Boost Spring Apps with MapStruct: Seamless Conversion Service Integration

Previously I recommended the MapStruct tool, which can replace BeanUtil for converting between DTO, VO, and PO objects using Java's compile‑time annotation processor.

MapStruct generates clean code, dramatically reducing manual mapping effort.

@Mapper(componentModel = "spring")
public interface AreaMapping {
    List<AreaInfoListVO> toVos(List<Area> areas);
}

The above interface converts a list of PO objects to a list of VO objects with just a few lines.

// spring bean
@Autowired
AreaMapping areaMapping;

// conversion source
List<Area> areas = …;
// conversion target
List<AreaInfoListVO> vos = areaMapping.toVos(areas);

Writing such conversion manually would take a lot of time.

Converter

Spring provides a Converter<S,T> interface:

@FunctionalInterface
public interface Converter<S,T> {
    @Nullable
    T convert(S source);

    default <U> Converter<S, U> andThen(Converter<? super T, ? extends U> after) {
        Assert.notNull(after, "After Converter must not be null");
        return (s) -> {
            T initialResult = this.convert(s);
            return initialResult != null ? after.convert(initialResult) : null;
        };
    }
}

It converts a source S to a target T, which aligns with MapStruct’s purpose.

Converters are registered to ConversionService via ConverterRegistry, allowing you to invoke:

<T> T convert(@Nullable Object source, Class<T> targetType);

MapStruct Spring Extensions

The official MapStruct Spring Extensions plugin makes any mapper that also implements Converter automatically register with ConversionService:

@Mapper(componentModel = "spring")
public interface CarMapper extends Converter<Car, CarDto> {
    @Mapping(target = "seats", source = "seatConfiguration")
    CarDto convert(Car car);
}

Usage:

@Autowired
private ConversionService conversionService;

Car car = …;
CarDto carDto = conversionService.convert(car, CarDto.class);

The plugin generates an adapter class that registers the mapper:

package org.mapstruct.extensions.spring.converter;

import cn.felord.mapstruct.entity.Car;
import cn.felord.mapstruct.entity.CarDto;
import org.springframework.context.annotation.Lazy;
import org.springframework.core.convert.ConversionService;
import org.springframework.stereotype.Component;

@Component
public class ConversionServiceAdapter {
    private final ConversionService conversionService;

    public ConversionServiceAdapter(@Lazy final ConversionService conversionService) {
        this.conversionService = conversionService;
    }

    public CarDto mapCarToCarDto(final Car source) {
        return (CarDto) this.conversionService.convert(source, CarDto.class);
    }
}

Custom Adapter Package and Name

By default the generated adapter resides in org.mapstruct.extensions.spring.converter with the name ConversionServiceAdapter. You can change the package and class name:

package cn.felord.mapstruct.config;

import org.mapstruct.MapperConfig;
import org.mapstruct.extensions.spring.SpringMapperConfig;

@MapperConfig(componentModel = "spring")
@SpringMapperConfig(conversionServiceAdapterPackage = "cn.felord.mapstruct.config",
        conversionServiceAdapterClassName = "MapStructConversionServiceAdapter")
public class MapperSpringConfig {
}

Specifying a ConversionService Bean

If multiple ConversionService beans exist, specify which one to use via conversionServiceBeanName:

package cn.felord.mapstruct.config;

import org.mapstruct.MapperConfig;
import org.mapstruct.extensions.spring.SpringMapperConfig;

@MapperConfig(componentModel = "spring")
@SpringMapperConfig(conversionServiceAdapterPackage = "cn.felord.mapstruct.config",
        conversionServiceAdapterClassName = "MapStructConversionServiceAdapter",
        conversionServiceBeanName = "myConversionService")
public class MapperSpringConfig {
}

Integrating Spring’s Built‑in Converters

Spring provides many useful Converter<S,T> implementations that are not directly exposed. Register them via externalConversions:

@MapperConfig(componentModel = "spring")
@SpringMapperConfig(
   externalConversions = @ExternalConversion(sourceType = String.class, targetType = Locale.class))
public interface MapstructConfig {}

The generated adapter will contain:

@Component
public class ConversionServiceAdapter {
    private final ConversionService conversionService;

    public ConversionServiceAdapter(@Lazy final ConversionService conversionService) {
        this.conversionService = conversionService;
    }

    public Locale mapStringToLocale(final String source) {
        return conversionService.convert(source, Locale.class);
    }
}

Summary

mapstruct-spring-annotations enables developers to use ConversionService with defined MapStruct mappers without importing each mapper individually, allowing loose coupling between mappers while preserving MapStruct’s core mechanisms.