RESTful Java Web Services - Third Edition

Conditional request processing with the ETag HTTP response header

Sometimes, you may find that the precision of the HTTP date object (precision in seconds) is not granular enough to decide the freshness of the cache. You can use Entity tags (ETag) present in the HTTP response header field for such scenarios. ETag is part of HTTP/1.1 and provides a way of incorporating caching into HTTP. This tag can be used for comparing the validity of a cached object on the client side with the original object on the server.

When a server returns a response to a client, it attaches an opaque identifier in the ETag HTTP header present in the response. This identifier represents the state of the resource entity returned in response to the client's request. ...