carstencogos / mapstruct

An annotation processor for generating type-safe bean mappers

What is MapStruct?

MapStruct is a Java annotation processor for the generation of type-safe bean mapping classes.

All you have to do is to define a mapper interface which declares any required mapping methods. During compilation, MapStruct will generate an implementation of this interface. This implementation uses plain Java method invocations for mapping between source and target objects, i.e. no reflection or similar.

Compared to writing mapping code from hand, MapStruct saves time by generating code which is tedious and error-prone to write. Following a convention over configuration approach, MapStruct uses sensible defaults but steps out of your way when it comes to configuring or implementing special behavior.

Compared to dynamic mapping frameworks, MapStruct offers the following advantages:

• Fast execution by using plain method invocations instead of reflection
• Compile-time type safety: Only objects and attributes mapping to each other can be mapped, no accidental mapping of an order entity into a customer DTO etc.
• Clear error-reports at build time, if entities or attributes can't be mapped

Hello World

The following shows a simple example for using MapStruct. Let's assume there is class representing cars (e.g. a JPA entity) and an accompanying data transfer object (DTO):

public class Car {

	private String make;
	private int numberOfSeats;
	
	//constructor, getters, setters etc.
}

public class CarDto {

	private String make;
	private int seatCount;

	//constructor, getters, setters etc.
}

Both types are rather similar, only the seat count attributes have different names. To generate a mapper for creating a CarDto object out of a Car object, a mapper interface needs to be defined:

@Mapper (1)
public interface CarMapper {

	CarMapper INSTANCE = Mappers.getMapper( CarMapper.class ); (3)

	@Mapping(source = "numberOfSeats", target = "seatCount")
	CarDto carToCarDto(Car car); (2)
}

1. The @Mapper annotation marks the interface as mapping interface and lets the MapStruct processor kick in during compilation
2. The actual mapping method expects the source object as parameter and returns the target object. Its name can be freely chosen. For attributes with different names in source and target object, the @Mapping annotation can be used to configure the names. Of course there can be multiple mapping methods in one interface.
3. An instance of the interface implementation can be retrieved from the Mappers class. By convention, the interface declares a member INSTANCE, providing access to the mapper implementation for clients

Based on the mapper interface, clients can perform object mappings in a very easy and type-safe manner:

@Test
public void shouldMapCarToDto() {
	//given
	Car car = new Car( "Morris", 2 );

	//when
	CarDto carDto = CarMapper.INSTANCE.carToCarDto( car );

	//then
	assertThat( carDto ).isNotNull();
	assertThat( carDto.getMake() ).isEqualTo( car.getMake() );
	assertThat( carDto.getSeatCount() ).isEqualTo( car.getNumberOfSeats() );
}

Advanced mappings

Reverse mappings

Often bi-directional mappings are required, e.g. from entity to DTO and from DTO to entity. For this purpose, simply declare a method with the required parameter and return type on the mapping interface which also declares the forward mapping method:

@Mapper
public interface CarMapper {

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

	@Mapping(source = "numberOfSeats", target = "seatCount")
	CarDto carToCarDto(Car car);
    
    Car carDtoToCar(CarDto carDto); (1)
}

1. The carDtoToCar() method is the reverse mapping method for carToCarDto(). Note that the attribute mappings only have to be specified at one of the two methods and will be applied to the corresponding reverse mapping method as well.

Mapping referenced objects and collections

Typically an object has not only primitive attributes but also references other objects. E.g. the Car class could contain a reference to a Person object (representing the car's driver) which should be mapped to a PersonDto object referenced by the CarDto class.

In this case just define a mapping method for the referenced object types as well:

@Mapper
public interface CarMapper {

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

	CarDto carToCarDto(Car car);
    
    PersonDto personToPersonDto(Person person);
    
    //reverse mapping methods
}

The generated code for the carToCarDto() method will invoke the personToPersonDto() method for mapping the driver attribute.

The same works for collections. E.g. the Car class could reference the list of all passengers:

public class Car {

    private List<Person> passengers;
	
	//...
}

To map this attribute, define a method accepting a list of persons and returning a list of person DTOs:

@Mapper
public interface CarMapper {

    //other members
    
    List<PersonDto> personsToPersonDtos(List<Person> prsons);
}

This method will be invoked by the generated implementation when mapping the passengers attribute.

NOTE: Collection mapping methods may be generated without declaration in the future (see issues #3 and #4).

Type mappings

Not always a mapped attribute has the same type in the source and target objects. MapStruct generates appropriate conversion code where possible (e.g. to map an int attribute to a String and vice versa by calling toString() and parseInt(), respectively.

Where this is not automatically possible, you can implement mapping methods yourself and make these known to MapStruct. E.g. the Car class might contain an attribute manufacturingDate while the corresponding DTO attribute is of type String.

In order to map this attribute, you could implement a mapper class like this:

public class DateMapper {

    public String asString(Date date) {
        return date != null ? new SimpleDateFormat( "yyyy-MM-dd" ).format( date ) : null;
    }

    public Date asDate(String date) {
        try {
            return date != null ? new SimpleDateFormat( "yyyy-MM-dd" ).parse( date ) : null;
        }
        catch ( ParseException e ) {
            throw new RuntimeException( e );
        }
    }
}

In the @Mapper annotation at the CarMapper interface reference this mapper class:

@Mapper(uses=DateMapper.class)
public class CarMapperMapper {

    CarDto carToCarDto(Car car);
    
    //other mapping methods
}

When generating code for the implementation of the carToCarDto() method, MapStruct will look for a method which maps a Date object into a String, find it on the DateMapper class and generate an invocation of asString() for mapping the manufacturingDate attribute.

Using MapStruct

MapStruct is a Java annotation processor based on JSR 269 and as such can be used within command line builds (javac, Ant, Maven etc.) as well as from within your IDE.

For Maven based projects add the following to your POM file in order to use MapStruct:

...
<properties>
    <org.mapstruct.version>[current MapStruct version]</org.mapstruct.version>
    
</properties>
...
<dependencies>
    <dependency>
        <groupId>org.mapstruct</groupId>
        <artifactId>mapstruct</artifactId>
        <version>${org.mapstruct.version}</version>
    </dependency>
</dependencies>
...
<build>
    <plugins>
        <plugin>
            <groupId>org.bsc.maven</groupId>
            <artifactId>maven-processor-plugin</artifactId>
            <version>2.0.2</version>
            <configuration>
                <defaultOutputDirectory>${project.build.directory}/generated-sources</defaultOutputDirectory>
                <processors>
                    <processor>org.mapstruct.ap.MappingProcessor</processor>
                </processors>
            </configuration>
            <executions>
                <execution>
                    <id>process</id>
                    <phase>generate-sources</phase>
                    <goals>
                        <goal>process</goal>
                    </goals>
                </execution>
            </executions>
            <dependencies>
                <dependency>
                    <groupId>org.mapstruct</groupId>
                    <artifactId>mapstruct-processor</artifactId>
                    <version>${org.mapstruct.version}</version>
                </dependency>
            </dependencies>
        </plugin>
    </plugins>
</build>

NOTE: In order to use MapStruct, you currently have to check out its sources and build it yourself as it is not yet available in any public Maven repository.

What's next

MapStruct is just in its very beginnings. Some ideas for further features:

• Support EL expressions to create derived attributes
• Support mapping of collections of objects (lists, arrays, maps etc.)
• Remove runtime dependencies to MapStruct by e.g. using the JDK service loader or dependency injection via CDI to retrieve mapper implementations
• etc.

Check out the issue list for more details.