How to map a Java Bean using mapstruct library ?

07 Nov 2021 in cs

Five min introduction on mapstruct for Java Bean Mapping.

1. Why mapping ?
2. Example Project Setup
3. How to map using mapstruct annotation ?
4. Testing
5. How it works ?
6. References
7. Conclusion

This blog, I am going to show a simple example for the understanding on Java Bean Mapping.

Why mapping ?

• Convert Java Bean/POJO to Another Bean/POJO with different names.
• Create Report Json with additional calculations.
• For using DTO Design Pattern.

Example Project Setup

git clone https://github.com/notesbyair/blog.mapstruct.git

In this example, we have a Java Bean - Car, we want to convert it to `CarDTO object where it is has different names for mapping.

How to map using mapstruct annotation ?

Define a mapper interface for above conversion, it will convert Car to CarDto. The mapstruct library generates the implementations for it.

In below interface source:Car, target: CarDto.

// file: "CarMapper.java"
import org.mapstruct.Mapper;
import org.mapstruct.Mapping;
import org.mapstruct.factory.Mappers;

@Mapper
public interface CarMapper {
    CarMapper INSTANCE = Mappers.getMapper( CarMapper.class );
    @Mapping(target = "seats", source = "seatConfiguration")
    CarDto convert(Car car);

    @Mapping(source = "numberOfSeats", target = "seatCount")
    @Mapping(source = "seatMaterial", target = "material")
    SeatConfigurationDto seatConfigurationToSeatConfigurationDto(SeatConfiguration seatConfiguration);
}

CarMapper for Converting Car to CarDTO

Testing

@Test
void testMapping() {
  Car car = new Car();
  car.setMake("Tesla");
  car.setType(CarType.SPORTS);
  car.setSeatConfiguration(new SeatConfiguration(1, SeatMaterial.LEATHER));
  CarDto carDTO = CarMapper.INSTANCE.convert(car);
  assertEquals(car.getMake(), carDTO.getMake());
  assertEquals(car.getType().toString(), carDTO.getType());
  assertEquals(car.getSeatConfiguration().getSeatMaterial().toString(),
      carDTO.getSeats().getMaterial());
}

How it works ?

1. If Source and Target bean contains same name, then it will convert directly, for example make is same name in both beans. For all other, you need to define a names using mapping annotation like shown in above.
2. You need to define another mapping, which will convert the seatConfiguration. For more, refer this section of the blog post
   • Mapping Beans with Child Beans

References

1. Quick Guide to MapStruct
2. https://mapstruct.org/
3. https://mapstruct.org/documentation/installation/

Conclusion

Using mapstruct, we can easily map Java Beans/POJO using less number of codes. Hope this gives nice introduction, let me know for any doubts here.