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package org.apache.juneau.http;

import org.apache.juneau.http.annotation.*;

/**
 * Represents a parsed <l>If-Unmodified-Since</l> HTTP request header.
 *
 * <p>
 * Only send the response if the entity has not been modified since a specific time.
 *
 * <h5 class='figure'>Example</h5>
 * <p class='bcode w800'>
 * 	If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 * </p>
 *
 * <h5 class='topic'>RFC2616 Specification</h5>
 *
 * The If-Unmodified-Since request-header field is used with a method to make it conditional.
 * If the requested resource has not been modified since the time specified in this field, the server SHOULD perform the
 * requested operation as if the If-Unmodified-Since header were not present.
 *
 * <p>
 * If the requested variant has been modified since the specified time, the server MUST NOT perform the requested
 * operation, and MUST return a 412 (Precondition Failed).
 *
 * <p class='bcode w800'>
 * 	If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
 * </p>
 *
 * <p>
 * An example of the field is:
 * <p class='bcode w800'>
 * 	If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 * </p>
 *
 * <p>
 * If the request normally (i.e., without the If-Unmodified-Since header) would result in anything other than a 2xx or
 * 412 status, the If-Unmodified-Since header SHOULD be ignored.
 *
 * <p>
 * If the specified date is invalid, the header is ignored.
 *
 * <p>
 * The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an
 * If-Modified-Since header fields is undefined by this specification.
 *
 * <ul class='seealso'>
 * 	<li class='extlink'>{@doc RFC2616}
 * </ul>
 */
@Header("If-Unmodified-Since")
public final class IfUnmodifiedSince extends HeaderDate {

	/**
	 * Returns a parsed <c>If-Unmodified-Since</c> header.
	 *
	 * @param value The <c>If-Unmodified-Since</c> header string.
	 * @return The parsed <c>If-Unmodified-Since</c> header, or <jk>null</jk> if the string was null.
	 */
	public static IfUnmodifiedSince forString(String value) {
		if (value == null)
			return null;
		return new IfUnmodifiedSince(value);
	}

	private IfUnmodifiedSince(String value) {
		super(value);
	}
}