parameter names are case sensitive
• The getParameter method is called the same way
  whether a get or a post
• An empty String is returned if the parameter
  exists but has no value
• and null is returned if there was no such
  parameter

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getParameterValues

• getParameterValues can be used instead of
  getParameter
• This returns an array of strings
• It is sometimes useful to get a full list
• Or use getParameterNames for this
• returns an Enumeration, each entry of which can
  be cast to a String and used in a getParameter
  call

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Example of Getting Parameters

• import
• public class ThreeParams extends HttpServlet
• public void doGet(HttpServletRequest request,
  HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• String title "Reading Three Request
  Parameters"
• out.println(ServletUtilities.headWithTitle(ti
  tle) "ltBODYgt\n" "ltH1 ALIGNCENTERgt" title
  "lt/H1gt\n" "ltULgt\n" " ltLIgtparam1 "
  request.getParameter("param1") "\n" "
  ltLIgtparam2 " request.getParameter("param2")
  "\n" " ltLIgtparam3 " request.getParameter("par
  am3") "\n" "lt/ULgt\n" "lt/BODYgtlt/HTMLgt")

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Can also have a post method

• public void doPost
• (HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
  doGet(request, response)

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Listing All Form Data

• PostForm.html and ShowParams.java example

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Steps Typically Performed to Handle an HTTP
request
HTTP request (e.g., get)
Read request headers
Decide on a response
Set status code and response headers
Generate HTML response
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Reading HTTP Request Headers

• When an HTTP client (e.g. a browser) sends a
  request, it is required to supply a request line
• Also sends headers, which are optional except for
  Content-Length with posts
• A servlet can read these headers

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Some Common Headers

• Accept The MIME types the browser prefers
• Accept-Charset The character set the browser
  expects.
• Accept-Encoding The types of data encodings (such
  as gzip) the browser knows how to decode
• Accept-Language The language the browser is
  expecting, in case the server has versions in
  more than one language

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Common Headers contd

• Authorization Authorization info, usually in
  response to a WWW-Authenticate header from the
  server.
• Connection Use persistent connection?
• Content-Length (for POST messages, how much data
  is attached)
• Cookie (well talk about this later)
• Host (host and port as listed in the original
  URL)
• If-Modified-Since (only return documents newer
  than this, otherwise send a 304 "Not Modified"
  response)

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Reading Request Headers in Servlets

• Just call the getHeader method of the
  HttpServletRequest
• returns a String if the header was supplied on
  this request, null otherwise
• The getCookies method returns the contents of the
  Cookie header, parsed and stored in an array of
  Cookie objects.
• The getAuthType and getRemoteUser methods break
  the Authorization header into its components
• The getDateHeader and getIntHeader methods read
  the specified header and then convert them to
  Date and int values, respectively

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Example

• public class ShowRequestHeaders extends
  HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• String title "Servlet Example Showing
  Request Headers"
• out.println(ServletUtilities.headWithTitle(tit
  le)
• "ltBODY BGCOLOR\"FDF5E6\"gt\n"
• "ltH1 ALIGNCENTERgt" title
  "lt/H1gt\n"
• "ltBgtRequest Method lt/Bgt"
• request.getMethod() "ltBRgt\n"
• "ltBgtRequest URI lt/Bgt"
• request.getRequestURI()
  "ltBRgt\n"
• "ltBgtRequest Protocol lt/Bgt"
• request.getProtocol()
  "ltBRgtltBRgt\n"
• "ltTABLE BORDER1 ALIGNCENTERgt\n"
  
• "ltTR BGCOLOR\"FFAD00\"gt\n"
• "ltTHgtHeader NameltTHgtHeader
  Value")

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• Enumeration headerNames request.getHeaderNames()
  
• while(headerNames.hasMoreElements())
• String headerName (String)headerNames.next
  Element()
• out.println("ltTRgtltTDgt" headerName)
• out.println(" ltTDgt"
• request.getHeader(headerName))
• out.println("lt/TABLEgt\nlt/BODYgtlt/HTMLgt")
• public void doPost(HttpServletRequest request,
• HttpServletResponse
  response)
• throws ServletException, IOException
• doGet(request, response)

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HTTP Status Codes

• When a Web server responds to a request from a
  browser or other Web client, the response
  typically begins with a status line
• HTTP/1.1 200 OK Content-Type text/plain
• Hello World

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The Status Line

• The status line consists of
• the HTTP version
• an integer that is interpreted as a status code
• and a very short status message
• In most cases, all of the headers are optional
  except for Content-Type, which specifies the MIME
  type of the document that follows.
• Servlets can perform a variety of tasks by
  manipulating the status line and the response
  headers

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Setting HTTP Status Codes

• The server sets
• The HTTP version
• The message (depending on the status code)
• A Servlet need only set the status code
• setStatus method of HttpServletResponse
• Status codes (passed to setStatus) are int
  constants named from the status message
• SC_OK
• SC_NOT_FOUND

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Caution

• Always set status code before sending any content
• Status code defaults to SC_OK

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Some Tasks Performed by Manipulating the Status
Line

• Forwarding the user to other sites
• Indicating that the attached document is
• an image
• Adobe Acrobat file
• or (most commonly) HTML file
• Telling the user that a password is required to
  access the document

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Specifying HTTP Response Headers

• A response from a Web server consists of
• a status line
• one or more response headers
• a blank line
• the document
• Specifying headers can be used to indicate
  errors, but can also be useful under normal
  circumstances
• can be used to specify cookies
• to supply the modification date (for caching)
• to instruct the browser to reload the page after
  a designated interval
• to say how long the file is so that persistent
  HTTP connections can be used

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Specifying Headers contd

• The most general way to specify headers is by the
  setHeader method of HttpServletResponse, which
  takes two strings
• the header name and the header value
• Like setting the status codes, this must be done
  before any document content is sent.
• Special methods of HttpServletResponse
• sendError
• sendRedirect

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Some Other Interesting Response Headers
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Example GZipped Web Pages

• import
• public class EncodedPage extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• String encodings request.getHeader("Accept-E
  ncoding")
• String encodeFlag request.getParameter("enco
  ding")
• PrintWriter out
• String title
• if ((encodings ! null)
• (encodings.indexOf("gzip") ! -1)
• !"none".equals(encodeFlag))
• title "Page Encoded with GZip"
• OutputStream out1 response.getOutputStream
  ()
• out new PrintWriter(new
  GZIPOutputStream(out1), false)
• response.setHeader("Content-Encoding",
  "gzip")
• else
• title "Unencoded Page"

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• out.println(ServletUtilities.headWithTitle(title)
  
• "ltBODY BGCOLOR\"FDF5E6\"gt\n"
• "ltH1 ALIGNCENTERgt" title
  "lt/H1gt\n")
• String line "Blah, blah, blah, blah, blah.
  "
• "Yadda, yadda, yadda, yadda."
• for(int i0 ilt10000 i)
• out.println(line)
• out.println("lt/BODYgtlt/HTMLgt")
• out.close()