Thread: [Py4j-users] An issue in reflection?

SourceForge Headquarters 225 Broadway Suite 1600 San Diego, CA 92101 +1 (858) 422-6466

So I'm trying to use Py4j to let a UI talk to a Java server. The java 
server uses 3 threads (entry-point, server socket, client-event 
dispatch). The python UI registers an event call back listener and that 
listener eventually gets dumped into a Vector. I had some initial 
trouble getting the addEventListener() working but fixed it (somehow). 
But now any calls to removeEventListener() don't work.

My first guess was something to do with different instances of the 
object when Vector does the lookup, but I'm not sure. I don't understand 
relfection concepts well enough and the Py->Java (callback) thing seems 
to suggest I should be fine since the original eventlistener hasn't been 
garbage collected yet. If you have any insight that would be great.

Error:
py4j.protocol.Py4JJavaError: An error occurred while calling 
t.removeEventListener.
: py4j.Py4JException: An exception was raised by the Python Proxy. 
Return Message: x
     at py4j.Protocol.getReturnValue(Protocol.java:417)
     at 
py4j.reflection.PythonProxyHandler.invoke(PythonProxyHandler.java:113)
     at com.sun.proxy.$Proxy0.equals(Unknown Source)
     at java.util.Vector.indexOf(Vector.java:404)
     at java.util.Vector.indexOf(Vector.java:378)
     at java.util.Vector.removeElement(Vector.java:637)
     at java.util.Vector.remove(Vector.java:795)
     at 
pctelelog.EventOperatorThread.removeEventListener(EventOperatorThread.java:20)
     at pctelelog.ServerThread.removeEventListener(ServerThread.java:16)
     at pctelelog.Launcher.removeEventListener(Launcher.java:17)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at 
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)

This code is a stripped down version of the development version but both 
are generating the same error.

package pctelelog;

public class Event {
     public String testData = "test";
}

===

package pctelelog;

public interface EventListener {
     public void onEvent(Event event);
}

===

package pctelelog;

import java.util.Vector;

public class EventOperatorThread extends Thread {

     private Vector<EventListener> m_listeners = new 
Vector<EventListener>();

     public void run() {
         while(true) {}
     }

     public synchronized void addEventListener(EventListener listener) {
         System.out.println("ADD");
         m_listeners.add(listener);
     }

     public synchronized void removeEventListener(EventListener listener) {
         System.out.println("REMOVE");
         m_listeners.remove(listener);
     }
}

===

package pctelelog;

public class ServerThread extends Thread {

     private EventOperatorThread m_operator = new EventOperatorThread();

     public void run() {
         m_operator.start();
         while(true) {}
     }

     public synchronized void addEventListener(EventListener listener) {
         m_operator.addEventListener(listener);
     }
     public synchronized void removeEventListener(EventListener listener) {
         m_operator.removeEventListener(listener);
     }

}

===

package pctelelog;

import py4j.GatewayServer;

public class Launcher {
     private ServerThread m_server = new ServerThread();

     public Launcher() {
         m_server.start();
         System.out.println("Server started.");
     }

     public void addEventListener(EventListener listener) {
         m_server.addEventListener(listener);
     }
     public void removeEventListener(EventListener listener) {
         m_server.removeEventListener(listener);
     }

     public static void main(String[] args) {
         GatewayServer gateway = new GatewayServer(new Launcher());
         gateway.start();
     }
}

===

from py4j.java_gateway import JavaGateway, GatewayClient

class EventListener(object):

     def onEvent(self, event):
         print event.testData;

     class Java:
         implements = ['pctelelog.EventListener']

class Launcher:

     def __init__(self):
         self.__gateway = JavaGateway(start_callback_server=True)
         listener = EventListener()
         self.__gateway.entry_point.addEventListener(listener)
         self.__gateway.entry_point.removeEventListener(listener)

if __name__ == '__main__':
     Launcher()
     pass

Thread: [Py4j-users] An issue in reflection?

py4j-users