I am testing what happens when Encode/Decode Exceptions occur during JSR-356 communication.  In our case, we do NOT use annotation, but we "extend Endpoint" and "implement Whole<ThingworxMessage>".  I was forcing a DecodeException to occur - expecting to my onError to be called with the actual DecodeException.  Instead, I got a Null.  

Stepping the following code in Tomcat:

in onMessage, the ((WsSession)session) is NULL.  As a result, the actual DecodeException (cause) is lost.

       // Can this message be decoded?
        Object payload;
        try {
            payload = decode(message);
        } catch (DecodeException de) {
            ((WsSession) session).getLocal().onError(session, de);
            return;
        }


Tracing this further up the stack, I found that Util.getMessageHandlers is initializing it and passing NULL in for the session:

            if (decoderMatch.getTextDecoders().size() > 0) {
                MessageHandlerResult result = new MessageHandlerResult(
                        new PojoMessageHandlerWholeText(listener, m, null,
                                endpointConfig,
                                decoderMatch.getTextDecoders(), new Object[1],
                                0, false, -1, -1),
                        MessageHandlerResultType.TEXT);
                results.add(result);
            }

Please let me know if I can provide any more details.

-bob