Assemblers

In Domain-Driven Design an interface layer is necessary to avoid coupling your domain to the outside world. A domain object is never exposed directly to the outside world but is mapped to a Data Transfer Object specifically tailored for the client needs. The assembler pattern allows to encapsulate this mapping responsibility in a separate object.

Default assembler

To have the business framework generate a default assembler, annotate your DTO class with @DtoOf:

@DtoOf(SomeAggregate.class)
public class SomeDto {
    
}

Then you can use a qualified injection of the parameterized Assembler interface. For ModelMapper, this would be:

public class SomeClass {
    @Inject
    @ModelMapper
    private Assembler<SomeAggregate, SomeDto> someDtoAssembler;
    
    public void someMethod(SomeAggregate someAggregate) {
        SomeDto someDto = someDtoAssembler.createDtoFromAggregate(someAggregate);
    }
}

Custom assembler

Sometimes it makes more sense to write your own custom assembler.

Declaration

To declare a custom assembler you have two alternatives:

public class SomeAssembler extends BaseAssembler<SomeAggregate, SomeDto> {
    @Override
    public void mergeAggregateIntoDto(A sourceAggregate, D targetDto) {
        // aggregate to dto mapping logic
    }

    @Override
    public void mergeDtoIntoAggregate(D sourceDto, A targetAggregate) {
        // dto to aggregate mapping logic
    }
}

public class SomeAssembler implements Assembler<SomeAggregate, SomeDto> {
    @Override
    public void mergeAggregateIntoDto(A sourceAggregate, D targetDto) {
        // aggregate to dto mapping logic
    }

    @Override
    public void mergeDtoIntoAggregate(D sourceDto, A targetAggregate) {
        // dto to aggregate mapping logic
    }

    @Override
    public Class<SomeDto> getDtoClass() {
        return SomeDto.class;
    }
}

Class configuration

When using default or generated assembler you have to explicitly specify the qualifier at the injection point, to choose the correct implementation.

To avoid specifying the qualifier in code, you can specify it as the defaultAssembler key in class configuration targetting the DTO:

classes:
  org:
    myorg:
      myapp:
        domain:
          model:
            someDto:
              defaultAssembler: org.seedstack.business.modelmapper.ModelMapper

Usage

To use your assembler directly, inject the Assembler interface:

public class SomeClass {
    @Inject
    private Assembler<SomeAggregate, SomeDto> someDtoAssembler;
    
    public void someMethod(SomeAggregate someAggregate) {
        SomeDto someDto = someDtoAssembler.createDtoFromAggregate(someAggregate);
    }
}