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See the License for the * 011// * specific language governing permissions and limitations under the License. * 012// *************************************************************************************************************************** 013package org.apache.juneau.http; 014 015import org.apache.juneau.http.annotation.*; 016 017/** 018 * Represents a parsed <l>If-Modified-Since</l> HTTP request header. 019 * 020 * <p> 021 * Allows a 304 Not Modified to be returned if content is unchanged. 022 * 023 * <h5 class='figure'>Example</h5> 024 * <p class='bcode w800'> 025 * If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 026 * </p> 027 * 028 * <h5 class='topic'>RFC2616 Specification</h5> 029 * 030 * The If-Modified-Since request-header field is used with a method to make it conditional: 031 * if the requested variant has not been modified since the time specified in this field, an entity will not be returned 032 * from the server; instead, a 304 (not modified) response will be returned without any message-body. 033 * 034 * <p class='bcode w800'> 035 * If-Modified-Since = "If-Modified-Since" ":" HTTP-date 036 * </p> 037 * 038 * <p> 039 * An example of the field is: 040 * <p class='bcode w800'> 041 * If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 042 * </p> 043 * 044 * <p> 045 * A GET method with an If-Modified-Since header and no Range header requests that the identified entity be transferred 046 * only if it has been modified since the date given by the If-Modified-Since header. 047 * The algorithm for determining this includes the following cases: 048 * <ol> 049 * <li>If the request would normally result in anything other than a 200 (OK) status, or if the passed 050 * If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. 051 * A date which is later than the server's current time is invalid. 052 * <li>If the variant has been modified since the If-Modified-Since date, the response is exactly the same as for a 053 * normal GET. 054 * <li>If the variant has not been modified since a valid If-Modified-Since date, the server SHOULD return a 304 055 * (Not Modified) response. 056 * </ol> 057 * 058 * <p> 059 * The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction 060 * overhead. 061 * 062 * <p> 063 * Note: The Range request-header field modifies the meaning of If-Modified-Since; see section 14.35 for full details. 064 * 065 * <p> 066 * Note: If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client. 067 * 068 * <p> 069 * Note: When handling an If-Modified-Since header field, some servers will use an exact date comparison function, 070 * rather than a less-than function, for deciding whether to send a 304 (Not Modified) response. 071 * To get best results when sending an If-Modified-Since header field for cache validation, clients are 072 * advised to use the exact date string received in a previous Last-Modified header field whenever possible. 073 * 074 * <p> 075 * Note: If a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the 076 * Last-Modified header for the same request, the client should be aware of the fact that this date is interpreted in 077 * the server's understanding of time. 078 * The client should consider unsynchronized clocks and rounding problems due to the different encodings of time between 079 * the client and server. 080 * This includes the possibility of race conditions if the document has changed between the time it was first requested 081 * and the If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the 082 * If-Modified-Since date is derived from the client's clock without correction to the server's clock. 083 * Corrections for different time bases between client and server are at best approximate due to network latency. 084 * The result of a request having both an If-Modified-Since header field and either an If-Match or an 085 * If-Unmodified-Since header fields is undefined by this specification. 086 * 087 * <ul class='seealso'> 088 * <li class='extlink'>{@doc RFC2616} 089 * </ul> 090 */ 091@Header("If-Modified-Since") 092public final class IfModifiedSince extends HeaderDate { 093 094 /** 095 * Returns a parsed <c>If-Modified-Since</c> header. 096 * 097 * @param value The <c>If-Modified-Since</c> header string. 098 * @return The parsed <c>If-Modified-Since</c> header, or <jk>null</jk> if the string was null. 099 */ 100 public static IfModifiedSince forString(String value) { 101 if (value == null) 102 return null; 103 return new IfModifiedSince(value); 104 } 105 106 private IfModifiedSince(String value) { 107 super(value); 108 } 109}