34.13. Configuring Mappers with a MapperFactory

Similar to previous examples of factories in the case study, the configuration of the PersistenceFacade with a set of IMapper objects can be achieved with a factory object, MapperFactory. However, as a slight twist, it is desirable to not name each mapper with a different operation. For example, this is not desirable:

						class MapperFactory
						{
						public IMapper getProductSpecificationMapper() {...}
						public IMapper getSaleMapper() {...}
						...
						}
					

This does not support Protected Variations with respect to a growing list of mappers—and it will grow. Consequently, the following is preferred:

						class MapperFactory
						{
						public Map getAllMappers() {...}
						...
						}
					

where the java.util.Map (probably implemented with a HashMap) keys are the Class objects (the persistent types), and the IMappers are the values.

Then, the facade can initialize its collection of IMappers as follows:

						class PersistenceFacade
						{
						private java.util.Map mappers =
						MapperFactory.getInstance().getAllMappers();
						...
						}
					

The factory can assign a set of IMappers using a data-driven design. That is, the factory can read system properties to discover which IMapper classes to instantiate. If a language with reflective programming capabilities is used, such as Java, then the instantiation can be based on reading in the class names as strings, and using something like a Class.newInstance operation for instantiation. Thus, the mapper set can be reconfigured without changing the source code.