Simplest example with py4J

Question

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)

Update 1

After I added: import py4j.GatewayServer; to the top of the file, I got a different error:

package py4j does not exist

Update 2

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?

Final update and solution:

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

I don't get it. I did exactly what you did here, but when I run: java -cp py4j0.10.8.1.jar AdditionApplication, I receive the following error:" Error: Could not find or load main class AdditionApplication". Any ideas? — Chiel
– Chiel, Commented Dec 21, 2018 at 21:59
For Mac/LInux: java -cp /usr/local/share/py4j/py4j0.10.5.jar :. AdditionApplication For Windows: java -cp /usr/local/share/py4j/py4j0.10.5.jar AdditionApplication — Sreenu Yatam
– Sreenu Yatam, Commented May 9, 2021 at 14:01
@Chiel a bit late to the party but i had the same issue today. You also need to include the folder which contains AdditionApplication.class to the classpath. On windows this worked for me: java -cp ".;C:\tools\miniconda3\envs\circe\share\py4j\py4j0.10.9.7.jar" AdditionApplication — Frank Martin
– Frank Martin, Commented Mar 8, 2023 at 10:50

Reimeus · Accepted Answer · 2014-03-13 18:11:30Z

6

Import GatewayServer from the py4j package so that the unqualified class can be used in the application

import py4j.GatewayServer;

edited Mar 13, 2014 at 18:11

answered Mar 13, 2014 at 17:33

Reimeus

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8 Comments

Amelio Vazquez-Reina Over a year ago

Thanks - I now get the error package py4j does not exist, but I installed py4j using pip install py4j from Python.

Reimeus Over a year ago

you need to include the py4j0.x.jar JAR file on your compile classpath, e.g. javac -cp /usr/share/py4j/py4j0.x.jar:. AdditionApplication.java

Amelio Vazquez-Reina Over a year ago

Thanks! The command now compiles to AdditionApplication.class. I imagine that in order to use it from Python it has to be running, since it's technically a server. How do I run it?

Reimeus Over a year ago

Never used p4j TBH - try java -cp /usr/share/py4j/py4j0.x.jar:. AdditionApplication

Amelio Vazquez-Reina Over a year ago

Thanks @Reimeus - When I run javac ... without the .class suffix, I got: error: Class names, 'AdditionApplication' are only accepted if annotation processing is explicitly requested

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