Selection of MessageBodyWriter - jax-rs

Question

My Application has 2 MessageBodyWriters:
MBW1 produces something/concrete; qs=0.6
MBW2 produces */*; qs=0.01
RestEasy correctly uses MBW1 for the following accept header values:
something/unsupported, */*
something/unsupported,*/*; q=1
something/unsupported;q=0.9,*/*;q=.9'
*/*
*/*; q=0.1
However it chooses MBW2 for the following accept-header value:
something/unsupported,*/*; q=0.99
I would like MBW2 to be chosen only when no other MBW can provide something acceptable to the client. However it seems that MBW2 is chosen even if the concrete type produced by MBW1 would be indeed acceptable by the client. I don't see the rationale that RestEsays returns something/concrete with 4 and 5 but not when the client adds an additional unsupported format (6). Is this an issue of the JAX-RS Spec or of RestEasy?

Answer 1

Following section 3.8 of the JAX-RS specification and assuming teh JAX-RS resource method and class have no #Produces annotation and that the two MessageBodyWriters support the class of returned entity object in step 2 we set
P = {"something/concrete; qs=0.6", "*/*; qs=0.01"}
and is step 4 we set
A = {"something/unsupported","*/*; q=0.99"}
by combining the values from P and A in step 5 we set
M = {"something/unsupported; qs=0.01", "something/concrete; q=0.99; qs=0.6", "*/*; q=0.99; qs=0.01"}
In step 7 M is sorted as follows, given that something/unsupported has an implicit q value of 1
M = {"something/unsupported; q=1; qs=0.01", "something/concrete; q=0.99; qs=0.6", "*/*; q=0.99; qs=0.01"}
In step 8 something/unsupported would be selected a MediaType of Response and consequentaly MBW2 would be chosen.
So it seems to be an issue of the JAX-RS spec as qs is not used to indicate quality and is only a tertiary sorting factor when the q-values are identical.
Here's a proposed changed to JAX-RS to allow for more flexible content negotiation: https://github.com/factsmission/jax-rs-spec/