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7.2 HTTP 1.1 Response Headers and Their Meaning

151

for a document that changes relatively frequently, to prevent the 

browser from displaying a stale cached value. For example, the follow 

ing would instruct the browser not to cache the document for longer 

than 10 minutes

long currentTime = System.currentTimeMillis();

long tenMinutes = 10*60*1000; // In milliseconds

response.setDateHeader("Expires", 

currentTime + tenMinutes);

Also see the 

max age

 value of the 

Cache Control

 header.

Last Modified

This very useful header indicates when the document was last changed. 

The client can then cache the document and supply a date by an 

If Mod 

ified Since

 request header in later requests. This request is treated as 

a conditional 

GET

, with the document only being returned if the 

Last Modified

 date is later than the one specified for 

If Modi 

fied Since

. Otherwise, a 304 (

Not Modified

) status line is returned, 

and the client uses the cached document. If you set this header explicitly, 

use the 

setDateHeader

 method to save yourself the bother of formatting 

GMT date strings. However, in most cases you simply implement the 

getLastModified

 method and let the standard 

service

 method handle 

If Modified Since

 requests. For an example, see Section 2.8 (An 

Example Using Servlet Initialization and Page Modification Dates).

Location

This header, which should be included with all responses that have a sta 

tus code in the 300s, notifies the browser of the document address. The 

browser automatically reconnects to this location and retrieves the new 

document. This header is usually set indirectly, along with a 302 status 

code, by the 

sendRedirect

 method of 

HttpServletResponse

. An 

example is given in Section 6.3 (A Front End to Various Search Engines).

Pragma

Supplying this header with a value of 

no cache

 instructs HTTP 1.0 cli 

ents not to cache the document. However, support for this header was 

inconsistent with HTTP 1.0 browsers. In HTTP 1.1,  

Cache Control: 

no cache

  is a more reliable replacement.

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