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012// ***************************************************************************************************************************
013package org.apache.juneau.http;
014
015/**
016 * Represents a parsed <l>Expect</l> HTTP request header.
017 * 
018 * <p>
019 * Indicates that particular server behaviors are required by the client.
020 * 
021 * <h5 class='figure'>Example</h5>
022 * <p class='bcode'>
023 *    Expect: 100-continue
024 * </p>
025 * 
026 * <h5 class='topic'>RFC2616 Specification</h5>
027 * 
028 * The Expect request-header field is used to indicate that particular server behaviors are required by the client.
029 * <p class='bcode'>
030 *    Expect       =  "Expect" ":" 1#expectation
031 *    expectation  =  "100-continue" | expectation-extension
032 *    expectation-extension =  token [ "=" ( token | quoted-string )
033 *                             *expect-params ]
034 *    expect-params =  ";" token [ "=" ( token | quoted-string ) ]
035 * </p>
036 * 
037 * <p>
038 * A server that does not understand or is unable to comply with any of the expectation values in the Expect field of a
039 * request MUST respond with appropriate error status.
040 * The server MUST respond with a 417 (Expectation Failed) status if any of the expectations cannot be met or, if there
041 * are other problems with the request, some other 4xx status.
042 * 
043 * <p>
044 * This header field is defined with extensible syntax to allow for future extensions.
045 * If a server receives a request containing an Expect field that includes an expectation-extension that it does not
046 * support, it MUST respond with a 417 (Expectation Failed) status.
047 * 
048 * <p>
049 * Comparison of expectation values is case-insensitive for unquoted tokens (including the 100-continue token), and is
050 * case-sensitive for quoted-string expectation-extensions.
051 * 
052 * <p>
053 * The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST return a 417 (Expectation Failed) status if it
054 * receives a request with an expectation that it cannot meet.
055 * However, the Expect request-header itself is end-to-end; it MUST be forwarded if the request is forwarded.
056 * 
057 * <p>
058 * Many older HTTP/1.0 and HTTP/1.1 applications do not understand the Expect header.
059 * 
060 * <p>
061 * See section 8.2.3 for the use of the 100 (continue) status.
062 * 
063 * <h5 class='section'>See Also:</h5>
064 * <ul class='doctree'>
065 *    <li class='extlink'><a class='doclink' href='https://www.w3.org/Protocols/rfc2616/rfc2616.html'>Hypertext Transfer Protocol -- HTTP/1.1</a>
066 * </ul>
067 */
068public final class Expect extends HeaderString {
069
070   /**
071    * Returns a parsed <code>Expect</code> header.
072    * 
073    * @param value The <code>Expect</code> header string.
074    * @return The parsed <code>Expect</code> header, or <jk>null</jk> if the string was null.
075    */
076   public static Expect forString(String value) {
077      if (value == null)
078         return null;
079      return new Expect(value);
080   }
081
082   private Expect(String value) {
083      super(value);
084   }
085}