How to add custom classes/records to reflect-config.json?

[jpomykala]

Hey there,

I'm using Jackson for reading JSONs with Picocli and GraalVM. Unfortunately, most of JSON libraries use reflection for JSON deserialization, and as a result, I have to add them to reflect-config.json. I wonder if there is any way to point records/classes that would be added to the reflect-config.json.

I saw there is a way to provide a custom factory class instead picocli.codegen.aot.graalvm.ReflectionConfigGenerator but before I start doing this, I wanted to ask if there is any easier way to achieve this.

Thanks!

[yuvkun10]

Great question! There are a few approaches to add your custom classes/records to reflect-config.json without writing a custom factory.

Option 1: Use the --additional-class flag on ReflectionConfigGenerator (simplest)

java -cp ... picocli.codegen.aot.graalvm.ReflectionConfigGenerator \
  --additional-class=com.example.MyRecord \
  --additional-class=com.example.AnotherClass \
  com.example.MyCommand

This is the easiest path if you have a few extra classes to add.

Option 2: Create a manual reflect-config.json and let GraalVM merge it

Create src/main/resources/META-INF/native-image/reflect-config.json:

[
  {
    "name": "com.example.MyRecord",
    "allDeclaredConstructors": true,
    "allDeclaredMethods": true,
    "allDeclaredFields": true
  }
]

GraalVM automatically merges all reflect-config.json files found on the classpath under META-INF/native-image/.

Option 3: Use the GraalVM Tracing Agent (most thorough)

Run your app with the tracing agent to auto-generate the config:

java -agentlib:native-image-agent=config-output-dir=src/main/resources/META-INF/native-image \
  -jar myapp.jar --your-flags

This generates all necessary config files including reflect-config.json for all classes actually used at runtime, which catches Jackson deserialization targets automatically.

The tracing agent approach is particularly good for Jackson since it exercises the actual reflection calls Jackson makes during deserialization.

See the picocli GraalVM docs and GraalVM reflection docs for full details.

Hope this helps! Let me know if you need clarification.