MapStruct 入门

介绍（What？Why？How？）

What?

MapStruct is a code generator that greatly simplifies the implementation of mappings between Java bean types based on a convention over configuration approach.

The generated mapping code uses plain method invocations and thus is fast, type-safe and easy to understand.

MapStruct是一个代码生成器，它极大地简化了Javabean类型之间的映射的实现，这是一种基于约定而非配置的方法。生成的映射代码使用简单的方法调用，因此快速、类型安全且易于理解。

Why?

Multi-layered applications often require to map between different object models (e.g. entities and DTOs). Writing such mapping code is a tedious and error-prone task. MapStruct aims at simplifying this work by automating it as much as possible.

In contrast to other mapping frameworks MapStruct generates bean mappings at compile-time which ensures a high performance, allows for fast developer feedback and thorough error checking.

多层应用程序通常需要在不同的对象模型（例如实体和DTO）之间进行映射。编写这样的映射代码是一项乏味且容易出错的任务。MapStruct旨在通过尽可能自动化来简化这项工作。与其他映射框架不同，MapStruct在编译时生成bean映射，这确保了高性能，允许快速的开发人员反馈和彻底的错误检查

How?

MapStruct is an annotation processor which is plugged into the Java compiler and can be used in command-line builds (Maven, Gradle etc.) as well as from within your preferred IDE.

MapStruct uses sensible defaults but steps out of your way when it comes to configuring or implementing special behavior.

MapStruct是一个插入Java编译器的注释处理器，可以在命令行构建（Maven、Gradle等）中使用，也可以在您首选的IDE中使用。MapStruct使用合理的默认值，但在配置或实现特殊行为时，它会避开您。

总结，简而言之就是这功能，Bean copy：

	BeanUtils.copyProperties(userDO,userDTO);

怎么用

1. 引入依赖

        <dependency>
            <groupId>org.mapstruct</groupId>
            <artifactId>mapstruct</artifactId>
            <version>1.4.2.Final</version>
        </dependency>
        <dependency>
            <groupId>org.mapstruct</groupId>
            <artifactId>mapstruct-processor</artifactId>
            <version>1.4.2.Final</version>
        </dependency>

1. User与UserQueryParm

@Data
public class User {
    String sno;
    String username;
    String isDelete;
}

@Data
public class UserQueryParam {
    String sno;
    String name;
}

1. 编写接口

@Mapper
public interface UserMapper {

    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    // User 与 UserQueryParam 字段不一致时映射
    @Mapping(source = "username",target = "name")
    UserQueryParam entity2queryParam(User user);

}

1. 使用

public class MapstructApplication {
    public static void main(String[] args) {
        User user = new User();
        user.setSno("20231212");
        user.setUsername("xiaozhang");

        List<UserQueryParam> collect = Arrays.asList(user).stream()
                .map(UserMapper.INSTANCE::entity2queryParam)
                .collect(Collectors.toList());
        for (UserQueryParam userQueryParam : collect) {
            System.out.println(JSONUtil.toJsonStr(userQueryParam));//{"sno":"20231212","name":"xiaozhang"}
        }
    }
}

答疑解惑

1. 为什么不用BeanUtils.copyProperties(userDO,userDTO)？
   
   当转换的对象是列表时，MapperStruct的优雅才得以展现。

2. MapStruct在编译时生成bean映射？
   
   
   
   编译时产生，安全性更高，编译不通过，则会运行失败。

3. 使用mapstruct.Mapper 转换对象属性为null ？
   
   lombok依赖和mapstruct依赖顺序问题，将lombok放前边。
   
   参考链接：https://blog.csdn.net/qq_41169544/article/details/127675917

参考

官网：https://mapstruct.org/

ABin-阿斌：https://blog.csdn.net/Mango_Bin/article/details/125168370