OutputStreams and Writers are trivially implemented using streams - writes to the remote object are simply propagated across to channel. Other types of streams are more difficult to implement this way. InputStream for example has two types of messages - a data chunk and an exception chunk. Because of this, some basic framing is necessary; with only support for streams, this framing must take place within the stream handler which is a pain to implement.

So, a stream serializer should be able to register itself as a byte stream handler (using a StreamChannel) or a message stream handler (using a MessageChannel).