Experiment with HAL as the basis for an application protocol about multibit-merchant

[gary-rowe]

HAL+JSON introduces a few discontinuities:

• the generic JAX-RS Response is needed with HalBuilder providing the entity body (unless a custom MessageBodyWriter is introduced) so automated Jackson marshalling to an entity doesn't really work
• promotes careful thought about the state machine behind the resources
• is much more lightweight than AtomPub and OData but may be too lightweight since there is no standard way of obtaining a meta-model (e.g. How does a User relate to Roles, and what transitions are available between them?)

Made an attempt at fixing the custom marshalling part of HalBuilder, but rapidly discovered that JAXB and Jackson produce XML and JSON that is not quite the same as required by HAL. There are several subtleties that the HalBuilder library deals with.

Overall, this simply means that the application Resource endpoints must be derivatives of Response.

[gary-rowe]

Merged this into develop