Auto Registering Converter

A Converter which registers itself with ConversionService

Introduction

The Data Transfer Object ( DTO ) pattern helps in wrapping the request body into a POJO. It is a good practice to keep the DB Model Entities separate from the DTOs although they end up having similar state fields. In this case the most common thing to do is creating a converter class which converts a DTO to ENTITY. Creating your own converter classes and then calling them on demand is a boilerplate code. Spring framework provides a Converter Interface which helps you get rid of the boilerplate code. A ConversionService helps in  easily locating an appropriate converter as per your need. However, conversion of a nested DTOs is tricky since a converter is registered with ConversionService and if we try to Autowire the Conversion Service into another converter then it creates a circular dependency. We will understand this use case in this blog.

Ideally we will need to Autowire the ConversionService into the UserDtoConverter to help locate the appropriate converter for AddressDto. You would think a simple way to use above converter is as shown below :

@Service

public class UserService {

    @Autowired

    private ConversionService conversionService;

    public void save(UserDto userDto){

        var mappedEntity = conversionService.convert(userDto, UserEntity.class);

    }

}

But if we Autowire ConversionService into UserDtoConverter we get below error :

***************************

APPLICATION FAILED TO START

***************************

Description:

Field conversionService in com.example.demo.converters.UserDtoConverter required a bean of type 'org.springframework.core.convert.ConversionService' that could not be found.

The injection point has the following annotations:

- @org.springframework.beans.factory.annotation.Autowired(required=true)

Auto Registering Converter

A converter which registers itself with ConversionService

To resolve this issue we need to create an abstract AutoRegisteringConverter which registers itself with the ConversionService post its own construction. Below is the required code for that :

Note : Below sample is suggested by André Wolf in this GitHub discussion

public abstract class AutoRegisteringConverter<S, T> implements Converter<S, T> {

    private ConversionService conversionService;

    public ConversionService getConversionService() {

        return conversionService;

    }

    @Autowired

    public void setConversionService(ConversionService conversionService) {

        this.conversionService = conversionService;

    }

    @PostConstruct

    private void register() {

        if (conversionService instanceof GenericConversionService) {

            ((GenericConversionService) conversionService).addConverter(this);

        }

    }

}

If you want such a setup then you don't need to implement the Converter Interface for that converter instead extend above AutoRegisteringConverter as shown below :

@Component

public class UserDtoConverter extends AutoRegisteringConverter<UserDto, UserEntity> {

    @Override

    public UserEntity convert(UserDto source) {

        var addressEntity = getConversionService().convert(source.address(), AddressEntity.class);

        return new UserEntity(source.name(), addressEntity);

    }

}

Please note for simple converters which do not need to delegate the mapping to other converters should be still implementing the Converter interface as in our case the AddressDtoConverter.

Summary

We learnt how to ensure that a converter follows a single responsibility principle of converting only the DTOs which it is supposed to and delegate the work to the other Converters.