Depending on the Mapstruct version you are using, this solution works:

• Create a named method that converts your value to a string in your mapper, and annotate it with @Named.

@Named("valueToString")
String valueToString(value){return value.toString()};

• Use qualifiedByName in your @Mapping annotation to indicate that you want to use this method in order to map your value:

@Mapping(source = "value.toString()", target = "date", qualifiedByName="valueToString")
String map(Foo value);

Voilà!