Java Servlet Programming

HTTP Headers

A servlet can set HTTP headers to provide extra information about its response. As we said in Chapter 4, a full discussion of all the possible HTTP 1.0 and HTTP 1.1 headers is beyond the scope of this book. Table 5.2 lists the HTTP headers that are most often set by servlets as a part of a response.

Table 5-2. HTTP Response Headers

Header	Usage
Cache-Control	Specifies any special treatment a caching system should give to this document. The most common values are no-cache (to indicate this document should not be cached), no-store (to indicate this document should not be cached or even stored by a proxy server, usually due to its sensitive contents), and max-age= seconds (to indicate how long before the document should be considered stale). This header was introduced in HTTP 1.1.
Pragma	The HTTP 1.0 equivalent of Cache-control, with no-cache as its only possible value.
Connection	Used to indicate whether the server is willing to maintain an open (persistent) connection to the client. If so, its value is set to keep-alive. If not, its value is set to close. Most web servers handle this header on behalf of their servlets, automatically setting its value to keep-alive when a servlet sets its Content-Length header.
Retry-After	Specifies a time when the server can again handle requests, used with the SC_SERVICE_UNAVAILABLE status code. Its value is either an int that represents the number of seconds or a date string that represents an actual time. ...