Py4J Listener Callback Example

ExampleListener.java

package py4j.examples;

public interface ExampleListener {

	Object notify(Object source);

}

listener_example.py

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()

ListenerApplication.java

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);
	}
}