CS651

HTTP Response Headers
(for HTTP 1.1)

An HTTResponse from a web-server consists of:

Status Line
Response Headers
Blank Line
Data/Document

EXAMPLE (from Core Servlets book)

HTTP/1.1 200 OK
Content-Type: text/plain

Hello Class

Meaning of example above:

HTTP/1.1 tells the protocol and version
Status code is given as 200.
A short meassage follows corresponding to the status code, here it is 200.
Next follows the HTTP Response Header, here there is only one listed, Content-Type. Specifies MIME type
Next is the blank line
Then the document follows, here it is simply a single line "Hello Class".

HTTP Response STATUS CODES:

Servlet can create a status line to return in its HTTP response by invoking

HttpServletResponse.setStatus(int)
This only specifies the status code, as the protocol and version are determine by the server and created automatically in the response
method takes an int for the code. Can use constants defined in HttpServletResponse.
MUST do before sending any headers and the data itself !!!!!

ALSO, there are two special methods you can use for status codes dealing with errors (e.g. 404) and redirection (303):

HttpServletResponse.sendError(int, String message);
HttpServletResponse.sendRedirect(String url);

NOTE: You may never create a status line in your Servlet's fabricated response!!! This is because there is a default status code (200) that is sent by a Servlet automatically....which says everything was successful!!!!
General groupings of codes

100-199: informational, client shour respond with some other action
200-299: request was successful
300-399: requested files have moved, often will respond with a Location header following that tells where the new location is.
400-499: Error by client
500-599: Error by server

HttpServletResponse Status Code Variables and Meaning
see reading and www.rfc-editor.org (look for RFC 2616) for more codes

100 (SC_CONTINUE)	Server Responds with this code when it has recieved a request from client with Request header Except and value 100-Continue This means that the client has requested to send a follow-up request. 100 server status code response tells client to go ahead are send the follow-up request.
417(Expectation Failed)	Server Responds with this code when it has recieved a request from client with Request header Except and value 100-Continuee This means that the client has requested to send a follow-up request. 417 server status code response tells client it will not accept any further follow-up requests.
101 (SC_SWITCHING_PROTOCOLS)	server will comply with a HTTP request header Upgrade, that asks it to change to a different protocol to communicate with client.
200 (SC_OK)	server responds with this to tell client that the request was successful. Usually following a GET, POST request. this is the default. for Servlet. So, you do not have to set in Servlet.
201 (SC_CREATED)	Server has created new document and the location is given by the HTTP response header LOCATION to follow.
202 (SC_ACCEPTED)	Server tells client that they are processing the request but, are not finished yet.
204 (SC_NO_CONTENT)	Server tells client browser to continue to display previous document as no new document is being sent.
205 (SC_RESET_CONTENT)	Server tells client that no new document comming, but, please refresh current document. Usefull to get HTML forms to clear,etc.
206 (SC_PARTIAL_CONTENT)	Server says the following data is only part of the data to come.
300 (SC_MULTIPLE_CHOICES)	Server says the requested document can be found in several places, which will be returned in the data the server is sending If there is a following HTTP response LOCATION header, this will specified the preferred choice
301 (SC_MOVED_PERMANENTLY)	Server tells client that the document requested has permanetly moved. Must send an HTTP response header LOCATION that gives the new location.
302 (SC_MOVED_TEMPORARILY)	same as 301 but, location in LOCATION response header is a temporary location, not a premanent position.
304 (SC_NOT_MODIFIED)	Response to give to say that the document has not change. A response to a client request which has the HTTP Request Header IF-MODIFIED-SINCE in it....which means the client is asking to send the requested document only if it is newer that the specified time in this header.
305 (SC_USE_PROXY)	Server is telling client that the requested document can only be retrieved using the proxy listed in the HTTP Response Header LOCATION.
401 (SC_UNAUTHORIZED)	Server telling client thehy are trying to access a password-protected page without proper id info in the HTTP request AUTHORIZATION header. You must also send an HTTP response header of WWW_Authenticate
403 (SC_FORBIDDEN)	Server refuses to fulfill the request. Often used because bad file permissions.
404 (SC_NOT_FOUND)	Server can not find the resource reqeusted.
500 (SC_INTERNAL_SERVER_ERROR)	Means servlet or CGI program, etc. has failed.

HTTP Response HEADERS

Can appear in any order, but, must follow directly the Servlet's creation of a status code.
To set call:

HttpServletResponse.setHeader("header", "value");
There are a few additional special methods as short-cuts you can use (see reading and below, and API)

See reading or www.rfc-editor.org (RFC 2616) for more headers.
NOTE: Some of these are identical to HTTP Request headers.

Header

Meaning

Allow

Specifies the request methods (GET, POST, etc) that the server supports

Cache-Control

Tells clients under what circumstances the response document can be SAFELY cached. VALUES

public = allways cacheable
no-cache = never should be cached.
no-store = never cached, nor storedin temporary location on disk....used to prevent copies of sensitive info.
must-revalidate: client must renew document with original server each time is used.
max-age = XXX: document considered stale after xxx seconds.

Connection

Values:

close = tells client not to use persistent HTTP connections

Content-Encoding

Tells client the encoding scheme used to encode the data being sent by server.

Content-Language

Tells client the language used in the data being sent

Content-Length

Indicates number of bytes of response data being sent.

Content-Type

Tells client the MIME type of the data being sent.

Date

Gives the date.

can use HttpServletResponse.setDateHeader()

Expires

gives time when data being sent is considered out of date

Last-Modified

gives date when the document being sent was last modified.

Location

indicates the location of the data/document (either a change,etc).

Refresh

gives number of seconds in which the client browser should update the page.

Set-Cookie

specifies a cookie associated with the page.

To set use: HttpServletResponse.addCookie(..)
Can not use setHeader method.

Exercise