public static class PropertySource.StubPropertySource
PropertySource to be used as a placeholder in cases where an actual
property source cannot be eagerly initialized at application context
creation time. For example, a ServletContext-based property source
must wait until the ServletContext object is available to its enclosing
ApplicationContext. In such cases, a stub should be used to hold the
intended default position/order of the property source, then be replaced
during context refresh.