Force null value check on field mapping in Mapstruct

Use the nullValueCheckStrategy = NullValueCheckStrategy.ALWAYS on the mapping property you want to be checked. In the following snippet you can see a PostalAdress entity is mapped to an entity of the same type, but the id is overwritten only when it is present in the source:

import java.time.LocalDateTime;

import org.mapstruct.Mapper;
import org.mapstruct.Mapping;
import org.mapstruct.NullValueCheckStrategy;

import com.example.entity.PostalAddress;

@Mapper(
    componentModel = "cdi",
    imports = {LocalDateTime.class})
public interface PostalAddressEntity2EntityMapperService {

  @Mapping(source = "id", target = "id", nullValueCheckStrategy = NullValueCheckStrategy.ALWAYS)
  PostalAddress toPostalAddressntityToPostalAddressEntity(
      PostalAddress PostalAddress);
}

This will generate something similar with the following

package com.example.entitytoentity;

import com.example.entity.PostalAddress;
import java.time.LocalDateTime;
import javax.annotation.processing.Generated;
import javax.enterprise.context.ApplicationScoped;

@Generated(
    value = "org.mapstruct.ap.MappingProcessor",
    date = "2021-09-01T14:28:11+0200",
    comments = "version: 1.4.2.Final, compiler: javac, environment: Java 11.0.12 (Azul Systems, Inc.)"
)
@ApplicationScoped
public class PostalAddressEntity2EntityMapperServiceImpl implements PostalAddressEntity2EntityMapperService {

    @Override
    public PostalAddress toPostalAddressEntity(PostalAddress PostalAddress) {
        if ( PostalAddress == null ) {
            return null;
        }

        PostalAddress PostalAddress1 = new PostalAddress();

        if ( PostalAddress.getId() != null ) {
            PostalAddress1.setId( PostalAddress.getId() );
        }
        PostalAddress1.setStreeNo( PostalAddress.getStreetNo() );
        PostalAddress1.setStreet( PostalAddress.getStreet() );
        PostalAddress1.setMainAdress( PostalAddress.getMainAdress() );

        return PostalAddress1;
    }
}

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