How do I discover what content types a HTTP POST service allows?

I am writing a client for OpenTox webservices, but several services they provide have optional support for certain MIME types with only a few obligatory, e.g. this Dataset service. So I need to discover which of those content types are supported by a particular running service dynamically...

Now, for GET services I can use the "Accepts" HTTP header and when I POST content I can use "Content-type" to set what format I am sending in. But I rather not default into trial-and-error, as these data sets can be large. Therefore, I like to know up front the best MIME type I can use.

How do I perform content ne开发者_Go百科gotiation with POST services? I am using the Apache HttpClient, but that is not important for the answer.

---

POSTed Content negotiation is not enshrined in the HTTP specification; instead what is advisable is to use the OPTIONS method. This is exactly the case in the OpenTox framework: OPTIONS has to be implemented by all web services. The service returns an RDF document (according to the Accept Header you specify) and a WADL-like content, i.e. it returns machine-readable documentation specifying the available MIME types, the possible HTTP status codes etc. Unfortunately so far there's only one implementation of OPTIONS in OpenTox, namely JAQPOT3. Check out this wiki entry.