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.
You can call a Python method from Java by implementing a Java interface on the python side.
The steps are:
gateway = JavaGateway(start_callback_server=True)
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)
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