Kotlin中MapStruct@AfterMapping报错：仅可应用于已实现类

问题场景 #

在Kotlin结合Spring Boot使用MapStruct进行对象映射时，需在映射完成后通过@AfterMapping调用Spring Repository和Service获取关联对象，但执行kaptKotlin任务时编译失败。

原代码 #

@Mapper(componentModel = "spring")
interface ObjectMapper {

    @Mappings(
        Mapping(source = "aCode", target = "a.code"),
        Mapping(source = "bCode", target = "b.code")
    )
    fun convertFormDtoToEntity(
        dto: ObjectFormDto,
        @Context aRepo: ARepository,
        @Context bService: BService
    ): Object

    @AfterMapping
    fun afterMappingFormDtoToEntity(
        dto: ObjectFormDto,
        @Context aRepo: ARepository,
        @Context bService: BService,
        @MappingTarget entity: Object
    ) {
        entity.a = aRepo.findByA(entity.a.code)
        entity.b = bService.getB(entity.b.code)
    }
}

报错信息 #

error: @AfterMapping can only be applied to an implemented class
    public abstract void afterMappingFormDtoToEntity(@org.jetbrains.annotations.NotNull()...

解决方案 #

方案1：将Mapper改为抽象类 #

MapStruct允许使用抽象类作为Mapper载体，抽象类中的方法可直接实现，不会触发该错误：

@Mapper(componentModel = "spring")
abstract class ObjectMapper {

    @Mappings(
        Mapping(source = "aCode", target = "a.code"),
        Mapping(source = "bCode", target = "b.code")
    )
    abstract fun convertFormDtoToEntity(
        dto: ObjectFormDto,
        @Context aRepo: ARepository,
        @Context bService: BService
    ): Object

    @AfterMapping
    fun afterMappingFormDtoToEntity(
        dto: ObjectFormDto,
        @Context aRepo: ARepository,
        @Context bService: BService,
        @MappingTarget entity: Object
    ) {
        entity.a = aRepo.findByA(entity.a.code)
        entity.b = bService.getB(entity.b.code)
    }
}

方案2：为接口中的@AfterMapping方法添加@JvmDefault注解 #

若坚持使用接口，需让Kotlin生成Java兼容的默认方法，添加@JvmDefault注解并开启对应配置：

1. 修改Mapper接口：

@Mapper(componentModel = "spring")
interface ObjectMapper {

    @Mappings(
        Mapping(source = "aCode", target = "a.code"),
        Mapping(source = "bCode", target = "b.code")
    )
    fun convertFormDtoToEntity(
        dto: ObjectFormDto,
        @Context aRepo: ARepository,
        @Context bService: BService
    ): Object

    @JvmDefault
    @AfterMapping
    fun afterMappingFormDtoToEntity(
        dto: ObjectFormDto,
        @Context aRepo: ARepository,
        @Context bService: BService,
        @MappingTarget entity: Object
    ) {
        entity.a = aRepo.findByA(entity.a.code)
        entity.b = bService.getB(entity.b.code)
    }
}

2. 在build.gradle.kts的kotlin配置中开启默认方法支持：

kotlin {
    sourceSets.all {
        languageSettings {
            optIn("kotlin.jvm.JvmDefault")
        }
    }
}

当前环境配置 #

MapStruct版本：1.5.2.Final，kapt版本：1.6.10，kapt配置如下：

kapt {
    arguments {
        arg("mapstruct.unmappedTargetPolicy", "IGNORE")
    }
    keepJavacAnnotationProcessors = true
}

内容的提问来源于stack exchange，提问作者3LexW