Feature request: Custom serializer for map keys

[WouterGritter]

When using a custom type in a map within a config, e.g.:

@Configuration
class MyMessagesConfig {
    Map<MyCustomType, String> translations;
}

With MyCustomType having a @SerializeWith(serializer = ...) annotation (with the serializer serializing to String), I am unable to serialize/deserialize this type:

myplugin.shaded.configlib.ConfigurationException: Cannot select serializer for type 'java.util.Map<myplugin.MyCustomType, java.lang.String>'.
Map keys can only be of simple or enum type.

I recognize that key types are more limited, I presume they can only be of type String and not of any other primitive type. But when having a (de)serializer that serializes to and from Strings, I think this should be possible.

In my usecase, MyCustomType is a glorified enum with some custom logic. The (de)serializer simply gets the name of the type, and fetches the type again by the name when deserializing.

[Exlll]

Hi @WouterGritter, thanks for your suggestion.

Besides String map keys can be enums and other "simple target types", like Boolean, Integer, Double, etc.
Is it perhaps possible for you to model your class as an enum class?

I agree, however, that applying custom serializers to map keys should be possible as long as valid target type are produced. I don't remember the exact reason why I added this restriction, but I remember there was one.

In the meantime, another (although more verbose) solution would be to write a serializer that applies to the whole translations map in your configuration class. In it, you can do whatever you like as long as it returns a vali target type. Here is an example:

@Configuration
static class MyMessagesConfig {
    @SerializeWith(serializer = TranslationMapSerializer.class)
    Map<MyCustomType, String> translations;
}

static final class TranslationMapSerializer
        implements Serializer<Map<MyCustomType, String>, Map<String, String>> {

    // your actual transformation functions...
    String myCustomTypeToString(MyCustomType type) {return null;}
    MyCustomType myCustomTypeFromString(String string) {return null;}

    @Override
    public Map<String, String> serialize(Map<MyCustomType, String> element) {
        return element.entrySet().stream().collect(Collectors.toMap(
                entry -> myCustomTypeToString(entry.getKey()),
                Map.Entry::getValue
        ));
    }

    @Override
    public Map<MyCustomType, String> deserialize(Map<String, String> element) {
        return element.entrySet().stream().collect(Collectors.toMap(
                entry -> myCustomTypeFromString(entry.getKey()),
                Map.Entry::getValue
        ));
    }
}