As Spring and some other libraries, it would be nice to support default property value. For sample :
Lookup will return the actual system property value of "property_name" if defined, otherwise "default_value".
The form without default value would work as normal and return a "null" value when this property is not defined.
The way of getting the default value will be similar to "ResourceBundleLookup" and both "EnvironmentLookup" and "SystemPropertiesLookup" will benefit this feature.