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package org.apache.juneau.http.header;

import java.util.function.*;

import org.apache.juneau.http.annotation.*;

/**
 * Represents a parsed <l>Vary</l> HTTP response header.
 *
 * <p>
 * Tells downstream proxies how to match future request headers to decide whether the cached response can be used rather
 * than requesting a fresh one from the origin server.
 *
 * <h5 class='figure'>Example</h5>
 * <p class='bcode w800'>
 *    Vary: *
 *    Vary: Accept-Language
 * </p>
 *
 * <h5 class='topic'>RFC2616 Specification</h5>
 *
 * The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh,
 * whether a cache is permitted to use the response to reply to a subsequent request without revalidation.
 * For uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to
 * select the representation.
 * A Vary field value of "*" implies that a cache cannot determine from the request headers of a subsequent request
 * whether this response is the appropriate representation.
 * See section 13.6 for use of the Vary header field by caches.
 * <p class='bcode w800'>
 *    Vary  = "Vary" ":" ( "*" | 1#field-name )
 * </p>
 *
 * <p>
 * An HTTP/1.1 server SHOULD include a Vary header field with any cacheable response that is subject to server-driven
 * negotiation.
 * Doing so allows a cache to properly interpret future requests on that resource and informs the user agent about the
 * presence of negotiation on that resource.
 * A server MAY include a Vary header field with a non-cacheable response that is subject to server-driven negotiation,
 * since this might provide the user agent with useful information about the dimensions over which the response varies
 * at the time of the response.
 *
 * <p>
 * A Vary field value consisting of a list of field-names signals that the representation selected for the response is
 * based on a selection algorithm which considers ONLY the listed request-header field values in selecting the most
 * appropriate representation.
 * A cache MAY assume that the same selection will be made for future requests with the same values for the listed
 * field names, for the duration of time for which the response is fresh.
 *
 * <p>
 * The field-names given are not limited to the set of standard request-header fields defined by this specification.
 * Field names are case-insensitive.
 *
 * <p>
 * A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network
 * address of the client), play a role in the selection of the response representation.
 * The "*" value MUST NOT be generated by a proxy server; it may only be generated by an origin server.
 *
 * <ul class='seealso'>
 *    <li class='extlink'>{@doc ExtRFC2616}
 * </ul>
 */
@Header("Vary")
public class Vary extends BasicStringHeader {

   private static final long serialVersionUID = 1L;

   /**
    * Convenience creator.
    *
    * @param value
    *    The header value.
    *    <br>Can be any of the following:
    *    <ul>
    *       <li>{@link String}
    *       <li>Anything else - Converted to <c>String</c> then parsed.
    *    </ul>
    * @return A new {@link Vary} object.
    */
   public static Vary of(Object value) {
      if (value == null)
         return null;
      return new Vary(value);
   }

   /**
    * Convenience creator using supplier.
    *
    * <p>
    * Header value is re-evaluated on each call to {@link #getValue()}.
    *
    * @param value
    *    The header value supplier.
    *    <br>Can be any of the following:
    *    <ul>
    *       <li>{@link String}
    *       <li>Anything else - Converted to <c>String</c> then parsed.
    *    </ul>
    * @return A new {@link Vary} object.
    */
   public static Vary of(Supplier<?> value) {
      if (value == null)
         return null;
      return new Vary(value);
   }

   /**
    * Constructor.
    *
    * @param value
    *    The header value.
    *    <br>Can be any of the following:
    *    <ul>
    *       <li>{@link String}
    *       <li>Anything else - Converted to <c>String</c> then parsed.
    *       <li>A {@link Supplier} of anything on this list.
    *    </ul>
    */
   public Vary(Object value) {
      super("Vary", value);
   }

   /**
    * Constructor
    *
    * @param value
    *    The header value.
    */
   public Vary(String value) {
      this((Object)value);
   }
}