I installed py4J
using pip on my conda
virtual environment in Python. I wrote a super simple example AdditionApplication.java
to test py4J
, but it fails to compile, i.e.
javac AdditionApplication.java
fails complaining that GatewayServer
is not defined.
I am knowledgeable in Python but unfortunately not in Java. What else do I need to provide?
public class AdditionApplication {
public int addition(int first, int second) {
return first + second;
}
public static void main(String[] args) {
AdditionApplication app = new AdditionApplication();
// app is now the gateway.entry_point
GatewayServer server = new GatewayServer(app);
server.start();
}
}
In case it matters I have the following version of Java installed:
java -version
java version "1.7.0_51"
Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)
After I added: import py4j.GatewayServer;
to the top of the file, I got a different error:
package py4j does not exist
pip install py4j
left a jar
file under <PATH_TO_CONDA_ENVIRONMENT>/share/py4j/py4j0.8.1.jar
. I have added it to my class path with:
javac -cp <PATH_TO_CONDA_ENVIRONMENT>/share/py4j/py4j0.8.1.jar AdditionApplication.java
and it output
AdditionApplication.class
How do I run it?
After applying the previous fixes, I finally run the code with:
java -cp <PATH_TO_CONDA_ENVIRONMENT>/share/py4j/py4j0.8.1.jar AdditionApplication
the code runs in the background. To test it:
>>> from py4j.java_gateway import JavaGateway
>>> gateway = JavaGateway() # connect to the JVM
>>> random = gateway.jvm.java.util.Random() # create a java.util.Random instance
>>> number1 = random.nextInt(10) # call the Random.nextInt method
>>> number2 = random.nextInt(10)
>>> print(number1,number2)
(2, 7)
>>> addition_app = gateway.entry_point # get the AdditionApplication instance
>>> addition_app.addition(number1,number2) # call the addition method
Import GatewayServer
from the py4j
package so that the unqualified class can be used in the application
import py4j.GatewayServer;