When a client offers multiple media types in an Accept header, for example, Accept: text/html, image,jpg, text/json, *
https://github.com/resteasy/Resteasy/blob/3.0.10.Final/jaxrs/resteasy-jaxrs/src/main/java/org/jboss/resteasy/core/registry/SegmentNode.java#L430 selects the most specific type and places it as an attribute without taking into consideration the MessageBodyWriters available.
When the type is retrieved in https://github.com/resteasy/Resteasy/blob/3.0.10.Final/jaxrs/resteasy-jaxrs/src/main/java/org/jboss/resteasy/core/ServerResponseWriter.java#L129 it is only the best type chosen previously, irrespective of current MessageBodyWriters available.
If I have a resource that can return different object types, for example, String & JsonObject. and in this case, my endpoint returns returns a JsonObject, I will get an error similar to:
Could not find MessageBodyWriter for response object of type: org.glassfish.json.JsonObjectBuilderImpl$JsonObjectImpl of media type: text/html
There isn't a writer registered for text/html but there is one for text/json, which would have been handled by the text/json or *