org.apache.juneau.http

Class IfMatch

java.lang.Object

org.apache.juneau.http.HeaderEntityValidatorArray

org.apache.juneau.http.IfMatch

public final class IfMatch
extends HeaderEntityValidatorArray

Represents a parsed If-Match HTTP request header.

Only perform the action if the client supplied entity matches the same entity on the server. This is mainly for methods like PUT to only update a resource if it has not been modified since the user last updated it.

Example

If-Match: "737060cd8c284d8af7ad3082f209582d"

RFC2616 Specification

The If-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously obtained from the resource can verify that one of those entities is current by including a list of their associated entity tags in the If-Match header field. Entity tags are defined in section 3.11. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. It is also used, on updating requests, to prevent inadvertent modification of the wrong version of a resource. As a special case, the value "*" matches any current entity of the resource.

If-Match = "If-Match" ":" ( "*" | 1#entity-tag )

If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request (without the If-Match header) on that resource, or if "*" is given and any current entity exists for that resource, then the server MAY perform the requested method as if the If-Match header field did not exist.

A server MUST use the strong comparison function (see section 13.3.3) to compare the entity tags in If-Match.

If none of the entity tags match, or if "*" is given and no current entity exists, the server MUST NOT perform the requested method, and MUST return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating method, such as PUT, from modifying a resource that has changed since the client last retrieved it.

If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status, then the If-Match header MUST be ignored.

The meaning of "If-Match: *" is that the method SHOULD be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see section 14.44) exists, and MUST NOT be performed if the representation does not exist.

A request intended to update a resource (e.g., a PUT) MAY include an If-Match header field to signal that the request method MUST NOT be applied if the entity corresponding to the If-Match value (a single entity tag) is no longer a representation of that resource. This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge.

Examples:

If-Match: "xyzzy" If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-Match: *

The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification.

See Also:

• Hypertext Transfer Protocol -- HTTP/1.1

Method Summary

Modifier and Type	Method and Description
static IfMatch	forString(String value) Returns a parsed If-Match header.

Methods inherited from class org.apache.juneau.http.HeaderEntityValidatorArray

asValidators, toString

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Method Detail

forString

public static IfMatch forString(String value)

Returns a parsed If-Match header.

Parameters:

value - The If-Match header string.

Returns:

The parsed If-Match header, or null if the string was null.