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.
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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) |
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| 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.
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| 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.
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| 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.
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| 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. |
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