Hi Reto,
My approach has been to only implement get size iff it is known in
advanced (e.g. File size). I consider it more of a hint to the HTTP
container to know whether it can use a Content-Length with that value
instead of:
1) using chunked transfer encoding;
2) buffering the content to determine the length; or
3) closing the connection after the response entity has been written.
I think the HTTP container should determine the best strategy of one
of the three above and the writers do not need to be involved in such
details.
If getSize returns a value > -1 then that value should be equal to the
value that is number of bytes that are written to the OutputStream of
the writeTo method for the same parameter values passed to both methods.
Paul.
On Jan 29, 2009, at 9:41 AM, Reto Bachmann-Gmür wrote:
> Hello
>
> I'm wondering what the best practice to write MBWs is. In my
> experience
> the getSize and the writeTo method often have very similar code, or
> the
> getSize-method looks similar to this:
>
> @Override
> public long getSize(GraphNode node, Class<?> clazz, Type t,
> Annotation[] as,
> MediaType m) {
> ByteArrayOutputStream baos = new ByteArrayOutputStream();
> try {
> writeTo(node, clazz, t, as, m, null, baos);
> } catch (IOException ex) {
> throw new RuntimeException(ex);
> } catch (WebApplicationException ex) {
> throw new RuntimeException(ex);
> }
> return baos.size();
> }
>
> One optimization might be to cache the resulting object- bytes mapping
> (in a ThreadLocal weak reference of a WeakHashMap) so that when the
> implementation invokes writeTo, the already computed bytes can be
> written to the OutputStream.
>
> Another approach could be to return an estimate of the size or -1 in
> the
> getSize method, and set the exact Content-Length to the headers-map
> when
> writeTo is invoked.
>
> Another issue is when the rendering of the object could change between
> invocation of getSize and writeTo. The class GraphNode represents a
> node
> in a graph (which is occasionally modified), the writers for different
> media-types expand the neighborhood of the node to different depths so
> that it would be awkward to do this expansion in the resource method
> and
> return a value-object. I think in such a scenario the writeTo method
> must either write the bytes pre-computed on invocation of getSize or
> (re)set the Content-Length header.
>
> Cheers,
> Reto
>
>
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