Package org.apache.juneau.http.header

Class IfNoneMatch

java.lang.Object
org.apache.juneau.http.header.BasicHeader
org.apache.juneau.http.header.BasicEntityTagsHeader
org.apache.juneau.http.header.IfNoneMatch
All Implemented Interfaces:
Serializable, Cloneable, Header, NameValuePair
@Header("If-None-Match") public class IfNoneMatch extends BasicEntityTagsHeader
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.

See Also:

  • Constructor Details

    • IfNoneMatch

      public IfNoneMatch(String value)
      Constructor.
      Parameters:
      value - The header value.
      Must be a comma-delimited list of entity validator values (e.g. "\"xyzzy\", \"r2d2xxxx\", \"c3piozzzz\"").
      Can be null.

    • IfNoneMatch

      public IfNoneMatch(EntityTags value)
      Constructor.
      Parameters:
      value - The header value.
      Can be null.

    • IfNoneMatch

      public IfNoneMatch(Supplier<EntityTags> value)
      Constructor with delayed value.

      Header value is re-evaluated on each call to BasicEntityTagsHeader.getValue().

      Parameters:
      value - The supplier of the header value.
      Can be null.

  • Method Details

    • of

      public static IfNoneMatch of(String value)
      Static creator.
      Parameters:
      value - The header value.
      Must be a comma-delimited list of entity validator values (e.g. "\"xyzzy\", \"r2d2xxxx\", \"c3piozzzz\"").
      Can be null.
      Returns:
      A new header bean, or null if the value is null.

    • of

      public static IfNoneMatch of(EntityTags value)
      Static creator.
      Parameters:
      value - The header value.
      Can be null.
      Returns:
      A new header bean, or null if the value is null.

    • of

      public static IfNoneMatch of(Supplier<EntityTags> value)
      Static creator with delayed value.

      Header value is re-evaluated on each call to BasicEntityTagsHeader.getValue().

      Parameters:
      value - The supplier of the header value.
      Can be null.
      Returns:
      A new header bean, or null if the value is null.