RESTEasy currently doesn't support using ParamConverters for any kind of primitive type. IMO this is a violation of the JAX-RS spec. The API docs of ParamConverterProvider doesn't restrict the types for which users can register custom converters. Jersey for example supports converters for all types, including primitives.

I think that all the problematic code can be found in the StringParameterInjector.

In case of primitive types, it doesn't even try to lookup the ParamConverter:

https://github.com/resteasy/Resteasy/blob/63862e7e5b109d56227a9450fd1dc209cc926597/resteasy-core/src/main/java/org/jboss/resteasy/core/StringParameterInjector.java#L462-L473

And later on it converts primitive types using StringToPrimitive BEFORE even checking if there is a ParamConverter:

https://github.com/resteasy/Resteasy/blob/63862e7e5b109d56227a9450fd1dc209cc926597/resteasy-core/src/main/java/org/jboss/resteasy/core/StringParameterInjector.java#L682-L698

Thoughts?