How to prevent py4j from resurrecting an older version of my JAR?

Question

I'm running a basic py4j program as follows:

from py4j.java_gateway import JavaGateway
gateway = JavaGateway.launch_gateway()
example = gateway.entry_point
results = example.foo()
print(results) // "foo"

Originally, my java program looked like this

package foo;
import py4j.GatewayServer;

public class Example {

    public static void main(String[] args) throws IOException {
        GatewayServer gatewayServer = new GatewayServer(new Example());
        gatewayServer.start();
        System.out.println("PyJ Gateway Server Started");
    }

    public String foo() {
        return "foo";
    }
}

Environment details: I'm running the python program via asdf+poetry python and building the java jar with maven. Python version 3.9.17, Java version 17.

After building and running my server a few times, I realized I needed a new method, so got rid of foo(), replacing it with bar(). So now my program looks like the following:

package foo;
import py4j.GatewayServer;

public class Example {

    public static void main(String[] args) throws IOException {
        GatewayServer gatewayServer = new GatewayServer(new Example());
        gatewayServer.start();
        System.out.println("PyJ Gateway Server Started");
    }

    public String bar(String input) {
        return input;
    }
}

However, now after rebuilding and relaunching my java jar for Example, I'm seeing the following results in my python program:

from py4j.java_gateway import JavaGateway
gateway = JavaGateway.launch_gateway()
example = gateway.entry_point
results = example.foo()
print(results) // "foo"
results = example.bar("bar") // py4j.protocol.Py4JError: An error occurred while calling t.bar. Trace: py4j.Py4JException: Target Object ID does not exist for this gateway :t

Note that these results persist, even if I locally kill all processes with "java" in the name, delete the original JAR, or anything else. Somehow py4j seems to have learned the first ever JAR I presented it with and will never let go.

How do I kill the deprecated java code and prevent python from speaking to it ever again?

Answer 1

It turns out that using JavaGateway.launch_gateway() was restarting some cached version of my java jar. Rather than using the generic launch_gateway, using a more specific JavaGateway constructor worked.

from py4j.java_gateway import JavaGateway
gateway = JavaGateway(gateway_parameters=GatewayParameters(port=12345))
example = gateway.entry_point
...