leejunkim

Mapper and MapStruct

3 min read

Overview

MapStruct is a library that helps in converting DTOs and Entities.

  • Automatically generates the implementation for a mapper interface at compile time, giving you fast, type-safe, and boilerplate-free code for your conversions.
  • If you use this well, your working time will decrease tremendously!

Initialization

dependencies {
    compileOnly 'org.projectlombok:lombok'
    annotationProcessor 'org.projectlombok:lombok'
 
	// add these
    implementation 'org.mapstruct:mapstruct:1.4.2.Final'
    annotationProcessor 'org.mapstruct:mapstruct-processor:1.4.2.Final'
}

Example

Product - The Entity

@Data
@Entity
@Getter
public class Product {
 
    @Id
    private Long id;
    private String name; // <<<
    private String description;
    private double price;
}

ProductDto - The Dto

@Data
public class ProductDto {
    private Long id;
    private String productName; // << Different name from the entity
    private String description;
    private double price;
}

The Mapper

import org.mapstruct.Mapper;
import org.mapstruct.Mapping;
import org.mapstruct.factory.Mappers;
 
@Mapper(componentModel = "spring")
public interface ProductMapper {
 
    // You can define an instance for non-Spring usage if needed
    // ProductMapper INSTANCE = Mappers.getMapper(ProductMapper.class);
 
    /**
     * Maps a Product Entity to a Product DTO.
     * The field 'name' in Product is mapped to 'productName' in ProductDto.
     * Fields 'id', 'description', and 'price' are mapped automatically by name.
     */
    @Mapping(source = "name", target = "productName")
    ProductDto toDto(Product product);
 
    /**
     * Maps a Product DTO back to a Product Entity.
     * The field 'productName' in the DTO is mapped back to 'name' in the Entity.
     */
    @Mapping(source = "productName", target = "name")
    Product toEntity(ProductDto productDto);
}
  • @Mapper(componentModel = "spring")
    • tells MapStruct to generate an implementation that is a Spring bean
    • You have to put this on top of every mapper
  • source, target
    • source = from
    • target = to

NOT a proxy

  • Mapstruct is an annotation processor that runs during your project’s compile time (ex. when you build your project using gradle or maven)

  • What happens

    • It scans your code with interfaces with @Mapper, reads the method signatures and @Mapping annotations inside that interface
    • It then generates a concrete Java implementation class for that interface! ⇒ Ex. ProductMapperImpl.java
    • This generated class is plain java that performs the getter & setter calls, and is also compiled into bytecode along with the rest of the code

  • AOP (Aspect Oriented Programming)

  • MapStruct vs ModelMapper

    • ModelMapper - 리플렉션을 써서 동적으로 가지고 와서 매핑해줌.
    • MapStruct - 훨씬 빠름. 클래스파일 만듬 → 빌드 → build folder에 classes/member/mapper/mapstruct → 구현 클래스가 만들어져있음!!!
    • 빌드되는 시점에 구현 클래스를 만들어줌
    • 클래스: 기본생성자 + setter / 전체생성자 / 빌더 << 다 대응을함
      • 클래스에 다 있으면, mapper은 뭘 쓸지 헷갈려함
        • 빌더 > 전체생성자 > 기본생성자
        • 아무것도 없으면 안만들어짐

  • proxy아님!

default 쓰는 것 대신 바로 알려주는 것

  • annotation 사용

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