Jersey(1.19.1)

Conditional GETs are a great way to reduce bandwidth, and potentially server-side performance, depending on how the information used to determine conditions is calculated. A well-designed web site may return 304 (Not Modified) responses for the many of the static images it serves.

JAX-RS provides support for conditional GETs using the contextual interface Request.

The following example shows conditional GET support from the sparklines sample:

public SparklinesResource(
    @QueryParam("d") IntegerList data,
    @DefaultValue("0,100") @QueryParam("limits") Interval limits,
    @Context Request request,
    @Context UriInfo ui) {
    
    if (data == null)
        throw new WebApplicationException(400);

    this.data = data;

    this.limits = limits;

    if (!limits.contains(data))
        throw new WebApplicationException(400);

    this.tag = computeEntityTag(ui.getRequestUri());
    if (request.getMethod().equals("GET")) {
        Response.ResponseBuilder rb = request.evaluatePreconditions(tag);
        if (rb != null)
            throw new WebApplicationException(rb.build());
    }
}

The constructor of the SparklinesResouce root resource class computes an entity tag from the request URI and then calls the request.evaluatePreconditions with that entity tag. If a client request contains an If-None-Match header with a value that contains the same entity tag that was calculated then the evaluatePreconditionsreturns a pre-filled out response, with the 304 status code and entity tag set, that may be built and returned. Otherwise, evaluatePreconditions returns null and the normal response can be returned.

Notice that in this example the constructor of a resource class can be used perform actions that may otherwise have to be duplicated to invoked for each resource method.