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

[Jeremy <ken...@gm...>]

Hah, turns out it was the missing 'self.' changed it to self.__eq__(obj) 
and it works! (Getting my Java and Python mixed up.)

Its great that I can just pass around the interface now.

There's one last thing that has been confusing me and that is launching 
the server using the java_gateway.launch_gateway().
I've been trying to start the app using that instead of manually calling 
the popen() myself.

My directory layout looks like this:

root
|
| -Launcher.py
| -MainWindow.py
| -Other stuff .py
| lib
    \
      | -common-codec.jar
      | -jackson-core.jar
      | -py4j-0.8.1.jar
      | -pctelelog-gateway-server.jar
      | -Other dependencies for gateway server

Looking at the documentation for launch_gateway() I decided to try using 
this in my Launcher.

self.__gateway = 
JavaGateway.launch_gateway(jarpath="./lib/pctelelog-gateway-server.jar", 
classpath="pctelelog.TeleLogPy4jLauncher", die_on_exit=True)

TeleLogPy4jLauncher.java (inside the pctelelog-gateway-server.jar)

package pctelelog;
/*Imports*/
public class TeleLogPy4jLauncher implements GatewayServerListener {

     private static Logger logger = 
LogManager.getLogger(TeleLogPy4jLauncher.class);
     private static TeleLogServer m_telelogServer = new TeleLogServer();

     public TeleLogPy4jLauncher() {
         logger.info("Logging started.");
         m_telelogServer.start();
     }

     public void addEventListener(EventListener listener) {
         logger.info("Listener added.");
         m_telelogServer.addEventListener(listener);
     }

     public void removeEventListener(EventListener listener) {
         logger.info("Listener removed.");
         m_telelogServer.removeEventListener(listener);
     }

     public static void main(String[] args) {
         TeleLogPy4jLauncher launcher = new TeleLogPy4jLauncher();
         GatewayServer gateway = new GatewayServer(launcher);
         gateway.addListener(launcher);
         gateway.start();
     }
     /* GatewayListener implementations */
}

When I run Launcher.py I get a StringIndexOutofBoundsException when the 
addEventListener is called in my MainWindow initCallback(). If I launch 
the JAR manually everything works fine. So I feel like there must be 
something I'm misunderstanding about the way to use launch_gateway. 
Maybe it won't work with my server inside a jar?

Thanks for the help and patience so far!

Jeremy

On 5/17/14, 5:17 PM, Barthelemy Dagenais wrote:
> Hi,
>
> regarding the equals method, I believe you were in the right direction:
>
>      def equals(self, obj):
>          # obj will be a JavaObject type, so obj is never equal to self
>          return obj is not None and obj.toString() == self.toString()
>
>      def toString(self):
>          return "Something Unique"
>
> In one of your examples, you call __eq__(obj), but it should be self.__eq__(obj)
>
> A good practice when you are not sure what is going on is to always
> wrap your code in:
>
> try:
>      # unsafe code
> except Exception:
>      from traceback import print_exc
>      print_exc()
>
> Sometimes, exceptions are swallowed or shadowed by other exceptions
> and this is a trick that saved me a lot of time in the past ;-)
>
> Finally, I'm not sure why you need the setter (vs the constructor with
> the parameter). I'll need to investigate and I'll try to get back to
> you this week.
>
> Thanks,
> Bart
>
>
> On Fri, May 16, 2014 at 7:35 PM, Jeremy <ken...@gm...> wrote:
>> Sorry for the delayed reply. Been busy, finally got time to work on my
>> project.
>>
>> I looked at the Vector.java code and can see that it calls the Object
>> class method, equals().
>> So taking your suggestion, I added an equals(Object) method to the
>> interface (not necessary likely) and tried to implement something in the
>> implementing class in Python. Something like:
>>
>> class equals(self, obj):
>>       if self == obj:
>>           return True
>>       return False
>>
>> But still got the same error. Tried adding in Python's global equal, and
>> that didn't work either. And even tried creating a UUID for each
>> EventListener, but that didn't work either. It just can't seem to find
>> the equals method. My final EventListener ended up looking like this.
>>
>> class EventListener(object):
>>       id = uuid.uuid4()
>>
>>       def onEvent(self, event):
>>           pass
>>
>>       def equals(self, obj):
>>           return __eq__(obj)
>>
>>       def __eq__(self, obj):
>>           if obj == None:
>>               return False
>>
>>           if obj.id == self.id:
>>               return True
>>           else:
>>               return False
>>
>>       class Java:
>>           implements = ['pctelelog.EventListener']
>>
>>       pass
>>
>>
>> My solution that seems to be working now has been to remove passing
>> around an interface and instead rename the interface EventCallback and
>> wrap it inside a new class (EventListener) and pass an object of that
>> class around. It works fine but I have a strange issue now, maybe I'm
>> doing it wrong.
>>
>> Here's my new EventListener.java
>> public class EventListener implements EventCallback {
>>
>>       private EventCallback m_callback = null;
>>
>>       public EventListener() {
>>       }
>>
>>       public EventListener(EventCallback callback) {
>>           assert callback != null;
>>
>>           m_callback = callback;
>>       }
>>
>>       public void setCallback(EventCallback callback) {
>>           m_callback = callback;
>>       }
>>
>>       public static EventListener createEventListener(EventCallback
>> callback) {
>>           return new EventListener(callback);
>>       }
>>
>>       @Override
>>       public void onEvent(AbstractEvent event) {
>>           m_callback.onEvent(event);
>>       }
>> }
>>
>> Here's the Python I use to get an instance of the class and then add it
>> to the listener pool:
>> (This method sits inside a PyQt Widget class.)
>>
>> def initCallback(self, func):
>>           callback = EventCallback()
>>           callback.onEvent = self.onEvent
>>           self.eventListener =
>> self.__gateway.jvm.pctelelog.EventListener(callback)
>> self.__gateway.entry_point.addEventListener(self.eventListener)
>>
>>
>> When I try to do it like that I get an error (Logger + Error):
>>
>> Callback Server Starting
>> Socket listening on ('127.0.0.1', 25334)
>> Command to send: r
>> u
>> pctelelog
>> rj
>> e
>>
>> Answer received: yp
>> Command to send: r
>> u
>> pctelelog.EventListener
>> rj
>> e
>>
>> Answer received: ycpctelelog.EventListener
>> Traceback (most recent call last):
>>     File
>> "/Users/Shared/eclipse/workspace/PCTeleLog-PyQt/pytelelog-pyqt/Launcher.py",
>> line 27, in <module>
>>       Launcher()
>>     File
>> "/Users/Shared/eclipse/workspace/PCTeleLog-PyQt/pytelelog-pyqt/Launcher.py",
>> line 23, in __init__
>>       mainWindow = MainWindow(self.__gateway)
>>     File
>> "/Users/Shared/eclipse/workspace/PCTeleLog-PyQt/pytelelog-pyqt/MainWindow.py",
>> line 25, in __init__
>>       self.initCallback(self.onEvent)
>>     File
>> "/Users/Shared/eclipse/workspace/PCTeleLog-PyQt/pytelelog-pyqt/MainWindow.py",
>> line 49, in initCallback
>>       self.eventListener =
>> self.__gateway.jvm.pctelelog.EventListener(callback)
>>     File "/usr/local/lib/python2.7/site-packages/py4j/java_gateway.py",
>> line 662, in __call__
>>       args_command = ''.join([get_command_part(arg) for arg in args])
>>     File "/usr/local/lib/python2.7/site-packages/py4j/protocol.py", line
>> 261, in get_command_part
>>       command_part = PYTHON_PROXY_TYPE + python_proxy_pool.put(parameter)
>> AttributeError: 'NoneType' object has no attribute 'put'
>>
>> ===
>> Now, if I change it around and instantiate with the void constructor and
>> set the callback with the setter. It works fine.
>>
>>       def initCallback(self, func):
>>           callback = EventCallback()
>>           callback.onEvent = self.onEvent
>>           self.eventListener = self.__gateway.jvm.pctelelog.EventListener()
>>           self.eventListener.setCallback(callback)
>> self.__gateway.entry_point.addEventListener(self.eventListener)
>>
>> I'd rather do without the setter if there was a way to manage it. Any
>> idea what's going on?
>>
>>
>> Thanks!
>>
>> On 5/7/14, 7:54 AM, Barthelemy Dagenais wrote:
>>> 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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