MapStruct Object Mapping

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Basic Mapper

@Mapper(componentModel = "spring", unmappedTargetPolicy = ReportingPolicy.IGNORE)
public interface UserMapper {

    UserResponse toResponse(User user);

    List<UserResponse> toResponseList(List<User> users);

    @Mapping(target = "id", ignore = true)
    @Mapping(target = "createdAt", ignore = true)
    @Mapping(target = "updatedAt", ignore = true)
    @Mapping(target = "password", ignore = true)
    User toEntity(CreateUserRequest dto);
}

Update Mapping

@Mapper(componentModel = "spring", unmappedTargetPolicy = ReportingPolicy.IGNORE)
public interface UserMapper {

    // Partial update - ignores null values
    @BeanMapping(nullValuePropertyMappingStrategy = NullValuePropertyMappingStrategy.IGNORE)
    void updateEntity(UpdateUserRequest dto, @MappingTarget User user);

    // Full update - sets all fields including nulls
    void updateEntityFull(UpdateUserRequest dto, @MappingTarget User user);
}

Field Mapping

@Mapper(componentModel = "spring")
public interface OrderMapper {

    // Different field names
    @Mapping(source = "customer.name", target = "customerName")
    @Mapping(source = "customer.email", target = "customerEmail")
    @Mapping(source = "items", target = "orderItems")
    OrderResponse toResponse(Order order);

    // Constant values
    @Mapping(target = "status", constant = "PENDING")
    @Mapping(target = "createdAt", expression = "java(java.time.LocalDateTime.now())")
    Order toEntity(CreateOrderRequest dto);

    // Expression
    @Mapping(target = "fullName", expression = "java(user.getFirstName() + \" \" + user.getLastName())")
    UserResponse toResponse(User user);

    // Date formatting
    @Mapping(source = "createdAt", target = "createdDate", dateFormat = "yyyy-MM-dd")
    OrderResponse toResponse(Order order);
}

Nested Objects

@Mapper(componentModel = "spring", uses = {AddressMapper.class, ItemMapper.class})
public interface OrderMapper {

    // Uses AddressMapper for address field
    // Uses ItemMapper for items collection
    OrderResponse toResponse(Order order);
}

@Mapper(componentModel = "spring")
public interface AddressMapper {
    AddressResponse toResponse(Address address);
}

@Mapper(componentModel = "spring")
public interface ItemMapper {
    ItemResponse toResponse(Item item);
}

Enum Mapping

@Mapper(componentModel = "spring")
public interface StatusMapper {

    @ValueMappings({
        @ValueMapping(source = "ACTIVE", target = "ENABLED"),
        @ValueMapping(source = "INACTIVE", target = "DISABLED"),
        @ValueMapping(source = MappingConstants.ANY_REMAINING, target = "UNKNOWN")
    })
    ExternalStatus toExternalStatus(InternalStatus status);
}

Custom Methods

@Mapper(componentModel = "spring")
public abstract class UserMapper {

    @Autowired
    protected RoleRepository roleRepository;

    public abstract UserResponse toResponse(User user);

    @Mapping(target = "roles", source = "roleIds")
    public abstract User toEntity(CreateUserRequest dto);

    // Custom mapping method
    protected Set<Role> mapRoles(Set<Long> roleIds) {
        if (roleIds == null) return new HashSet<>();
        return roleIds.stream()
            .map(id -> roleRepository.findById(id)
                .orElseThrow(() -> new ResourceNotFoundException("Role", "id", id)))
            .collect(Collectors.toSet());
    }
}

Collection Mapping

@Mapper(componentModel = "spring")
public interface ProductMapper {

    ProductResponse toResponse(Product product);

    List<ProductResponse> toResponseList(List<Product> products);

    Set<ProductResponse> toResponseSet(Set<Product> products);

    // Page mapping
    default Page<ProductResponse> toResponsePage(Page<Product> products) {
        return products.map(this::toResponse);
    }
}

After/Before Mapping

@Mapper(componentModel = "spring")
public abstract class UserMapper {

    @AfterMapping
    protected void afterMapping(@MappingTarget UserResponse response, User user) {
        response.setDisplayName(user.getFirstName() + " " + user.getLastName().charAt(0) + ".");
    }

    @BeforeMapping
    protected void beforeMapping(CreateUserRequest dto) {
        if (dto.getEmail() != null) {
            dto.setEmail(dto.getEmail().toLowerCase().trim());
        }
    }
}

Conditional Mapping

@Mapper(componentModel = "spring")
public abstract class UserMapper {

    @Condition
    public boolean isNotEmpty(String value) {
        return value != null && !value.trim().isEmpty();
    }

    // Only maps non-empty strings
    UserResponse toResponse(User user);
}

Maven Configuration

<properties>
    <mapstruct.version>1.6.2</mapstruct.version>
    <lombok.version>1.18.30</lombok.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.mapstruct</groupId>
        <artifactId>mapstruct</artifactId>
        <version>${mapstruct.version}</version>
    </dependency>

    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>${lombok.version}</version>
        <optional>true</optional>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.13.0</version>
            <configuration>
                <source>17</source>
                <target>17</target>
                <annotationProcessorPaths>
                    <!-- Order matters: Lombok first -->
                    <path>
                        <groupId>org.projectlombok</groupId>
                        <artifactId>lombok</artifactId>
                        <version>${lombok.version}</version>
                    </path>
                    <path>
                        <groupId>org.mapstruct</groupId>
                        <artifactId>mapstruct-processor</artifactId>
                        <version>${mapstruct.version}</version>
                    </path>
                    <path>
                        <groupId>org.projectlombok</groupId>
                        <artifactId>lombok-mapstruct-binding</artifactId>
                        <version>0.2.0</version>
                    </path>
                </annotationProcessorPaths>
                <compilerArgs>
                    <arg>-Amapstruct.defaultComponentModel=spring</arg>
                </compilerArgs>
            </configuration>
        </plugin>
    </plugins>
</build>

Key Annotations

Annotation	Purpose
`@Mapper`	Define mapper interface/abstract class
`@Mapping`	Field-level mapping configuration
`@MappingTarget`	Update existing object
`@BeanMapping`	Bean-level mapping settings
`@AfterMapping`	Post-processing method
`@BeforeMapping`	Pre-processing method
`@Condition`	Conditional mapping
`@ValueMapping`	Enum value mapping

Best Practices

Use componentModel = "spring" for Spring injection
Use unmappedTargetPolicy = ReportingPolicy.IGNORE for DTOs with fewer fields
Always exclude id, createdAt, updatedAt when mapping from DTOs
Use @MappingTarget for partial updates
Order annotation processors: Lombok → MapStruct → Binding
Use abstract classes instead of interfaces for custom logic
Use @Condition for conditional mapping logic

When NOT to Use This Skill

Scenario	Use Instead
Java language features	`java` skill
Lombok annotations	`lombok` skill
Spring configuration	`backend-spring-boot` skill
JPA entity operations	JPA-specific skills
Simple copying	Manual mapping or BeanUtils

Anti-Patterns

Anti-Pattern	Why It's Bad	Correct Approach
Mapping entities to entities	Breaks change tracking	Map DTO to entity
Not using @MappingTarget	Inefficient updates	Use for partial updates
Complex logic in expressions	Hard to test	Use custom methods
Ignoring all unmapped	Misses fields silently	Use WARN or ERROR policy
Not excluding audit fields	Overwrites metadata	Exclude id, timestamps
Circular references	StackOverflowError	Break cycles with custom mapping
Using interfaces for custom logic	Can't inject dependencies	Use abstract classes
Not testing mappers	Runtime mapping errors	Write mapper tests

Quick Troubleshooting

Issue	Cause	Solution
"Cannot find implementation"	Annotation processing failed	Check processor configuration
Lombok fields not found	Wrong processor order	Lombok before MapStruct
Mapper not autowired	Wrong componentModel	Use componentModel = "spring"
Circular dependency	Mappers reference each other	Use @Lazy or refactor
UnmappedTargetProperty warning	Missing mapping	Add @Mapping or ignore policy
NullPointerException in mapping	Null source	Add null checks or NullValuePropertyMappingStrategy
Custom method not called	Wrong signature	Match method parameters exactly
Generated code not updated	IDE cache	Clean and rebuild project

Reference Documentation

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: mapstruct for comprehensive documentation.