This class provides an example of adding trailers to serialized
AttributedMessage s. The
MessageWriter computes a checksum
(as a long) and sends the eight bytes after the message content.
The
MessageReader computes its own checksum and compares to
the one that was sent. The CHECKSUM_VALID_ATTR records whether or
not they matched. This attribute is stored in the
AttributedMessage and is available to all receive-side Aspects.