CS651

HTTP Request Headers
(for HTTP 1.1)

Request Headers are sent by the browser immediately following the initial GET or POST line. These headers can direct the Servlet how to react.

Servlet needs to explicity reat the request headers.
Example of information set in a GET HTTP request (from your Core book) from submitting a book-search at http://www.somebookstore.com/findbooks.html which invokes the servket at http://www.somebookstore.com/search:

GET    /search?keywords=servlets+jsp HTTP/1.1
Accept: image/gif, image/jpg, */*
Accept-Encoding: gzip
Connection: Keep-Alive
Cookie:   userID=id45678
Host:   www.somebookstore.com
Referer: http://www.somebookstore.com/findbooks.html
User-Agent: Mozilla/4.7 [en] (Win98; U)

Click here to find out about Methods of HttpServletRequest that are useful to get general information about the HTTP request.

How to get Headers:

call getHeader() method of HttpServletRequest passing the name of the Header you wish to retrieve. Will return the value as a String.

request.getHeader("Connection");

For the more commonly used headers there are methods you can use to access the values:

getCookies	returns an array of Cookies pased
getAuthType	breaks the Authorization header into its component pieces.
getContentLength	returns the value of Content-Length as an int
getContentType	returns value as a String (e.g. "text/html")
getDateHeader	returns value as an instance of the java class Date
getIntHeader	returns value as an int
getHeaderNames	returns as an instance of Enumeration class all of the header names received by this request.
getHeaders	use this if there may be more than one value associated with a header (this is Servlet version sensitive...see API)

Header

Meaning

Accept

Specifies MIME types that the browser can handle (e.g. image/jpg). Servlet can look at this to decide what kind of information it can return in its response.

Accept-Charset

This tells the Servlet what the character set that can be accepted by the client's browser

Accept-Encoding

Indicates the encoding types the client can handle. If the Servlet decides to return data using an encoding type, it must send an Content-Encoding response header. Note: this is different than the Content-Type response header...which indicates the kind of data in the document. The Content-Encoding type is the encoding applied to the content (of type Content-Type) after the content is created. On the client browser, the recieved data is first decoded before it uses the Content-Type to know how to view the decoded data.

Typically used for compressing data.

Accept-Language

Specifies client's preferred languages. See RFC 1766 for name of languuages (e.g. en-us = U.S. english, en = British english, etc).

Authorization

Used by clients to identify themselves when accessing password-protected Web-pages

Cache-Control

Client uses to specify how pages should be cached by proxy servers.

Connection

Client uses to indicate if can handle persistent HTTP connections. Such a connection allows client browser to retrieve multiple files (images, html, etc.) with a single socket connection, saving the overhead of negotiating independent connections for each.
values:

keep-alive = means persistent connection allowable
close = means persistent connection NOT allowable

HTTP1.1: persistent connections are the default.

Content-Length

Gives the size of a POST method request in bytes.

Content-Type

Usually used in a HTTP response sent from the Servlet. Can be given by client to Servlet when the client attaches a document as POST data.

Cookie

Client uses this to return cookies to the server.

From

Can be used to send as a value the email address of the requestor. Not sent by browsers, but, other applications sending HTTP requests may use this.

Host

Client sends the host and port as given in the URL.

HTTP 1.1: Browsers are required to send this information.

If-Modified-Since

Client sends to indicate they want the requested page (in our case generated by the Servlet), ONLY if it has been changed since the date specified (or in the case of Servlets, if the data used to construct the web-page is newer than this date)

If-Unmodified-Since

Opposite of If-Modified-Since

Referer

Indicates the URL of referring web-page.

User-Agent

Identifies client browser (or application) that made the request.
Can use this when want to know what browser or version of it the client has. Be careful on how to parse results.

SEE HTTP specs for other HTTP Request headers.

Exercise