Calling Python Functions from Java Using Jython

General Discussions
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1

Is it possible to call Python functions from Java code using Jython, or is Jython only useful for calling Java code from Python? I’m specifically looking to call Python from Java.

2

Absolutely, you can call Python from Java using Jython, provided your Python code doesn’t rely on C extensions that Jython doesn’t support. Jython is a powerful tool for this scenario, as it’s designed to enable seamless interaction between Java and Python.

For example, here’s how you can use Jython to invoke Python functions from Java:

PythonInterpreter interpreter = new PythonInterpreter();  
interpreter.exec("import sys\nsys.path.append('pathToModules')\nimport yourModule");  
// Call a Python function that takes a string and returns a string  
PyObject someFunc = interpreter.get("funcName");  
PyObject result = someFunc.__call__(new PyString("Test!"));  
String realResult = (String) result.__tojava__(String.class);

This way, you can leverage Python scripts directly within your Java application without needing external processes or scripts.

3

That’s a great approach with Jython, but if you’re not constrained by the Jython ecosystem and want to call Python from Java using external scripts, you can achieve this with Java’s Runtime.exec() method. Here’s how:

try {  
    String command = "python yourScript.py arg1 arg2";  
    Process process = Runtime.getRuntime().exec(command);  
    BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));  
    String line;  
    while ((line = reader.readLine()) != null) {  
        System.out.println(line);  
    }  
} catch (IOException e) {  
    e.printStackTrace();  
}

This method directly invokes the Python script as a separate process, giving you the flexibility to work with Python installations independent of Jython. It’s especially useful when you need compatibility with C extensions or non-Pure Python libraries.

4

Great points so far! If you’re exploring external script execution to call Python from Java, an alternative (and often preferred) approach is using ProcessBuilder. It’s a bit more versatile and provides better control over process inputs and outputs.

Here’s how you can use it:

ProcessBuilder pb = new ProcessBuilder("python", "yourScript.py", "arg1", "arg2");  
pb.redirectErrorStream(true);  
Process process = pb.start();  
BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));  
String line;  
while ((line = reader.readLine()) != null) {  
    System.out.println(line);  
}

Compared to Runtime.exec(), ProcessBuilder lets you customize the process environment, manage directories, and streamline error handling. It’s a reliable way to integrate Python scripts with Java applications efficiently.