Automatic content negotiation

[mkovatsc]

Content-Format and Accept handling is currently fully up to the resource implementer. Californium could provide an Accept option check before delivering requests or check the Content-Format after a request was processed and could automatically react with 4.06 Not Acceptable.

This could be added as Layer for the CoapStack.

[martinlanter]

I have implemented a proposal for this issue (https://github.com/Lanterm/californium/tree/accept-and-etag). I have added an interface AcceptSupport and a default implementation AcceptDefaultSupport. Each resource has such an object. By default, the support object lets any request pass and it will be up to the code in handleGET() to process it. This is exactly the behavior as of now. However, if the resource calls for instance getAcceptSupport().addAcceptable(MediaTypeRegistry.APPLICATION_XML);, the support object will check for the accept options of requests. If the accept option asks for a supported format, it passes. Otherwise, the resource automatically responds with a 4.06 and does NOT invoke handleGET(). With this mechanism, the developer is able to prevent requests with unsupported accept options from reaching the handleGET() method with minimal effort.

I did not choose to make this a layer in the CoapStack because it is not yet clear at this stage which resource is going to be the target, and therefore which content format it supports.