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

[Barthelemy D. <bar...@in...>]

Hi!

Thanks for using Py4J! I'll try to look at this issue more deeply
during the weekend, but from the stack trace, it seems the Java side
is sending a method call to the Python side and the Python side is
replying with an exception:

Error:
py4j.protocol.Py4JJavaError: An error occurred while calling
t.removeEventListener.
: py4j.Py4JException: An exception was raised by the Python Proxy.
Return Message: x

Also, from the stack trace, it appears that when deleting the listener
from the Java vector, the vector calls the "equals" method on the
listener. This is the first time I have to deal with this scenario and
I believe the equals method is delegated to the python side. Because
it is not implemented, I believe this may be the cause of failure. A
workaround might be to try to implement an "equals" method in your
Python listener.

Do you have logging enabled on the Python side? Any stack trace visible there?

Btw, you should subscribe to the mailing list, it would be easier to
manage the replies :-)

Bart

On Tue, May 6, 2014 at 7:58 PM, Jeremy <ken...@gm...> wrote:
> 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
>
>
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