Py4J Callback Interface throws "Invalid interface name" when the packaged .jar is used as a plugin

Question

My code is exactly the same as the example shown on py4j website:

Implementing Java Interfaces from Python

Except my classes are all in the same src.main.java package

(see below for the code)

Problem: If I do a gradle fatjar build with ListenerApplication as main, then execute the jar, everything works fine. If I do a gradle fatjar build and instead access the code via a plugin interface, I get the following error:

Py4JError: An error occurred while calling o0.registerListener. Trace:
py4j.Py4JException: Invalid interface name: ExampleListener
    at py4j.Protocol.getPythonProxy(Protocol.java:429)
    at py4j.Protocol.getObject(Protocol.java:311)
    at py4j.commands.AbstractCommand.getArguments(AbstractCommand.java:82)
    at py4j.commands.CallCommand.execute(CallCommand.java:77)
    at py4j.GatewayConnection.run(GatewayConnection.java:238)
    at java.lang.Thread.run(Thread.java:748)

Question: Why does Py4J have problems finding "ExampleListener" when the .jar is run as a plugin and not as an application? I can even add :

public String classtest() throws Exception {
    System.out.println("classtest called");
    Class<?> py = Class.forName("ExampleListener");
    return py.toString();
}

to the ListenerApplication, which will return the correct interface both when run as plugin and as application! The interesting thing is, if I run the program plus plugin from netbeans IDE, everything works fine! Does Netbeans somehow expose the interface, while the application run directly, does not?

Plugin interface

import org.micromanager.MenuPlugin;
import org.micromanager.Studio;
import org.scijava.plugin.Plugin;
import org.scijava.plugin.SciJavaPlugin;
import py4j.GatewayServer;

@Plugin(type = MenuPlugin.class)
public class Py4JPluginInterface implements MenuPlugin, SciJavaPlugin{
    private static final String menuName = "Simpletest_gradle";
    private static final String tooltipDescription = "py4j gateway";
    private static final String version =  "0.1";
    private static final String copyright = "copyright";

    @Override
    public String getSubMenu() {
        return "Simpletest_gradle";
    }

    @Override
    public void onPluginSelected() {
        GatewayServer gatewayServer = new GatewayServer(new ListenerApplication());
        gatewayServer.start();
        System.out.println("Gateway Started at IP:port = "+gatewayServer.getAddress()+":"+gatewayServer.getPort());
    }

    @Override
    public void setContext(Studio app) {
    }

    @Override
    public String getName() {
        return menuName;
    }

    @Override
    public String getHelpText() {
        return tooltipDescription;
    }

    @Override
    public String getVersion() {
        return version;
    }

    @Override
    public String getCopyright() {
        return copyright;
    }

}

The interface:

//py4j/examples/ExampleListener.java
package py4j.examples;

public interface ExampleListener {

    Object notify(Object source);

}

The application:

package py4j.examples;

import py4j.GatewayServer;

import java.util.ArrayList;
import java.util.List;

public class ListenerApplication {

    List<ExampleListener> listeners = new ArrayList<ExampleListener>();

    public void registerListener(ExampleListener listener) {
        listeners.add(listener);
    }

    public void notifyAllListeners() {
        for (ExampleListener listener: listeners) {
            Object returnValue = listener.notify(this);
            System.out.println(returnValue);
        }
    }

    @Override
    public String toString() {
        return "<ListenerApplication> instance";
    }

    public static void main(String[] args) {
        ListenerApplication application = new ListenerApplication();
        GatewayServer server = new GatewayServer(application);
        server.start(true);
    }
}

The python listener

from py4j.java_gateway import JavaGateway, CallbackServerParameters


class PythonListener(object):

    def __init__(self, gateway):
        self.gateway = gateway

    def notify(self, obj):
        print("Notified by Java")
        print(obj)
        gateway.jvm.System.out.println("Hello from python!")

        return "A Return Value"

    class Java:
        implements = ["py4j.examples.ExampleListener"]

if __name__ == "__main__":
    gateway = JavaGateway(
        callback_server_parameters=CallbackServerParameters())
    listener = PythonListener(gateway)
    gateway.entry_point.registerListener(listener)
    gateway.entry_point.notifyAllListeners()
    gateway.shutdown()

Answer 1

For those who are interested, this was a class loader issue, which is apparently common for plugin/OSGI apps.

See the maintainer's response: https://github.com/bartdag/py4j/issues/339#issuecomment-473655738

I simply added the following to the Java-side ListenerApplication constructor:

    RootClassLoadingStrategy rmmClassLoader = new RootClassLoadingStrategy();
    ReflectionUtil.setClassLoadingStrategy(rmmClassLoader);