How do I create a custom data type converter?

Datatype converters can either implement Converter (preferably) or GenericConverter if one needs runtime class information to govern the conversion strategy. All beans of both types are passed the conversionService at runtime.

Note that even though Converter is a functional interface, it is NOT possible defined a bean implementation as a lambda due to type erasure of generated lambdas, as such they need to be anonymous classes.

Note that the GenericConverter support strategy are not inherently by-directional, so either register both a String -> Type converter AND a distinct Type -> String converter, or register a single converter that supports both directions (and then derive the direction within the convert method by type checking the source object). Additionally it is also possible to specify a type (other than string) when accessing a property, in which case a converter between othe object’s property type and the desired type would be invoked.

A note on overriding existing converters: The GenericConverter interface is untyped, as such the last loaded bean of a same name will be registered. A good practice is to namespace any converters appropriately (e.g myBankConverter vs dateConverter) and restructure the config instantiation order as needed if core converters need to be overridden.

The ConversionConfig.java configuration class contains some examples of registered type converters that are applied by default.

For usage tips please see the following unit test cases: