Scaling JAX-RS Applications - RESTful Java with JAX-RS 2.0 - page 168

server will send back a 200, “OK,” response with the new representation of the resource. If it
hasn't been changed, the server will respond with 304, “Not Modified,” and return no repres-
entation. In both cases, the server should send an updated Cache-Control and ETag header
if appropriate.

One final thing about ETag s is they come in two flavors: strong and weak. A strong ETag
should change whenever any bit of the resource's representation changes. A weak ETag
changes only on semantically significant events. Weak ETag s are identified with a W/ prefix.
For example:

HTTP HTTP / 1.1 200 OOK

Content-Type : application/xml
Cache-Control : max-age=1000
ETag: W/"3141271342554322343200"

<customer id="123">...</customer>

Weak ETag s give applications a bit more flexibility to reduce network traffic, as a cache can
be revalidated when there have been only minor changes to the resource.

JAX-RS has a simple class called javax.ws.rs.core.EntityTag that represents the ETag
header:

public public class class EntityTag EntityTag {

public public EntityTag ( String value ) {...}

public

public EntityTag ( String value , boolean boolean weak ) {...}

public public static static EntityTag valueOf ( String value )

throws throws IllegalArgumentException {...}

public public boolean boolean isWeak () {...}

public public String getValue () {...}

}

It is constructed with a string value and optionally with a flag telling the object if it is a weak
ETag or not. The getValue() and isWeak() methods return these values on demand.

JAX-RS and conditional GETs

To help with conditional GETs, JAX-RS provides an injectable helper class called
javax.ws.rs.core.Request :

public public interface interface Request Request {

...

ResponseBuilder evaluatePreconditions ( EntityTag eTag );
ResponseBuilder evaluatePreconditions ( Date lastModified );