Core Servlets chapter 5: HTTP request headers

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Core Servlets chapter 5 HTTP request headers
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Requesting header information

• It is possible that information the servlet needs
  is not in the form data but is in the request
  headers, which contain Accept, Accept-Encoding,
  Connection, Cookie, Host, Referer and User-Agent.
• The servlet needs to explicitly read these.
• To read, call (String) getHeader method of
  HTTPServletRequest with the name of the specific
  header passed in.
• Always check for null on return because the
  header may not have been sent.
• The header names are not case sensitive.

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Requesting header information

• Some headers are used so frequently thay have
  their own specifically defined methods
• getCookies. Will parse and store in an array of
  Cookie objects
• getAuthType, getRemoteUser
• getContentLength an int
• getContentType returns String
• getDateHeader and getIntHeader return Date and
  int values
• getHeaderNames returns an Enumeration of all
  headers
• getHeaders returns an Enumeration of all
  occurences of a header

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The request line itself

• Methods in HTTPServletRequest return information
  from the first line of the request
• getMethod returns GET, POST, or other method
• getRequestURI returns the part of the request
  after the host and port number but before the
  form data
• getQueryString returns form data
• getProtocol returns HTTP/1.0 or HTTP/1.1 and
  should generally be checked before specifying
  response headers specific to HTTP/1.1

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A servlet which returns all header information

• Calling getHeaderNames returns an Enumeration of
  all headers. Looping through this with calls to
  getHeader returns values. These are used to
  populate a table.

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A table of all request header values
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From wikipediaMIME

• Multipurpose Internet Mail Extensions (MIME) is
  an Internet standard that extends the format of
  e-mail to support
• text in character sets other than US-ASCII
• non-text attachments
• message bodies with multiple parts and
• header information in non-ASCII character sets.
• MIME's use, however, has grown beyond describing
  the content of e-mail to describing content type
  in general.

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Understanding request headers

• Accept -all MIME types accepted by the browser..
  For example not all browsers support png format.
• Accept-Charset what char set the broswer can
  use
• Accept-Encoding eg., gzip and zip
• Accept-Language en, en-us, and so on
• Authorization used by client for identification
  when accessing pw/protected pages
• Connection
• Content-length - applicable only to post
  requests, it gives the size of the POST data in
  bytes
• Cookie- returns cookies to servers that sent them
  to the browser Dont read this directly instead
  use request.getCookies()
• Host
• User-Agent The browser or client making the
  request. It can be used to return different
  content to different browsers. For example, you
  might send wml to a phone or html to a
  conventional broswer.
• If-Modified-since
• If-unmodified since
• Referer provides URL of referring webpage.
  Tells you where a request came from, for example,
  tracking advertisers referring customers to your
  site, modifying response if the request came from
  within or outside your site, etc. Many broswers
  filter out this header (Norton, Opera)

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Sending compressed data note from the table
above the browser accepts gzip format
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Using gzip utilities three static functions

• package coreservlets
• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• import java.util.zip.
• public class GzipUtilities
• / Does the client support gzip? /
• public static boolean isGzipSupported
• (HttpServletRequest request)
• String encodings request.getHeader("Accept-E
  ncoding")
• return((encodings ! null)
• (encodings.indexOf("gzip") ! -1))
• / Has user disabled gzip (e.g., for
  benchmarking)? /
• public static boolean isGzipDisabled
• (HttpServletRequest request)
• String flag request.getParameter("disableGzi
  p")
• return((flag ! null) (!flag.equalsIgnoreCa
  se("false")))
• / Return gzipping PrintWriter for response.
  /
• public static PrintWriter getGzipWriter

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A servlet that generates lots of content

• package coreservlets
• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• public class LongServlet extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• // Change the definition of "out"
  depending on whether or not gzip is supported.
• PrintWriter out
• if (GzipUtilities.isGzipSupported(request)
• !GzipUtilities.isGzipDisabled(request))
• out GzipUtilities.getGzipWriter(response)
  
• response.setHeader("Content-Encoding",
  "gzip")
• else out response.getWriter()
  
• // Once "out" has been assigned
  appropriately, the rest of the page has no
  dependencies on the type of writer being used.
• String docType
• "lt!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML
  4.0 "

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Previous example

• In this example, the servlet checked to see if
  the browser accepted a zipped format for content,
  and, if it did, it sent the page using gzip.

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Checking browser type

• In this as in all examples, be sure to check that
  request.getHeader() is not null.
• Do not use this generally, as many browsers allow
  you to change the value sent by user-agent.
• The next example sends browser-specific insults
  to the user.

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The servlet

• package coreservlets
• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• public class BrowserInsult extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• String title, message
• // Assume for simplicity that Netscape and IE
  are the only two browsers
• String userAgent request.getHeader("User-Age
  nt")
• if ((userAgent ! null)
• (userAgent.indexOf("MSIE") ! -1))
• title "Microsoft Minion"
• message "Welcome, O spineless slave to
  the "
• "mighty empire."

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Visiting URL using two different browsers
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Webclient sends no header- I added code to catch
null pointer exception
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Customizing display based on Referer

• The next example checks if the header Referer
  contains one of three strings and displays
  different images accordingly.
• I am not sure where the images are in terms of
  the text. I just found my own gif files.
• This would be useful if you wanted to
• use a display type similar to the sender
• Wanted different display for someone visiting
  from outside than for someone arriving here from
  an internal link

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3 html files identical except name this is
JRunreferer.html

• lt!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
  Transitional//EN"gt
• ltHTMLgtltHEADgtltTITLEgtReferer Testlt/TITLEgtlt/HEADgt
• ltBODY BGCOLOR"FDF5E6"gt
• ltH1 ALIGN"CENTER"gtReferer Testlt/H1gt
• Click
• ltA HREF"http//localhost/servlet/coreservlets.Cus
  tomizeImage"gtherelt/Agt
• to visit the servlet.
• lt/BODYgtlt/HTMLgt

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Most of the servlet

• public class CustomizeImage extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• String referer request.getHeader("Referer")
  
• if (referer null)
• referer "ltIgtnonelt/Igt"
• String title "Referring page " referer
• String imageName
• if (contains(referer, "JRun"))
• imageName "jrun-powered.gif"
• else if (contains(referer, "Resin"))
• imageName "resin-powered.gif"
• else imageName "tomcat-powered.ico"
  
• String imagePath "../request-headers/images/
  " imageName
• String docType
• "lt!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML
  4.0 "

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Checking refering page
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Obviously I found my own images
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Accessing the standard CGI variables

• These are variables based on header information,
  on the socket (name and IP of the requesting
  host), or from server installation parameters
  like mapping URLs to physical paths.
• Common Gateway Interface programmers (Perl, eg.)
  would be familiar with these.

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More remarks on CGI vars

• The text suggests unless you have a CGI
  background, you can skim this section,
• But note you can use
• getServletContext().getRealPath to map a URI (the
  part of the URL after the host/port info) to an
  actual path
• Request.getRemoteHost and request.getRemoteAddress
  to get the name and IP of the client.

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A servlet to display CGI variables
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(Most of) ShowCGIVariables

• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• String variables
• "AUTH_TYPE", request.getAuthType() ,
• "CONTENT_LENGTH",
• String.valueOf(request.getContentLength(
  )) ,
• "CONTENT_TYPE", request.getContentType()
  ,
• "DOCUMENT_ROOT",
• getServletContext().getRealPath("/") ,
• "PATH_INFO", request.getPathInfo() ,
• "PATH_TRANSLATED", request.getPathTransl
  ated() ,
• "QUERY_STRING", request.getQueryString()
  ,
• "REMOTE_ADDR", request.getRemoteAddr()
  ,
• "REMOTE_HOST", request.getRemoteHost()
  ,
• //more like this
• "SERVER_PROTOCOL", request.getProtocol()
  ,