Quarkus utilizes ahead-of-time (AOT) compilation to build blazing fast native executables. However, AOT works through closed-world analysis which eliminates unused code paths. This can break functionality relying on runtime reflection like dependency injection, bytecode manipulation, and integration with certain libraries.

Registering for Reflection

When building a native executable, GraalVM operates under a closed-world assumption, analyzing the call tree and eliminating unused classes, methods, and fields. To include elements requiring reflective access, explicit registration becomes crucial.

• Using the @RegisterForReflection annotation. The simplest way to register a class for reflection is through the @RegisterForReflection annotation:

@RegisterForReflection
public class MyClass {
}

For classes in third-party JARs, an empty class can host the @RegisterForReflection annotation:

@RegisterForReflection(targets={DemoReflection1.class, DemoReflection2.class})
public class MyReflectionConfiguration {
}

Note that DemoReflection1 and DemoReflection2 will be registered for reflection, but not MyReflectionConfiguration.

• Using a Configuration File Configuration files can also be used to register classes for reflection. For instance, to register all methods of com.demo.MyClass, create reflection-config.json:

[
   {
      "name":"com.demo.MyClass",
      "allDeclaredConstructors":true,
      "allPublicConstructors":true,
      "allDeclaredMethods":true,
      "allPublicMethods":true,
      "allDeclaredFields":true,
      "allPublicFields":true
   }
]

Make the configuration file known to the native-image executable by adding the following to application.properties:

quarkus.native.additional-build-args=-H:ReflectionConfigurationFiles=reflection-config.json

Quarkus Extension Support for Native Mode

To enable native mode support for a custom extension, Quarkus simplifies the registration of reflection through ReflectiveClassBuildItem. This class is used in the build process to specify classes requiring reflective access.

Understanding ReflectiveClassBuildItem

ReflectiveClassBuildItem is a Quarkus-specific class utilized in the extension development process. It plays a crucial role in indicating which classes should be made available for reflective access at runtime. This is especially relevant when certain operations, such as dependency injection or
bytecode manipulation, require runtime reflection.

Usage in Quarkus Extensions

When creating a Quarkus extension, you can seamlessly integrate the registration of reflective classes using ReflectiveClassBuildItem. The @BuildStep annotation signifies a build step, a fundamental concept in Quarkus extension development.

public class MyClass {

    @BuildStep
    ReflectiveClassBuildItem reflection() {
        return new ReflectiveClassBuildItem(false, false, "com.demo.DemoClass");
    }

}

In this snippet, MyClass is a placeholder for the actual extension class you are developing. The reflection() method, annotated with @BuildStep, creates an instance of ReflectiveClassBuildItem, indicating that the class com.demo.DemoClass requires reflective access. The false arguments for methods and fields indicate that reflective access is needed only for the constructor.

I showcase a Quarkus extension that leverages the ReflectiveClassBuildItem to dynamically register classes for reflection. The extension focuses on identifying classes implementing a specific interface (CustomFeature in this case) and also explicitly registers some standard Java classes for reflective access.

import io.quarkus.deployment.annotations.BuildStep;
import io.quarkus.deployment.builditem.CombinedIndexBuildItem;
import io.quarkus.deployment.builditem.nativeimage.ReflectiveClassBuildItem;
import org.jboss.jandex.ClassInfo;

import java.text.DecimalFormat;
import java.text.DecimalFormatSymbols;
import java.text.SimpleDateFormat;

public class ReflectionExtension {

    // Interface to identify classes for reflection
    private static final DotName CUSTOM_FEATURE_INTERFACE = DotName.createSimple(CustomFeature.class.getName());

    @BuildStep
    void registerForReflection(CombinedIndexBuildItem combinedIndex,
                               BuildProducer<ReflectiveClassBuildItem> reflectiveClasses) {

        for (ClassInfo implClassInfo : combinedIndex.getIndex().getAllKnownImplementors(CUSTOM_FEATURE_INTERFACE)) {
            String combinedIndexName = implClassInfo.name().toString();
            log.debugf("CustomFeature class implementation '[%s]' registered for reflection", combinedIndexName);

            reflectiveClasses.produce(new ReflectiveClassBuildItem(true, true, combinedIndexName));
        }

    }

}

Explanation

1. CombinedIndexBuildItem: This build item provides access to the combined index of all classes in the application. In this example, it is used to retrieve all known implementors of the Conversion interface.

2. Iterating Over Implementors: The extension iterates over all classes implementing the Conversion interface and registers them for reflection using ReflectiveClassBuildItem.

3. DotName is a class representing a dotted name, which is essentially a fully qualified class name in a format where package names and class names are separated by dots. The DotName class is part of the Jandex library, which is a tool used by Quarkus for indexing and querying Java classes. DotName is used to represent and work with fully qualified class names in the Jandex indexing system. It's a lightweight and efficient way to refer to classes within the Jandex index.

Considerations
While ReflectiveClassBuildItem provides a mechanism to address reflective access requirements, it's crucial to use it judiciously. Excessive reliance on reflective access can undermine the performance benefits of Quarkus' AOT compilation approach. Therefore, it's recommended to leverage this tool sparingly and explore alternative strategies whenever possible.

Conclusion

In summary, understanding and effectively using ReflectiveClassBuildItem is key to optimizing Quarkus extensions for native mode. By selectively indicating classes that necessitate reflective access, developers can strike a balance between the advantages of AOT compilation and the unavoidable realities of certain runtime operations.