PropertySourcesPlaceholderConfigurer accommodates recent changes in
Environment and PropertySource APIs, e.g. no longer assuming enumerability
of property names.
PSPC reuses as much functionality as possible from
AbstractPropertyPlaceholderConfigurer, but overrides
postProcessBeanFactory() and defines its own variation on
processProperties() in order to accept a PropertyResolver rather than
a PropertySource.
AbstractPropertyPlaceholderConfigurer introduces doProcessProperties()
method to encapsulate that which is actually common, such as the
visiting of each bean definition once a StringValueResolver has been
created in the subclass.
Chris Beams