How to Resolve the MapStruct Requires Implementation Class Error in Java

What does the error 'MapStruct requires implementation class' mean and how can I fix it?

The error message 'MapStruct requires implementation class' occurs when using the MapStruct framework in Java to generate mapping implementations for data transfer objects (DTOs) automatically. MapStruct needs an implementation class to fulfill the mappings specified in your interface. If this implementation is not generated correctly, you'll encounter this error.

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

    CarDto carToCarDto(Car car);

    Car carDtoToCar(CarDto carDto);
}

Causes

• The MapStruct annotation processor isn't properly configured in your build process.
• The required dependencies for MapStruct are missing from your project.
• The annotation processor might not be generating the implementation class due to IDE misconfigurations.
• The mapping interface may not be present in the expected package.

Solutions

• Ensure that the MapStruct dependency is correctly added to your `pom.xml` for Maven or `build.gradle` for Gradle. Example for Maven: ```xml <dependency> <groupId>org.mapstruct</groupId> <artifactId>mapstruct</artifactId> <version>1.5.2.Final</version> </dependency> <dependency> <groupId>org.mapstruct</groupId> <artifactId>mapstruct-processor</artifactId> <version>1.5.2.Final</version> <scope>provided</scope> </dependency> ```
• Add the annotation processor to your IDE's settings, such as IntelliJ IDEA or Eclipse, to enable automatic generation of implementation classes.
• Verify the package structure of your mapping interfaces and ensure they are at the correct location as expected by MapStruct.

Mistake: Forgetting to include the MapStruct annotation processor.

Solution: Ensure you have both 'mapstruct' and 'mapstruct-processor' dependencies in your project.

Mistake: Not running the build task after making changes to the mappings.

Solution: Always run the build process to enable MapStruct to generate the necessary implementation classes.

Mistake: Having incorrect import statements in the Java file.

Solution: Check your imports and ensure that you are using the correct Maven/Gradle versions of MapStruct.