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146

Chapter 7 Generating the Server Response: HTTP Response Headers

Accept Ranges

This header, which is new in HTTP 1.1, tells the client whether or not 

you accept 

Range

 request headers. You typically specify a value of 

bytes

 to indicate that you accept 

Range

 requests, and a value of 

none

to indicate that you do not.

Age

This header is used by proxies to indicate how long ago the document 

was generated by the original server. It is new in HTTP 1.1 and is rarely 

used by servlets.

Allow

The 

Allow

 header specifies the request methods (

GET

, 

POST

, etc.) that 

the server supports. It is required for 405 (

Method Not Allowed

) 

responses. The default 

service

 method of servlets automatically gener 

ates this header for 

OPTIONS

 requests.

Cache Control

This useful header tells the browser or other client the circumstances in 

which the response document can safely be cached. It has the following 

possible values:

public

: Document is cacheable, even if normal rules (e.g., for 

password protected pages) indicate that it shouldn't be.

private

: Document is for a single user and can only be stored 

in private (nonshared) caches.

no cache

: Document should never be cached (i.e., used to 

satisfy a later request). The server can also specify 

no cache="header1,header2,...,headerN"

  to indicate 

the headers that should be omitted if a cached response is later 

used. Browsers normally do not cache documents that were 

retrieved by requests that include form data. However, if a 

servlet generates different content for different requests even 

when the requests contain no form data, it is critical to tell the 

browser not to cache the response. Since older browsers use the 

Pragma

 header for this purpose, the typical servlet approach is 

to set both headers, as in the following example.

response.setHeader("Cache Control", "no cache");

response.setHeader("Pragma", "no cache");

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