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007// *  http://www.apache.org/licenses/LICENSE-2.0                                                                             *
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012// ***************************************************************************************************************************
013package org.apache.juneau.http;
014
015import org.apache.juneau.http.annotation.*;
016
017/**
018 * Represents a parsed <l>If-None-Match</l> HTTP request header.
019 *
020 * <p>
021 * Allows a 304 Not Modified to be returned if content is unchanged.
022 *
023 * <h5 class='figure'>Example</h5>
024 * <p class='bcode w800'>
025 *    If-None-Match: "737060cd8c284d8af7ad3082f209582d"
026 * </p>
027 *
028 * <h5 class='topic'>RFC2616 Specification</h5>
029 *
030 * The If-None-Match request-header field is used with a method to make it conditional.
031 * A client that has one or more entities previously obtained from the resource can verify that none of those entities
032 * is current by including a list of their associated entity tags in the If-None-Match header field.
033 * The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction
034 * overhead.
035 * It is also used to prevent a method (e.g. PUT) from inadvertently modifying an existing resource when the client
036 * believes that the resource does not exist.
037 *
038 * <p>
039 * As a special case, the value "*" matches any current entity of the resource.
040 *
041 * <p class='bcode w800'>
042 *    If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
043 * </p>
044 *
045 * <p>
046 * If any of the entity tags match the entity tag of the entity that would have been returned in the response to a
047 * similar GET request (without the If-None-Match header) on that resource, or if "*" is given
048 * and any current entity exists for that resource, then the server MUST NOT perform the requested method, unless
049 * required to do so because the resource's modification date fails to match that supplied in an If-Modified-Since
050 * header field in the request.
051 * Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response,
052 * including the cache- related header fields (particularly ETag) of one of the entities that matched.
053 * For all other request methods, the server MUST respond with a status of 412 (Precondition Failed).
054 *
055 * <p>
056 * See section 13.3.3 for rules on how to determine if two entities tags match.
057 * The weak comparison function can only be used with GET or HEAD requests.
058 *
059 * <p>
060 * If none of the entity tags match, then the server MAY perform the requested method as if the If-None-Match header
061 * field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request.
062 * That is, if no entity tags match, then the server MUST NOT return a 304 (Not Modified) response.
063 *
064 * <p>
065 * If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status,
066 * then the If-None-Match header MUST be ignored.
067 * (See section 13.3.4 for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the
068 * same request.)
069 *
070 * <p>
071 * The meaning of "If-None-Match: *" is that the method MUST NOT be performed if the representation selected by the
072 * origin server (or by a cache, possibly using the Vary mechanism, see section 14.44) exists, and SHOULD be performed
073 * if the representation does not exist.
074 * This feature is intended to be useful in preventing races between PUT operations.
075 *
076 * <p>
077 * Examples:
078 * <p class='bcode w800'>
079 *    If-None-Match: "xyzzy"
080 *    If-None-Match: W/"xyzzy"
081 *    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
082 *    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
083 *    If-None-Match: *
084 * </p>
085 *
086 * <p>
087 * The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since
088 * header fields is undefined by this specification.
089 *
090 * <ul class='seealso'>
091 *    <li class='extlink'>{@doc RFC2616}
092 * </ul>
093 */
094@Header("If-None-Match")
095public final class IfNoneMatch extends HeaderEntityValidatorArray {
096
097   /**
098    * Returns a parsed <c>If-None-Match</c> header.
099    *
100    * @param value The <c>If-None-Match</c> header string.
101    * @return The parsed <c>If-None-Match</c> header, or <jk>null</jk> if the string was null.
102    */
103   public static IfNoneMatch forString(String value) {
104      if (value == null)
105         return null;
106      return new IfNoneMatch(value);
107   }
108
109   private IfNoneMatch(String value) {
110      super(value);
111   }
112}