The set of "value" types that can be used in any place a value
can be specified.  This set includes the <ref> and <idref> elements, any of the
component types (<bean>, <service>, etc.) as inline components, the
generic <value> element for types sourced from string values, any of the
collection types (<set>, <list>, <array>, <map>, <props>), and the
<null> type to inject a null value.