py4j - How would I go about on calling a python method in java

Question

I've recently discovered py4j and was able to call static java methods from python. Now I want to call python methods from java. I couldn't find much documentation so this is the last place I can think of that might tell me if it's possible, and how.

Answer 1

You can call a Python method from Java by implementing a Java interface on the python side.

The steps are:

1. Create an interface in Java, e.g., py4j.examples.Operator
2. In Python, create a class and inside the class, create a Java class with an "implements" field.
3. In Python, instantiate a gateway with start_callback_server=True, e.g., gateway = JavaGateway(start_callback_server=True)
4. In Python, instantiate the class implementing a Java interface and send it to the Java side.
5. In Java, call the interface.

Example adapted from the Py4J documentation:

Java code:

// File 1
package py4j.examples;

public interface Operator {
        public int doOperation(int i, int j);
        public int doOperation(int i, int j, int k);
}

// File 2
package py4j.examples;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

import py4j.GatewayServer;

public class OperatorExample {

    // To prevent integer overflow
    private final static int MAX = 1000;

    public List<Integer> randomBinaryOperator(Operator op) {
        Random random = new Random();
        List<Integer> numbers = new ArrayList<Integer>();
        numbers.add(random.nextInt(MAX));
        numbers.add(random.nextInt(MAX));
        numbers.add(op.doOperation(numbers.get(0), numbers.get(1)));
        return numbers;
    }
}

Python code:

from py4j.java_gateway import JavaGateway

class Addition(object):
    def doOperation(self, i, j, k = None):
        if k == None:
            return i + j
        else:
            return i + j + k

    class Java:
        implements = ['py4j.examples.Operator']

if __name__ == '__main__':
    gateway = JavaGateway(start_callback_server=True)
    operator = Addition()
    operator_example = gateway.jvm.py4j.examples.OperatorExample()

    # "Sends" python object to the Java side.
    numbers = operator_example.randomBinaryOperator(operator)