MapStruct 是一個編譯時代碼生成器，用於簡化Java Bean 之間的映射。 1. 它通過定義接口自動生成實現(xiàn)類，避免手動編寫冗長的set/get 映射代碼；2. 具備類型安全、無運行時開銷、支持自動映射同名字段、自定義表達式、嵌套對象和集合映射等特性；3. 可與Spring 集成，使用@Mapper(componentModel = "spring") 將mapper 注入為Spring bean；4. 配置簡單，只需引入mapstruct 依賴和annotationProcessorPaths 插件；5. 支持@Named 等註解實現(xiàn)可複用的自定義映射邏輯，提升代碼整潔性與可維護性，是Java 中處理對象映射的最佳實踐之一。

If you've ever found yourself writing endless boilerplate code to copy data from one Java object (like a DTO) to another (like an entity), you know how tedious and error-prone it can be. That's where MapStruct comes in — a compile-time code generator that makes bean mapping in Java not just easier, but practically painless.

What Is MapStruct?

MapStruct is a code generator that simplifies mapping between Java beans (eg, from a JPA entity to a REST DTO). Instead of manually writing setX(getX()) for every field, you define an interface, and MapStruct generates the implementation at compile time — no reflection, no runtime overhead, just fast, clean, readable code.

Why MapStruct Over Manual Mapping or Other Libraries?

• ? Type-safe : Errors caught at compile time, not runtime.
• ? No runtime overhead : Generated code is plain Java — no reflection or proxies.
• ? Minimal boilerplate : Write an interface, get an implementation.
• ? Smart defaults : Automatically maps fields with the same name.
• ? Customizable : Handle name mismatches, nested objects, collections, and even custom logic.

Quick Example

Say you have:

 public class User {
    private Long id;
    private String firstName;
    private String lastName;
    private String email;
    // getters setters
}

public class UserDto {
    private String fullName; // maps firstName lastName
    private String email;
}

With MapStruct, create a mapper interface:

 @Mapper
public interface UserMapper {
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    @Mapping(target = "fullName", expression = "java(user.getFirstName() \" \" user.getLastName())")
    UserDto toDto(User user);
}

Then use it:

 User user = new User();
user.setFirstName("John");
user.setLastName("Doe");
user.setEmail("john@example.com");

UserDto dto = UserMapper.INSTANCE.toDto(user);
// dto.getFullName() → "John Doe"

Key Features You'll Love

1. Auto-Mapping by Name

If field names match (like email ), no extra config needed.

2. Custom Mapping Logic

Use @Mapping with:

• expression for inline Java expressions
• qualifiedByName for reusable custom methods
• @AfterMapping for post-processing logic

3. Nested Objects & Collections

MapStruct handles List<User> ? List<UserDto> automatically — no extra code.

4. Integration with Spring

Just add componentModel = "spring" :

 @Mapper(componentModel = "spring")
public interface UserMapper {
    UserDto toDto(User user);
}

Now it's a Spring bean — inject it like any other.

Setup (Maven)

Add to pom.xml :

 <dependency>
    <groupId>org.mapstruct</groupId>
    <artifactId>mapstruct</artifactId>
    <version>1.5.5.Final</version>
</dependency>

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <configuration>
        <annotationProcessorPaths>
            <path>
                <groupId>org.mapstruct</groupId>
                <artifactId>mapstruct-processor</artifactId>
                <version>1.5.5.Final</version>
            </path>
        </annotationProcessorPaths>
    </configuration>
</plugin>

Pro Tip: Use @Named for Reusable Logic

 @Mapper
public interface UserMapper {
    @Mapping(target = "fullName", qualifiedByName = "toFullName")
    UserDto toDto(User user);

    @Named("toFullName")
    default String toFullName(User user) {
        return user.getFirstName() " " user.getLastName();
    }
}

This keeps your mappings clean and reusable across multiple mappers.

MapStruct removes the friction from object mapping — no more manual copying, no more runtime surprises. Once you start using it, you'll wonder how you ever lived without it.
Basically, if you're doing bean mapping in Java and not using MapStruct, you're doing it the hard way.

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