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012// ***************************************************************************************************************************
013package org.apache.juneau.http;
014
015import org.apache.juneau.http.annotation.*;
016
017/**
018 * Represents a parsed <l>Expires</l> HTTP response header.
019 *
020 * <p>
021 * Gives the date/time after which the response is considered stale (in "HTTP-date" format as defined by RFC 7231).
022 *
023 * <h5 class='figure'>Example</h5>
024 * <p class='bcode w800'>
025 *    Expires: Thu, 01 Dec 1994 16:00:00 GMT
026 * </p>
027 *
028 * <h5 class='topic'>RFC2616 Specification</h5>
029 *
030 * The Expires entity-header field gives the date/time after which the response is considered stale.
031 * A stale cache entry may not normally be returned by a cache (either a proxy cache or a user agent cache) unless it is
032 * first validated with the origin server
033 * (or with an intermediate cache that has a fresh copy of the entity).
034 * See section 13.2 for further discussion of the expiration model.
035 *
036 * <p>
037 * The presence of an Expires field does not imply that the original resource will change or cease to exist at, before,
038 * or after that time.
039 *
040 * <p>
041 * The format is an absolute date and time as defined by HTTP-date in section 3.3.1; it MUST be in RFC 1123 date format:
042 *
043 * <p class='bcode w800'>
044 *    Expires = "Expires" ":" HTTP-date
045 * </p>
046 *
047 * <p>
048 * An example of its use is...
049 * <p class='bcode w800'>
050 *    Expires: Thu, 01 Dec 1994 16:00:00 GMT
051 * </p>
052 *
053 * <p>
054 * Note: if a response includes a Cache-Control field with the max-age directive (see section 14.9.3), that directive
055 * overrides the Expires field.
056 *
057 * <p>
058 * HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value "0", as in the past
059 * (i.e., "already expired").
060 *
061 * <p>
062 * To mark a response as "already expired," an origin server sends an Expires date that is equal to the Date header
063 * value.
064 * (See the rules for expiration calculations in section 13.2.4.)
065 *
066 * <p>
067 * To mark a response as "never expires," an origin server sends an Expires date approximately one year from the time
068 * the response is sent.
069 * HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future.
070 *
071 * <p>
072 * The presence of an Expires header field with a date value of some time in the future on a response that otherwise
073 * would by default be non-cacheable indicates that the response is cacheable, unless indicated otherwise by a
074 * Cache-Control header field (section 14.9).
075 *
076 * <ul class='seealso'>
077 *    <li class='extlink'>{@doc RFC2616}
078 * </ul>
079 */
080@Header("Expires")
081public final class Expires extends HeaderDate {
082
083   /**
084    * Returns a parsed <c>Expires</c> header.
085    *
086    * @param value The <c>Expires</c> header string.
087    * @return The parsed <c>Expires</c> header, or <jk>null</jk> if the string was null.
088    */
089   public static Expires forString(String value) {
090      if (value == null)
091         return null;
092      return new Expires(value);
093   }
094
095   private Expires(String value) {
096      super(value);
097   }
098}