Prentice Hall and Sun Microsystems. Personal use only; do not redistribute.
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
.
Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.
Servlet and JSP training courses by book's author: courses.coreservlets.com.






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