In my previous blog post I evaluated the feasibility of Java for lightweight system programming thanks to the GraalVM native image generation tool. Despite the initial results look promising, I felt disappointed when they pointed me out that GraalVM does not support reflection by default (which IMHO is a wonderful and powerful tool to enhance the expressiveness of our software and to reduce boilerplate). However, you can actually configure the ahead-of-time compiler to incorporate a user-provided reflection metadata.

Let's see how.

Simple reflection example: read KickAss Configuration data

As a quick example, let's write a simple program that can read external configuration data using KAConf: KickAss Configuration, a library made by myself to easily configure software through Java Annotations (inspired in the Spring Framework). With KAConf, you can configure your data annotating your fields with a @Property annotation:

import info.macias.kaconf.Property;

public class Config {
   private String name = "world";

   public String getName() {
  	return name;

You can also annotate static fields:

public class StaticConfig {
   public static String GREETING = "Hello";

Then, you only have to create a Configurator object, tell it about the sources of configuration data (e.g. environment) and which objects or classes are configurable:

public static void main(String[] args) {
   Configurator c = new ConfiguratorBuilder()

   Config cfg = new Config();

   System.out.println(StaticConfig.GREETING + ", " + cfg.getName() + "!");

If you compile and run the above code as a normal JVM bytecode (e.g. a JAR), it will make use of the default configuration:

$ java -jar aot-reflection-test.jar
Hello, world!

Or will read the configuration from the environment:

$ GREETING="What's up" NAME="gang" java -jar aot-reflection-test.jar
What's up, gang!

The KAConf library makes intensive use of Java Reflection to automatically set the configurable values to their respective fields. However, if you use the native-image GraalVM command, you can run the program but the generated binary is not able to modify the Config and StaticConfig values at runtime, showing always the default Hello, world! message.

How to pass the reflection configuration to native-image

First, you need to create a JSON file (e.g. reflectconfig.json) where you specify the classes and fields reflection information:

	"name" : "info.macias.aotrt.StaticConfig",
	"allDeclaredFields": true
	"name" : "info.macias.aotrt.Config",
	"allDeclaredFields": true

For the sake of simplicity, we have used the allDeclaredFields property as a wildcard, but you can actually specify which concrete fields, constructors and methods can be used for reflection (see the online GraalVM documentation for more details).

Then, you have to compile the bytecode into native by passing the -H:ReflectionConfigurationFiles parameter to the native-image command:

$ native-image --no-fallback -H:ReflectionConfigurationFiles=reflectconfig.json\
    	-jar build/libs/aot-reflection-test.jar aot-test

And, voilĂ ! your native application can make use of the coolest configuration library around the world.

$ GREETING="What's up" NAME="gang" ./aot-test
What's up, gang!

Known limitations

The ideal way of using KAConf is along with immutable properties, e.g.:

private final String name = "world";

public static final String GREETING = KA.def("Hello");

However, the GraalVM native-image AoT compiler (at least the version 1.0.0 RC16), doesn't seem to like final fields, even if you enable writing with the "allowWriting": true property inside the reflectconfig.json file.

Example source code