I have code that attempts to redefine a class at runtime using a ClassFileTransformer and an instance of Instrumentation.
However, I’ve noticed that the transform method of ClassFileTransformer fails to throw any exceptions.

Here’s an example (tested in java 8 and 17):

import net.bytebuddy.agent.ByteBuddyAgent;

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.Instrumentation;
import java.security.ProtectionDomain;

public class Main {
    public static void main(String[] args) {

        // implementation 'net.bytebuddy:byte-buddy-agent:1.14.13'
        ByteBuddyAgent.install();
        Instrumentation instrumentation = ByteBuddyAgent.getInstrumentation();

        instrumentation.addTransformer(new Transformer(), true);

        try {
            instrumentation.retransformClasses(Klass.class);
        }catch (Exception e) {
            System.out.println("An error occurred: " + e.getMessage());
            return;
        }

        System.out.println("Finished successfully");

    }

    private static class Transformer implements ClassFileTransformer {

        @Override
        public byte[] transform(ClassLoader cl, String name, Class<?> klass, ProtectionDomain pd, byte[] classfileBuffer) {
            System.out.println("Transforming class " + klass.getName());
            throw new RuntimeException("Example exception");
        }
    }

    private static class Klass {}

}

This code throws an exception and should logically trigger the “An error occurred: ” message.

Instead, no exception is caught by the catch block:

Transforming class Main$Klass
Finished successfully

Do you know what causes this behavior and whether it can be avoided?