MapStruct

Java bean mappings, the easy way!

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What is it?

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.

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.

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.


News

MapStruct 1.1.0.CR1 released

Sjaak Derksen, 07 September 2016

MapStruct 1.1.0.CR1 consolodates the functionality introduced in the preceeding MapStruct 1.1.0 Beta releases. A number of bugs were solved: Automatic generation of an iterable mapping methods where the property selection is...

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MapStruct 1.1.0.Beta2 released

Sjaak Derksen, 22 July 2016

It has been a while since the latest release of MapStruct. Via our mailing list, issue list and other channels we have received valuable feedback. We have tried to incorporate...

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MapStruct 1.1.0.Beta1 released

Andreas Gudian, 16 March 2016

It has been four months since the release of the first final version of MapStruct. We’ve received a lot of great feedback and noticed a substantial increase of interest in...

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MapStruct in 2 minutes

The following shows how map two objects using MapStruct.

Let’s assume we have a class representing cars (e.g. a JPA entity) and an accompanying data transfer object (DTO).

Both types are rather similar, only the seat count attributes have different names and the type attribute is of a special enum type in the Car class but is a plain string in the DTO.

public class Car {

    private String make;
    private int numberOfSeats;
    private CarType type;

    //constructor, getters, setters etc.
}
public class CarDto {

    private String make;
    private int seatCount;
    private String type;

    //constructor, getters, setters etc.
}

The mapper interface

To generate a mapper for creating a CarDto object out of a Car object, a mapper interface needs to be defined:

  1. @Mapper 1
  2. public interface CarMapper {
  3.  
  4. CarMapper INSTANCE = Mappers.getMapper( CarMapper.class ); 3
  5.  
  6. @Mapping(source = "numberOfSeats", target = "seatCount")
  7. CarDto carToCarDto(Car car); 2
  8. }

The @Mapper annotation 1 marks the interface as mapping interface and lets the MapStruct processor kick in during compilation.

The actual mapping method 2 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.

Where required and possible a type conversion will be executed for attributes with different types in source and target, e.g. the type attribute will be converted from the enumeration type into a string.

Of course there can be multiple mapping methods in one interface, for all of which an implementation will be generated by MapStruct.

An instance of the interface implementation can be retrieved from the Mappers class. By convention, the interface declares a member INSTANCE 3, providing clients access to the mapper implementation.


Using the mapper

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", 5, CarType.SEDAN );

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

    //then
    assertThat( carDto ).isNotNull();
    assertThat( carDto.getMake() ).isEqualTo( "Morris" );
    assertThat( carDto.getSeatCount() ).isEqualTo( 5 );
    assertThat( carDto.getType() ).isEqualTo( "SEDAN" );
}

Tell me more!

You like what you see? Then check out the reference documentation to learn how to get started with MapStruct and which advanced features there are. In case you need help or want to propose a new feature just drop by on the mapstruct-users group.

You want to contribute to the development of MapStruct? That’s great, this page has all the information you need.