You can try adding a @javax.ws.rs.core.Context javax.ws.rs.core.HttpHeaders field/property to your root resource class, resource method parameter, or to a custom javax.ws.rs.ext.ExceptionMapper and calling HttpHeaders.getMediaType().
You can try adding a @javax.ws.rs.core.Context javax.ws.rs.core.HttpHeaders field/property to your root resource class, resource method parameter, or to a custom javax.ws.rs.ext.ExceptionMapper and calling HttpHeaders.getMediaType().