How to Call a Python Script from Java

Integrating Python scripts with Java applications can be a powerful way to leverage Python’s rich ecosystem while maintaining Java’s performance and scalability. Whether you're working on data analysis, machine learning, or automation, calling Python from Java is easier than you might think.

Why Call Python from Java?

Python excels in scripting, data science, and AI, while Java is widely used for enterprise applications. Combining both allows you to:

Use Python libraries like pandas, numpy, or tensorflow in a Java environment.
Run Python-based machine learning models in a Java backend.
Automate Python scripts within a larger Java application.

Method 1: Using Runtime.exec()

The simplest way to call a Python script from Java is by using Runtime.exec(). This method executes the Python interpreter directly from Java.

Step-by-Step Implementation

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class RunPythonScript {
    public static void main(String[] args) {
        try {
            // Command to execute Python script
            String command = "python3 /path/to/your_script.py arg1 arg2";
            
            // Execute the command
            Process process = Runtime.getRuntime().exec(command);
            
            // Read output
            BufferedReader reader = new BufferedReader(
                new InputStreamReader(process.getInputStream())
            );
            
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
            
            // Wait for the process to complete
            int exitCode = process.waitFor();
            System.out.println("Exited with code: " + exitCode);
            
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Pros and Cons

Pros: Simple, no extra dependencies.
Cons: Limited interaction, requires Python to be installed on the system.

Method 2: Using Jython (For Python 2.x)

Jython is a Java implementation of Python 2.7, allowing seamless integration between Java and Python code.

How to Use Jython

import org.python.util.PythonInterpreter;

public class JythonExample {
    public static void main(String[] args) {
        PythonInterpreter interpreter = new PythonInterpreter();
        interpreter.exec("print('Hello from Python!')");
    }
}

Limitations

Only supports Python 2.x (not Python 3).
May not work with all Python libraries.

Method 3: Using ProcessBuilder (More Control)

For better control over the execution environment, use ProcessBuilder.

ProcessBuilder pb = new ProcessBuilder("python3", "script.py", "arg1", "arg2");
pb.directory(new File("/path/to/script"));
Process process = pb.start();

Method 4: Using Py4J (Recommended for Python 3)

Py4J is a popular library that enables Python-Java interoperability with minimal overhead.

Setting Up Py4J

Install Py4J in Python: pip install py4j
Add the Py4J JAR to your Java project.

Example Code

// Java side
import py4j.GatewayServer;

public class JavaApp {
    public String greet(String name) {
        return "Hello, " + name + " from Java!";
    }

    public static void main(String[] args) {
        GatewayServer server = new GatewayServer(new JavaApp());
        server.start();
    }
}

# Python side
from py4j.java_gateway import JavaGateway

gateway = JavaGateway()
java_app = gateway.entry_point
print(java_app.greet("Python"))  # Output: Hello, Python from Java!

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