Java Reference
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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 / 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 that represents the ETag
public class
class EntityTag
EntityTag {
public EntityTag ( String value ) {...}
public EntityTag ( String value , boolean
boolean weak ) {...}
public static
static EntityTag valueOf ( String value )
throws IllegalArgumentException {...}
public boolean
boolean isWeak () {...}
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 :
public interface
interface Request
Request {
ResponseBuilder evaluatePreconditions ( EntityTag eTag );
ResponseBuilder evaluatePreconditions ( Date lastModified );
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