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013package org.apache.juneau.http;
014
015/**
016 * Represents a parsed <l>Upgrade</l> HTTP request header.
017 * 
018 * <p>
019 * Ask the client to upgrade to another protocol.
020 * 
021 * <h5 class='figure'>Example</h5>
022 * <p class='bcode'>
023 *    Upgrade: HTTP/2.0, HTTPS/1.3, IRC/6.9, RTA/x11, websocket
024 * </p>
025 * 
026 * <h5 class='topic'>RFC2616 Specification</h5>
027 * 
028 * The Upgrade general-header allows the client to specify what additional communication protocols it supports and
029 * would like to use if the server finds it appropriate to switch protocols.
030 * The server MUST use the Upgrade header field within a 101 (Switching Protocols) response to indicate which
031 * protocol(s) are being switched.
032 * 
033 * <p class='bcode'>
034 *    Upgrade        = "Upgrade" ":" 1#product
035 * </p>
036 * 
037 * <p>
038 * For example,
039 * <p class='bcode'>
040 *    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
041 * </p>
042 * 
043 * <p>
044 * The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other,
045 * incompatible protocol.
046 * It does so by allowing the client to advertise its desire to use another protocol, such as a later version of HTTP
047 * with a higher major version number, even though the current request has been made using HTTP/1.1.
048 * This eases the difficult transition between incompatible protocols by allowing the client to initiate a request in
049 * the more commonly supported protocol while indicating to the server that it would like to use a "better" protocol if
050 * available (where "better" is determined by the server, possibly according to the nature of the method and/or resource
051 * being requested).
052 * 
053 * <p>
054 * The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer
055 * connection.
056 * Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional.
057 * The capabilities and nature of the application-layer communication after the protocol change is entirely dependent
058 * upon the new protocol chosen, although the first action after changing the protocol MUST be a response to the initial
059 * HTTP request containing the Upgrade header field.
060 * 
061 * <p>
062 * The Upgrade header field only applies to the immediate connection.
063 * Therefore, the upgrade keyword MUST be supplied within a Connection header field (section 14.10) whenever Upgrade is
064 * present in an HTTP/1.1 message.
065 * 
066 * <p>
067 * The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection.
068 * For that purpose, it is more appropriate to use a 301, 302, 303, or 305 redirection response.
069 * 
070 * <p>
071 * This specification only defines the protocol name "HTTP" for use by the family of Hypertext Transfer Protocols, as
072 * defined by the HTTP version rules of section 3.1 and future updates to this specification.
073 * Any token can be used as a protocol name; however, it will only be useful if both the client and server associate
074 * the name with the same protocol.
075 * 
076 * <h5 class='section'>See Also:</h5>
077 * <ul class='doctree'>
078 *    <li class='extlink'><a class='doclink' href='https://www.w3.org/Protocols/rfc2616/rfc2616.html'>Hypertext Transfer Protocol -- HTTP/1.1</a>
079 * </ul>
080 */
081public final class Upgrade extends HeaderStringArray {
082
083   /**
084    * Returns a parsed <code>Upgrade</code> header.
085    * 
086    * @param value The <code>Upgrade</code> header string.
087    * @return The parsed <code>Upgrade</code> header, or <jk>null</jk> if the string was null.
088    */
089   public static Upgrade forString(String value) {
090      if (value == null)
091         return null;
092      return new Upgrade(value);
093   }
094
095   private Upgrade(String value) {
096      super(value);
097   }
098}