001// ***************************************************************************************************************************
002// * Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements.  See the NOTICE file *
003// * distributed with this work for additional information regarding copyright ownership.  The ASF licenses this file        *
004// * to you under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance            *
005// * with the License.  You may obtain a copy of the License at                                                              *
006// *                                                                                                                         *
007// *  http://www.apache.org/licenses/LICENSE-2.0                                                                             *
008// *                                                                                                                         *
009// * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an  *
010// * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the License for the        *
011// * specific language governing permissions and limitations under the License.                                              *
012// ***************************************************************************************************************************
013package org.apache.juneau.http;
014
015/**
016 * Represents a parsed <l>If-None-Match</l> HTTP request header.
017 * 
018 * <p>
019 * Allows a 304 Not Modified to be returned if content is unchanged.
020 * 
021 * <h5 class='figure'>Example</h5>
022 * <p class='bcode'>
023 *    If-None-Match: "737060cd8c284d8af7ad3082f209582d"
024 * </p>
025 * 
026 * <h5 class='topic'>RFC2616 Specification</h5>
027 * 
028 * The If-None-Match request-header field is used with a method to make it conditional.
029 * A client that has one or more entities previously obtained from the resource can verify that none of those entities
030 * is current by including a list of their associated entity tags in the If-None-Match header field.
031 * The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction
032 * overhead.
033 * It is also used to prevent a method (e.g. PUT) from inadvertently modifying an existing resource when the client
034 * believes that the resource does not exist.
035 * 
036 * <p>
037 * As a special case, the value "*" matches any current entity of the resource.
038 * 
039 * <p class='bcode'>
040 *    If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
041 * </p>
042 * 
043 * <p>
044 * If any of the entity tags match the entity tag of the entity that would have been returned in the response to a
045 * similar GET request (without the If-None-Match header) on that resource, or if "*" is given
046 * and any current entity exists for that resource, then the server MUST NOT perform the requested method, unless
047 * required to do so because the resource's modification date fails to match that supplied in an If-Modified-Since
048 * header field in the request.
049 * Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response,
050 * including the cache- related header fields (particularly ETag) of one of the entities that matched.
051 * For all other request methods, the server MUST respond with a status of 412 (Precondition Failed).
052 * 
053 * <p>
054 * See section 13.3.3 for rules on how to determine if two entities tags match.
055 * The weak comparison function can only be used with GET or HEAD requests.
056 * 
057 * <p>
058 * If none of the entity tags match, then the server MAY perform the requested method as if the If-None-Match header
059 * field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request.
060 * That is, if no entity tags match, then the server MUST NOT return a 304 (Not Modified) response.
061 * 
062 * <p>
063 * If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status,
064 * then the If-None-Match header MUST be ignored.
065 * (See section 13.3.4 for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the
066 * same request.)
067 * 
068 * <p>
069 * The meaning of "If-None-Match: *" is that the method MUST NOT be performed if the representation selected by the
070 * origin server (or by a cache, possibly using the Vary mechanism, see section 14.44) exists, and SHOULD be performed
071 * if the representation does not exist.
072 * This feature is intended to be useful in preventing races between PUT operations.
073 * 
074 * <p>
075 * Examples:
076 * <p class='bcode'>
077 *    If-None-Match: "xyzzy"
078 *    If-None-Match: W/"xyzzy"
079 *    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
080 *    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
081 *    If-None-Match: *
082 * </p>
083 * 
084 * <p>
085 * The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since
086 * header fields is undefined by this specification.
087 * 
088 * <h5 class='section'>See Also:</h5>
089 * <ul class='doctree'>
090 *    <li class='extlink'><a class='doclink' href='https://www.w3.org/Protocols/rfc2616/rfc2616.html'>Hypertext Transfer Protocol -- HTTP/1.1</a>
091 * </ul>
092 */
093public final class IfNoneMatch extends HeaderEntityValidatorArray {
094
095   /**
096    * Returns a parsed <code>If-None-Match</code> header.
097    * 
098    * @param value The <code>If-None-Match</code> header string.
099    * @return The parsed <code>If-None-Match</code> header, or <jk>null</jk> if the string was null.
100    */
101   public static IfNoneMatch forString(String value) {
102      if (value == null)
103         return null;
104      return new IfNoneMatch(value);
105   }
106
107   private IfNoneMatch(String value) {
108      super(value);
109   }
110}