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4.3 HTTP 1.1 Request Headers

99

can examine the 

Accept

 header to decide which format to use. For exam 

ple, images in PNG format have some compression advantages over those 

in GIF, but only a few browsers support PNG. If you had images in both 

formats, a servlet could call 

request.getHeader("Accept")

, check for 

image/png

, and if it finds it, use 

xxx.png

 filenames in all the 

IMG

 ele 

ments it generates. Otherwise it would just use 

xxx.gif

. 

See Table 7.1 in Section 7.2 (HTTP 1.1 Response Headers and Their 

Meaning) for the names and meanings of the common MIME types.

Accept Charset

This header indicates the character sets (e.g., ISO 8859 1) the browser 

can use.

Accept Encoding

This header designates the types of encodings that the client knows how 

to handle. If it receives this header, the server is free to encode the page 

by using the format specified (usually to reduce transmission time), 

sending the 

Content Encoding

 response header to indicate that it has 

done so. This encoding type is completely distinct from the MIME type 

of the actual document (as specified in the 

Content Type

 response 

header), since this encoding is reversed before the browser decides what 

to do with the content. On the other hand, using an encoding the 

browser doesn't understand results in totally incomprehensible pages. 

Consequently, it is critical that you explicitly check the 

Accept Encod 

ing

 header before using any type of content encoding. Values of 

gzip

or 

compress

 are the two standard possibilities.

Compressing pages before returning them is a very valuable service 

because the decoding time is likely to be small compared to the savings 

in transmission time. See Section 4.4 (Sending Compressed Web 

Pages) for an example where compression reduces download times by 

a factor of 10.

Accept Language

This header specifies the client's preferred languages, in case the servlet 

can produce results in more than one language. The value of the header 

should be one of the standard language codes such as 

en

, 

en us

, 

da

, 

etc. See RFC 1766 for details.

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