A Message Splitter implementation that evaluates the specified SpEL
expression. The result of evaluation will typically be a Collection or
Array. If the result is not a Collection or Array, then the single Object
will be returned as the payload of a single reply Message.
Subclasses must override this method to split the received Message. The return value may be a Collection or
Array. The individual elements may be Messages, but it is not necessary. If the elements are not Messages, each
will be provided as the payload of a Message. It is also acceptable to return a single Object or Message. In that
case, a single reply Message will be produced.