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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.header; 014 015import java.time.*; 016import java.util.function.*; 017 018import org.apache.juneau.http.annotation.*; 019 020/** 021 * Represents a parsed <l>If-Modified-Since</l> HTTP request header. 022 * 023 * <p> 024 * Allows a 304 Not Modified to be returned if content is unchanged. 025 * 026 * <h5 class='figure'>Example</h5> 027 * <p class='bcode'> 028 * If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 029 * </p> 030 * 031 * <h5 class='topic'>RFC2616 Specification</h5> 032 * 033 * The If-Modified-Since request-header field is used with a method to make it conditional: 034 * if the requested variant has not been modified since the time specified in this field, an entity will not be returned 035 * from the server; instead, a 304 (not modified) response will be returned without any message-body. 036 * 037 * <p class='bcode'> 038 * If-Modified-Since = "If-Modified-Since" ":" HTTP-date 039 * </p> 040 * 041 * <p> 042 * An example of the field is: 043 * <p class='bcode'> 044 * If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 045 * </p> 046 * 047 * <p> 048 * A GET method with an If-Modified-Since header and no Range header requests that the identified entity be transferred 049 * only if it has been modified since the date given by the If-Modified-Since header. 050 * The algorithm for determining this includes the following cases: 051 * <ol> 052 * <li>If the request would normally result in anything other than a 200 (OK) status, or if the passed 053 * If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. 054 * A date which is later than the server's current time is invalid. 055 * <li>If the variant has been modified since the If-Modified-Since date, the response is exactly the same as for a 056 * normal GET. 057 * <li>If the variant has not been modified since a valid If-Modified-Since date, the server SHOULD return a 304 058 * (Not Modified) response. 059 * </ol> 060 * 061 * <p> 062 * The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction 063 * overhead. 064 * 065 * <p> 066 * Note: The Range request-header field modifies the meaning of If-Modified-Since; see section 14.35 for full details. 067 * 068 * <p> 069 * Note: If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client. 070 * 071 * <p> 072 * Note: When handling an If-Modified-Since header field, some servers will use an exact date comparison function, 073 * rather than a less-than function, for deciding whether to send a 304 (Not Modified) response. 074 * To get best results when sending an If-Modified-Since header field for cache validation, clients are 075 * advised to use the exact date string received in a previous Last-Modified header field whenever possible. 076 * 077 * <p> 078 * Note: If a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the 079 * Last-Modified header for the same request, the client should be aware of the fact that this date is interpreted in 080 * the server's understanding of time. 081 * The client should consider unsynchronized clocks and rounding problems due to the different encodings of time between 082 * the client and server. 083 * This includes the possibility of race conditions if the document has changed between the time it was first requested 084 * and the If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the 085 * If-Modified-Since date is derived from the client's clock without correction to the server's clock. 086 * Corrections for different time bases between client and server are at best approximate due to network latency. 087 * The result of a request having both an If-Modified-Since header field and either an If-Match or an 088 * If-Unmodified-Since header fields is undefined by this specification. 089 * 090 * <h5 class='section'>See Also:</h5><ul> 091 * <li class='link'><a class="doclink" href="../../../../../index.html#juneau-rest-common">juneau-rest-common</a> 092 * <li class='extlink'><a class="doclink" href="https://www.w3.org/Protocols/rfc2616/rfc2616.html">Hypertext Transfer Protocol -- HTTP/1.1</a> 093 * </ul> 094 * 095 * @serial exclude 096 */ 097@Header("If-Modified-Since") 098public class IfModifiedSince extends BasicDateHeader { 099 100 //----------------------------------------------------------------------------------------------------------------- 101 // Static 102 //----------------------------------------------------------------------------------------------------------------- 103 104 private static final long serialVersionUID = 1L; 105 private static final String NAME = "If-Modified-Since"; 106 107 /** 108 * Static creator. 109 * 110 * @param value 111 * The header value. 112 * <br>Must be an RFC-1123 formated string (e.g. <js>"Sat, 29 Oct 1994 19:43:31 GMT"</js>). 113 * <br>Can be <jk>null</jk>. 114 * @return A new header bean, or <jk>null</jk> if the name is <jk>null</jk> or empty or the value is <jk>null</jk>. 115 */ 116 public static IfModifiedSince of(String value) { 117 return value == null ? null : new IfModifiedSince(value); 118 } 119 120 /** 121 * Static creator. 122 * 123 * @param value 124 * The header value. 125 * <br>Can be <jk>null</jk>. 126 * @return A new header bean, or <jk>null</jk> if the name is <jk>null</jk> or empty or the value is <jk>null</jk>. 127 */ 128 public static IfModifiedSince of(ZonedDateTime value) { 129 return value == null ? null : new IfModifiedSince(value); 130 } 131 132 /** 133 * Static creator with delayed value. 134 * 135 * <p> 136 * Header value is re-evaluated on each call to {@link #getValue()}. 137 * 138 * @param value 139 * The supplier of the header value. 140 * <br>Can be <jk>null</jk>. 141 * @return A new header bean, or <jk>null</jk> if the name is <jk>null</jk> or empty or the value is <jk>null</jk>. 142 */ 143 public static IfModifiedSince of(Supplier<ZonedDateTime> value) { 144 return value == null ? null : new IfModifiedSince(value); 145 } 146 147 //----------------------------------------------------------------------------------------------------------------- 148 // Instance 149 //----------------------------------------------------------------------------------------------------------------- 150 151 /** 152 * Constructor. 153 * 154 * @param value 155 * The header value. 156 * <br>Must be an RFC-1123 formated string (e.g. <js>"Sat, 29 Oct 1994 19:43:31 GMT"</js>). 157 * <br>Can be <jk>null</jk>. 158 */ 159 public IfModifiedSince(String value) { 160 super(NAME, value); 161 } 162 163 /** 164 * Constructor. 165 * 166 * @param value 167 * The header value. 168 * <br>Can be <jk>null</jk>. 169 */ 170 public IfModifiedSince(ZonedDateTime value) { 171 super(NAME, value); 172 } 173 174 /** 175 * Constructor with delayed value. 176 * 177 * <p> 178 * Header value is re-evaluated on each call to {@link #getValue()}. 179 * 180 * @param value 181 * The supplier of the header value. 182 * <br>Can be <jk>null</jk>. 183 */ 184 public IfModifiedSince(Supplier<ZonedDateTime> value) { 185 super(NAME, value); 186 } 187}