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012// ***************************************************************************************************************************
013package org.apache.juneau.http;
014
015import org.apache.juneau.http.annotation.*;
016
017/**
018 * Represents a parsed <l>Vary</l> HTTP response header.
019 *
020 * <p>
021 * Tells downstream proxies how to match future request headers to decide whether the cached response can be used rather
022 * than requesting a fresh one from the origin server.
023 *
024 * <h5 class='figure'>Example</h5>
025 * <p class='bcode w800'>
026 *    Vary: *
027 *    Vary: Accept-Language
028 * </p>
029 *
030 * <h5 class='topic'>RFC2616 Specification</h5>
031 *
032 * The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh,
033 * whether a cache is permitted to use the response to reply to a subsequent request without revalidation.
034 * For uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to
035 * select the representation.
036 * A Vary field value of "*" implies that a cache cannot determine from the request headers of a subsequent request
037 * whether this response is the appropriate representation.
038 * See section 13.6 for use of the Vary header field by caches.
039 * <p class='bcode w800'>
040 *    Vary  = "Vary" ":" ( "*" | 1#field-name )
041 * </p>
042 *
043 * <p>
044 * An HTTP/1.1 server SHOULD include a Vary header field with any cacheable response that is subject to server-driven
045 * negotiation.
046 * Doing so allows a cache to properly interpret future requests on that resource and informs the user agent about the
047 * presence of negotiation on that resource.
048 * A server MAY include a Vary header field with a non-cacheable response that is subject to server-driven negotiation,
049 * since this might provide the user agent with useful information about the dimensions over which the response varies
050 * at the time of the response.
051 *
052 * <p>
053 * A Vary field value consisting of a list of field-names signals that the representation selected for the response is
054 * based on a selection algorithm which considers ONLY the listed request-header field values in selecting the most
055 * appropriate representation.
056 * A cache MAY assume that the same selection will be made for future requests with the same values for the listed
057 * field names, for the duration of time for which the response is fresh.
058 *
059 * <p>
060 * The field-names given are not limited to the set of standard request-header fields defined by this specification.
061 * Field names are case-insensitive.
062 *
063 * <p>
064 * A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network
065 * address of the client), play a role in the selection of the response representation.
066 * The "*" value MUST NOT be generated by a proxy server; it may only be generated by an origin server.
067 *
068 * <h5 class='section'>See Also:</h5>
069 * <ul class='doctree'>
070 *    <li class='extlink'>{@doc RFC2616}
071 * </ul>
072 */
073@Header("Vary")
074public final class Vary extends HeaderString {
075
076   /**
077    * Returns a parsed <code>Vary</code> header.
078    *
079    * @param value The <code>Vary</code> header string.
080    * @return The parsed <code>Vary</code> header, or <jk>null</jk> if the string was null.
081    */
082   public static Vary forString(String value) {
083      if (value == null)
084         return null;
085      return new Vary(value);
086   }
087
088   private Vary(String value) {
089      super(value);
090   }
091}