Springboot Generic JPA AttributeConverter - spring-data-jpa

Question

I have below sample code, i am trying to write generic JPA converter which could convert,
Collection of user defined objects to Json
vice versa
Below is sample code I was trying to achieve the result but looks like it's not correct.
Please take a look.
To be more clear i need like below
List To string
Json String to List
Please suggest
#Converter(autoApply = true)
public class SetJsonConverter<E extends Collections> implements AttributeConverter<E, Object> {
#Override
public Object convertToDatabaseColumn(E e) {
return null;
}
#Override
public E convertToEntityAttribute(Object o) {
ObjectMapper objectMapper=new ObjectMapper();
return null;
}
}

Answer 1

JPA will not automatically handle generic converters. Each collection type and element type will require subclassing. You will need to define the base converter the following way:
public class AbstractJsonConverter<T, C extends Collection<T>> implements AttributeConverter<C, String> {
private final ObjectMapper objectMapper;
private final TypeReference<C> collectionType;
public AbstractJsonConverter(ObjectMapper objectMapper, Class<T> elementType, TypeReference<C> collectionType) {
this.objectMapper = objectMapper;
this.collectionType = collectionType;
}
#Override
public String convertToDatabaseColumn(C collection) {
try {
return objectMapper.writeValueAsString(collection);
} catch (JsonProcessingException e) {
throw new RuntimeException(e);
}
}
#Override
public C convertToEntityAttribute(String jsonString) {
try {
return objectMapper.readValue(jsonString, collectionType);
} catch (IOException e) {
throw new RuntimeException();
}
}
}
You then define specific converters as:
#Converter(autoApply = true)
public class UserSetConverter extends AbstractJsonConverter<User, Set<User>> {
public UserSetConverter(ObjectMapper objectMapper) {
super(objectMapper, User.class, new TypeReference<Set<User>>() {});
}
}