org.apache.juneau.http.IfNoneMatch Maven / Gradle / Ivy
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package org.apache.juneau.http;
/**
* Represents a parsed If-None-Match HTTP request header.
*
*
* Allows a 304 Not Modified to be returned if content is unchanged.
*
*
Example
*
* If-None-Match: "737060cd8c284d8af7ad3082f209582d"
*
*
* RFC2616 Specification
*
* The If-None-Match request-header field is used with a method to make it conditional.
* A client that has one or more entities previously obtained from the resource can verify that none of those entities
* is current by including a list of their associated entity tags in the If-None-Match header field.
* The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction
* overhead.
* It is also used to prevent a method (e.g. PUT) from inadvertently modifying an existing resource when the client
* believes that the resource does not exist.
*
*
* As a special case, the value "*" matches any current entity of the resource.
*
*
* If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
*
*
*
* If any of the entity tags match the entity tag of the entity that would have been returned in the response to a
* similar GET request (without the If-None-Match header) on that resource, or if "*" is given
* and any current entity exists for that resource, then the server MUST NOT perform the requested method, unless
* required to do so because the resource's modification date fails to match that supplied in an If-Modified-Since
* header field in the request.
* Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response,
* including the cache- related header fields (particularly ETag) of one of the entities that matched.
* For all other request methods, the server MUST respond with a status of 412 (Precondition Failed).
*
*
* See section 13.3.3 for rules on how to determine if two entities tags match.
* The weak comparison function can only be used with GET or HEAD requests.
*
*
* If none of the entity tags match, then the server MAY perform the requested method as if the If-None-Match header
* field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request.
* That is, if no entity tags match, then the server MUST NOT return a 304 (Not Modified) response.
*
*
* If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status,
* then the If-None-Match header MUST be ignored.
* (See section 13.3.4 for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the
* same request.)
*
*
* The meaning of "If-None-Match: *" is that the method MUST NOT be performed if the representation selected by the
* origin server (or by a cache, possibly using the Vary mechanism, see section 14.44) exists, and SHOULD be performed
* if the representation does not exist.
* This feature is intended to be useful in preventing races between PUT operations.
*
*
* Examples:
*
* If-None-Match: "xyzzy"
* If-None-Match: W/"xyzzy"
* If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
* If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
* If-None-Match: *
*
*
*
* The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since
* header fields is undefined by this specification.
*
*
Additional Information
*
*/
public final class IfNoneMatch extends HeaderEntityValidatorArray {
/**
* Returns a parsed If-None-Match header.
*
* @param value The If-None-Match header string.
* @return The parsed If-None-Match header, or null if the string was null.
*/
public static IfNoneMatch forString(String value) {
if (value == null)
return null;
return new IfNoneMatch(value);
}
private IfNoneMatch(String value) {
super(value);
}
}