An easy way to call Java from Python

Introduction

How to call java method from python using py4j. I had a hard time with all the old Japanese documents, so I summarized them.

environment

OS X Yosemite 10.10.5 anaconda 3.5.0(python 3.5.2)

Installation

After installing py4j with pip, copy the jar so that Java can read py4j.

$ pip install py4j
$ sudo cp ~/.pyenv/versions/(Virtual environment name)/share/py4j/py4j(version).jar /Library/Java/Extensions/

Implementation

As in the official tutorial, let's create a simple sample of addition in Java. First, create the following Java program.

`AdditionApplication.java`


import py4j.GatewayServer;

public class AdditionApplication {

  public int addition(int first, int second) {
    return first + second;
  }

  public static void main(String[] args) {
    AdditionApplication app = new AdditionApplication();
    //app gateway.entry_Set to point
    GatewayServer server = new GatewayServer(app);
    server.start();
    System.out.println("Gateway Server Started")
  }
}

After creating it, start the server with the following command.

$ javac AdditionApplication.java
$ java AdditionApplication

Next, let's write a python program to call AdditionApplication.

from py4j.java_gateway import JavaGateway

#Connect to JVM
gateway = JavaGateway() 
# java.util.Create a Random instance
random = gateway.jvm.java.util.Random()   
# Random.Call nextInt
number1 = random.nextInt(10)              
number2 = random.nextInt(10)
print(number1,number2) # (2, 7)
#Get an instance of AdditionApplication
addition_app = gateway.entry_point        
#Call addition
sum_num=addition_app.addition(number1,number2)    
print(sum_num) # 9

For the time being, it can be called from python. However, it is a little troublesome to start and stop the server from the terminal. Anyway, let's run it in python from start to finish.

#Execute by specifying the classpath
args=(["java","-cp",'/path/to/class/','AdditionApplication'])
p=subprocess.Popen(args)
#Prevent processing from going down before starting the server
time.sleep(3) 
gateway = JavaGateway(start_callback_server=True)
random = gateway.jvm.java.util.Random()
n1 = random.nextInt(10)
n2 = random.nextInt(10)
print(number1,number2) # (2, 7)
addition_app = gateway.entry_point
print(addition_app.addition(n1, n2)) # 9
#Kill the process
gateway.shutdown()

You can now run it consistently with python until the end. It can be used when inheriting a project of a Javer person, when there is a Java library you want to use, or when you want to implement some processing that requires speed in Java.