Why Is MapStruct Not Generating Implementation Classes?

What could be the reasons for MapStruct not generating implementation classes?

MapStruct is a powerful code generator in Java that simplifies the process of mapping between Java bean types. However, developers sometimes encounter issues where MapStruct does not generate the required implementation classes, leading to confusion and runtime errors. Understanding the roots of this issue and how to resolve it is crucial for effective usage of this library.

import org.mapstruct.Mapper;
import org.mapstruct.factory.Mappers;

@Mapper
public interface MyMapper {
    MyMapper INSTANCE = Mappers.getMapper(MyMapper.class);
    Target map(Source source);
}

Causes

• Missing or incorrect `@Mapper` annotation: Ensure that your mapper interface is correctly annotated with `@Mapper`. It is essential for MapStruct to recognize the interface as a source for generating implementation classes.
• Maven or Gradle configuration issues: If you're using Maven or Gradle, project dependencies or incorrect compiler settings may prevent MapStruct from executing correctly.
• Annotation processing not enabled: Java annotation processing needs to be activated in your IDE or build tool's configuration for MapStruct to function properly.
• Incompatibility with the generated output folder: Check that the implementation is being output to the correct folder, and that your IDE is configured to recognize compilation outputs from annotation processors.

Solutions

• Annotate your mapper interfaces with `@Mapper` to ensure that MapStruct can identify them for implementation generation.
• Verify your `pom.xml` (for Maven) or `build.gradle` (for Gradle) configurations to include MapStruct dependencies and annotation processor configurations correctly.
• In IDEs like IntelliJ or Eclipse, enable annotation processing under settings to ensure that MapStruct can generate the necessary code.
• Check your project structure and output settings to confirm that there are no issues with compiler outputs.

Mistake: Not configuring the annotation processor correctly in the build tool.

Solution: Ensure you have the correct plugin and dependencies for annotation processing based on your build tool.

Mistake: Forget to include the `@Mapper` annotation on the interface.

Solution: Always annotate your mapper interfaces with `@Mapper`.

Mistake: Using old versions of MapStruct that have bugs.

Solution: Update to the latest version of MapStruct to benefit from bug fixes and enhancements.