Running python in Java

[Glenda Karen]

I have some code written in python that use libraries such as pandas, numpy, sci-kit learn.
I'm wondering if there is a way to run this code in Java?

I looked into using Jython but those libraries can't be used in it as they are also written in C.

Would ProcessBuilder or Appache Commons Exec allow me to run the files? I found an article using these but it was for a very simple python program.

Any ideas would be great, thanks!

[Zachary Griggs]

Interfacing Java with other stuff is always a pain.

The first thing I would try is running a simple Shell command from Java in order to run a command like "python myScript.py".
I've written this code in the past to run a shell command in java. I've used this to run C++ apps from a Java program.

    public static String executeCommand(String command) throws IOException {
        Runtime rt = Runtime.getRuntime();
        System.out.println("Sending command: " + command);
        Process process = rt.exec(command);

        BufferedReader stdInput = new BufferedReader(new
                InputStreamReader(process.getInputStream()));

        BufferedReader stdError = new BufferedReader(new
                InputStreamReader(process.getErrorStream()));

        StringBuilder result = new StringBuilder();

        addStreamToStringBuilder(stdInput, result);
        addStreamToStringBuilder(stdError, result);

        return result.toString();
    }

    private static void addStreamToStringBuilder(BufferedReader stream, StringBuilder result) throws IOException {
        String s = null;
        while ((s = stream.readLine()) != null) {
            result.append(s).append("\n");
        }
    }

[Rob Spoor]

Although that code tries to read from both the output stream and the error stream, it's still prone to deadlock. If the process outputs a lot to its error stream, and the error stream buffers become full, your code is still trying to read from the output stream. However, there's nothing coming to that because the process is blocked on its error stream, which you're not consuming yet. It also won't end because the process doesn't end.

There are two solutions:
1) Do the reads in two threads, one for the output and one for the input. This is what When Runtime.exec() won't teaches us. Be careful about interleaving the two though.
2) Use ProcessBuilder and call redirectErrorStream(true). You'll then only need to read from the process' output stream.

I prefer the latter, because it lets the JVM and/or OS deal with merging the two streams.