MapStructMergeSourcesThenMap.java

package com.grokhard.exploring.java.mapstruct;

import static org.junit.jupiter.api.Assertions.assertEquals;

import org.junit.jupiter.api.Test;
import org.mapstruct.BeanMapping;
import org.mapstruct.Mapper;
import org.mapstruct.MappingTarget;
import org.mapstruct.NullValueCheckStrategy;
import org.mapstruct.NullValuePropertyMappingStrategy;
import org.mapstruct.factory.Mappers;

import lombok.AllArgsConstructor;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;


public class MapStructProblemTest {

  /**
   * PROBLEM
   * ----
   * I need an implementation such that it is possible to merge one instance of
   * SourceInput into another, then convert into FinalOutput.
   * 
   * Sort of like 
   * 
   * ```
   * FinalOutput converted = convert(merge(SourceInput a, SourceInput b))
   * ```
   */

  @Test
  public void should_merge_then_map() {
    SourceInput one = new SourceInput("One", null);
    SourceInput two = new SourceInput(null, "2023-09-07");

    FinalOutput finalOutput = MergingMapper.INSTANCE.mergeFrom(one, two);
    assertEquals(finalOutput.getName(), one.getName());
    assertEquals(finalOutput.getBirthday(), two.getBirthday());
  }


  @Mapper
  public static interface MergingMapper {

    public static final MergingMapper INSTANCE = Mappers.getMapper(MergingMapper.class);

    // EXPLANATION:
    // This method is the glue of other generated methods by MapStruct.
    // Its implementation is kept as simple as possible.
    public default FinalOutput mergeFrom(SourceInput a, SourceInput b) {
      SourceInput merged = new SourceInput();
      copy(a, merged);
      copy(b, merged);
      return toFinalOutput(merged);
    }

    // EXPLANATION:
    // This method does the usual mapping from SourceInput to FinalOutput.
    FinalOutput toFinalOutput(SourceInput source);

    // See: https://stackoverflow.com/questions/68682925/using-mapstruct-to-fill-in-same-classes
    @BeanMapping(
      nullValuePropertyMappingStrategy = NullValuePropertyMappingStrategy.IGNORE,
      nullValueCheckStrategy = NullValueCheckStrategy.ALWAYS
    )
    // EXPLANATION:
    // This method serves as a copy-properties method from one SourceInput to another.
    // It's important to note that the latter argument will be modified in-place and is returned.
    SourceInput copy(SourceInput source, @MappingTarget SourceInput target);
  }


  @Getter
  @Setter
  @NoArgsConstructor
  @AllArgsConstructor
  public static class FinalOutput {
    private String name;
    private String birthday;
  }

  @Getter
  @Setter
  @NoArgsConstructor
  @AllArgsConstructor
  public static class SourceInput {
    private String name;
    private String birthday;
  }
}