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012// ***************************************************************************************************************************
013package org.apache.juneau.http;
014
015/**
016 * Represents a parsed <l>Last-Modified</l> HTTP response header.
017 * 
018 * <p>
019 * The last modified date for the requested object (in "HTTP-date" format as defined by RFC 7231).
020 * 
021 * <h5 class='figure'>Example</h5>
022 * <p class='bcode'>
023 *    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
024 * </p>
025 * 
026 * <h5 class='topic'>RFC2616 Specification</h5>
027 * 
028 * The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was
029 * last modified.
030 * 
031 * <p class='bcode'>
032 *    Last-Modified  = "Last-Modified" ":" HTTP-date
033 * </p>
034 * 
035 * <p>
036 * An example of its use is...
037 * <p class='bcode'>
038 *    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
039 * </p>
040 * 
041 * <p>
042 * The exact meaning of this header field depends on the implementation of the origin server and the nature of the
043 * original resource.
044 * For files, it may be just the file system last-modified time.
045 * For entities with dynamically included parts, it may be the most recent of the set of last-modify times for its
046 * component parts.
047 * For database gateways, it may be the last-update time stamp of the record.
048 * For virtual objects, it may be the last time the internal state changed.
049 * 
050 * <p>
051 * An origin server MUST NOT send a Last-Modified date which is later than the server's time of message origination.
052 * In such cases, where the resource's last modification would indicate some time in the future, the server MUST replace
053 * that date with the message origination date.
054 * 
055 * <p>
056 * An origin server SHOULD obtain the Last-Modified value of the entity as close as possible to the time that it
057 * generates the Date value of its response.
058 * This allows a recipient to make an accurate assessment of the entity's modification time, especially if the entity
059 * changes near the time that the response is generated.
060 * 
061 * <p>
062 * HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
063 * 
064 * <h5 class='section'>See Also:</h5>
065 * <ul class='doctree'>
066 *    <li class='extlink'><a class='doclink' href='https://www.w3.org/Protocols/rfc2616/rfc2616.html'>Hypertext Transfer Protocol -- HTTP/1.1</a>
067 * </ul>
068 */
069public final class LastModified extends HeaderDate {
070
071   /**
072    * Returns a parsed <code>Last-Modified</code> header.
073    * 
074    * @param value The <code>Last-Modified</code> header string.
075    * @return The parsed <code>Last-Modified</code> header, or <jk>null</jk> if the string was null.
076    */
077   public static LastModified forString(String value) {
078      if (value == null)
079         return null;
080      return new LastModified(value);
081   }
082
083   private LastModified(String value) {
084      super(value);
085   }
086}