Coming Up for Air

Initializing JAX-RS Sub-resources

This morning, I was reading through the Proposed Final Draft for JAX-RS 2.0 specification, when I found a little nugget that could have saved me some work, specificially in initializing subresources.

This is kind of sad to admit (though, surely — hopefully — I’m not alone in this :), but I have been initializing subresource manually. For example:


    
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public <T> T getSubResource(Class<T> clazz) {
    try {
        T resource = clazz.newInstance();
        BaseResource br = (BaseResource)resource;
        br.uriInfo = uriInfo;
        br.securityContext = securityContext;
        br.requestHeaders = requestHeaders;

        return resource;
    } catch (Exception ex) {
        throw new WebApplicationException(ex, Status.INTERNAL_SERVER_ERROR);
    }
}

While this works, the problems should be pretty obvious: it’s extremely inflexible. Fortunately, the JAX-RS Expert Group, who has clearly thought about this more than I, has a better solution: ResourceContext. Take this example, pulled from the spec:


    
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@Path("widgets")
public class WidgetsResource {
    @Context
    private ResourceContext rc;

    @Path("{id}")
    public WidgetResource findWidget(@PathParam("id") String id) {
        return rc.initResource(new WidgetResource(id));
    }
}

public class WidgetResource {
    @Context
    private HttpHeaders headers;

    public WidgetResource(String id) {...}

    @GET
    public Widget getDetails() {...}
}

Notice the method WidgetsResource.findWidget(). The parent resource, WidgetsResource, has an instance of ResourceContext injected, which is then used to initialize the subresource instantiated in findWidget(). The benefit of this approach over the approach above is that everthing is injected, regardless of what I’m expecting to find.

I’ve been working with JAX-RS for over 3 years now, and I’m just now learning this. Shows that there’s always something to learn, and that reading specs pays off. :)

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