Received incorrect object type from Py4J gateway

[Josh]

I am using Java callback functions in Python using Py4J.

Java openjdk 11.0.3

Python 3

Py4J 0.10.8.1

I have successfully used Py4J in the past to use Java callback functions in Python. However, I recently added a new callback class using the same pattern I used before, but now I get the following error with the new class.

AttributeError: 'JavaMember' object has no attribute '_get_object_id'

In my attempts to track down the issue, I printed out the Java objects in Python. The old class StatusCallback shows an object type of TestsJNI$StatusCallback@548ebebc, but the new class shows an object type of <py4j.java_gateway.JavaMember object at 0x7f8dc69817f0>.

This explains the AttributeError I got. But I can't figure out why I am not getting py4j.java_gateway.JavaMember instead of something like TestsJNI$UploadCallback.

Java Code

import py4j.GatewayServer;

public class TestsJNI
{
    public static boolean functionCalled = false;

    public static class UploadCallback
    {
        public static void callback()
        {
            functionCalled = true;
        }

        // I have also tried these functions
        //public static void callback(int a)
        //{
        //  functionCalled = true;
        //}

        //public static void callback(int a, boolean b)
        //{
        //  functionCalled = true;
        //}
    }
    public static UploadCallback uploadCallback = new UploadCallback();

    public static class StatusCallback
    {
        public static void callback()
        {
            functionCalled = true;
        }
    }
    public static StatusCallback statusCallback = new StatusCallback();

    public static void main(String args[])
    {
        TestsJNI testApp = new TestsJNI();

        // Py4J server
        GatewayServer server = new GatewayServer(testApp);
        server.turnLoggingOff();
        server.start();
    }
}

Python Code

from py4j.java_gateway import JavaGateway, GatewayParameters, get_field

javaCmd = ("java -cp /mnt/c/Workspace/tests/java/:.:/home/fred/.local/share/py4j/py4j0.10.8.1.jar TestsJNI")
print(javaCmd)
self.jvmProcess = Popen(javaCmd, shell=True, preexec_fn=os.setsid)
time.sleep(1.0)

# Connect to JVM via Py4J gateway
logging.getLogger("py4j").setLevel(logging.ERROR)
self.gateway = JavaGateway(gateway_parameters=GatewayParameters(auto_field=True))

entryPoint = self.gateway.entry_point
self.jni = entryPoint.autoguidanceJNI
self.jniObj = self.gateway.entry_point

testObj = self.jniObj.uploadCallback
print("upload testObj = " + str(testObj))
ret = False

testObj = self.jniObj.StatusCallback
print("status testObj = " + str(testObj));

Python Output

upload testObj = <py4j.java_gateway.JavaMember object at 0x7f8dc69817f0>
status testObj = TestsJNI$StatusCallback@548ebebc

My Question

Why does Py4J give me a py4j.java_gateway.JavaMember object for the UploadCallback class and a TestsJNI$StatusCallback object for the StatusCallback class?

[Barthelemy Dagenais]

Hi,

I'll need to perform some tests. Maybe the auto_field is confused by the static declarations, but I believe I would have encountered this bug before.

A few observations:

- This does not look like the whole code. I see reference to self. in the python code and to autoguidanceJNI, which is not declared anywhere.

- I see a reference to jniObj.StatusCallback (with capital S) and not jniObj.statusCallback. Is this on purpose?

- Did you try accessing the fields using get_field instead of auto_field: https://www.py4j.org/faq.html#how-to-access-a-field

HTH,

Bart

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[Josh]

Correct it is not the whole code, it would be too large to paste here.

The capital S is just a typo here, sorry.

Yes, I have tried using `get_field` but I got the same results.

[Barthelemy Dagenais]

Hi,

I did some additional tests and could not reproduce your issue with get_field.

I recreated this test:

public class ExampleClass {

   public static class UploadCallback {
      public static void callback() {

         System.out.println("Callback called");

      }
   }
   
   public static UploadCallback uploadCallback = new UploadCallback();

Then:

In [1]: from py4j.java_gateway import JavaGateway, get_field

In [2]: gateway = JavaGateway()

In [3]: example = gateway.entry_point.getNewExample()

In [4]: upload = get_field(example, "uploadCallback")

In [5]: upload.callback()

And the JVM prints "Callback called"

Is it possible that something else is interfering ?

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[Josh Henrie]

It is very possible that something was interfering.

In an attempt to resolve the issue I reverted to using Py4J 0.10.7 and at the same time changed a bunch of other things. And the error went away.

Unfortunately, I am not sure what changes I made to fix the issue.

I do know that I have a machine with Py4J 0.10.8.1 and the scripts run fine, so I don't think that downgrading Py4J was the solution.

To view this discussion on the web visit https://groups.google.com/a/py4j.org/d/msgid/py4j/CAM_%3DWAad8E4Gr8ru7zzRsgwqzq8i507xZdbzUhUe%3DtXx4NOg%3Dw%40mail.gmail.com.