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102

Chapter 4 Handling the Client Request: HTTP Request Headers

If Match

This rarely used header applies primarily to 

PUT

 requests. The client 

can supply a list of entity tags as returned by the 

ETag

 response header, 

and the operation is performed only if one of them matches.

If Modified Since

This header indicates that the client wants the page only if it has been 

changed after the specified date. This option is very useful because it 

lets browsers cache documents and reload them over the network only 

when they've changed. However, servlets don't need to deal directly 

with this header. Instead, they should just implement the 

getLastMod 

ified

 method to have the system handle modification dates automati 

cally. An illustration is given in Section 2.8 (An Example Using Servlet 

Initialization and Page Modification Dates).

If None Match

This header is like 

If Match

, except that the operation should be per 

formed only if no entity tags match.

If Range

This rarely used header lets a client that has a partial copy of a docu 

ment ask for either the parts it is missing (if unchanged) or an entire 

new document (if it has changed since a specified date).

If Unmodified Since

This header is like 

If Modified Since

 in reverse, indicating that the 

operation should succeed only if the document is older than the speci 

fied date. Typically, 

If Modified Since

 is used for 

GET

 requests ( give 

me the document only if it is newer than my cached version ), whereas 

If Unmodified Since

 is used for 

PUT

 requests ( update this docu 

ment only if nobody else has changed it since I generated it ).

Pragma

A 

Pragma

 header with a value of 

no cache

 indicates that a servlet that 

is acting as a proxy should forward the request even if it has a local copy. 

The only standard value for this header is 

no cache

.

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