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Run python script from java as subprocess

I'm trying to execute python code (live from the console, not just opening a single file).

ProcessBuilder builder = new ProcessBuilder("python");
Process process = builder.start();
new Thread(() -> {
    try {
        process.waitFor();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}).start();
new Thread(() -> {
    String line;
    final BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
    // Ignore line, or do something with it
    while (true) try {
        if ((line = reader.readLine()) == null) break;
        else System.out.println(line);
    } catch (IOException e) {
        e.printStackTrace();
    }
}).start();

final PrintWriter writer = new PrintWriter(new OutputStreamWriter(process.getOutputStream()));
writer.println("1");
writer.println("2 * 2");

I tried this code, but after trying to push the following expressions 1 and 2*2, I don't get a response (evaluation of my expressions).

Does anyone know what the issue is?

Answer*

Your `python` code doesn't appear to print anything, and handling multiple threads to read and write to another process is a tricky topic; luckily, the functionality is built-in. You could do

	ProcessBuilder builder = new ProcessBuilder("/usr/bin/env", "python",
			"-c", "print(2*2); exit()");
	builder.inheritIO();
	try {
		Process process = builder.start();
		process.waitFor();
	} catch (Exception e) {
		e.printStackTrace();
	}

Which outputs

    4

and terminates. For less trivial Python and Java integration, I would strongly suggest you look [here](https://wiki.python.org/moin/IntegratingPythonWithOtherLanguages#Java). As for your existing code, you never `exit()` python and you never `flush()` your `PrintWriter`. And you write on the `main` thread. And you need to pass `-i` to python, or it won't assume stdin is a console. Changing your code to

	ProcessBuilder builder = new ProcessBuilder("/usr/bin/env", "python", "-i");
	Process process = builder.start();

	new Thread(() -> {
		String line;
		final BufferedReader reader = new BufferedReader(
				new InputStreamReader(process.getInputStream()));
		// Ignore line, or do something with it
		while (true)
			try {
				if ((line = reader.readLine()) == null)
					break;
				else
					System.out.println(line);
			} catch (IOException e) {
				e.printStackTrace();
			}
	}).start();
	new Thread(() -> {
		final PrintWriter writer = new PrintWriter(
				new OutputStreamWriter(process.getOutputStream()));
		writer.println("1");
		writer.println("2 * 2");
		writer.println("exit()");
		writer.flush();
	}).start();

Seems to work properly.

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1

I was to have a long running python terminal, that I can at any point run commands on

Tobiq
– Tobiq

2020-02-11 20:18:02 +00:00
Commented Feb 11, 2020 at 20:18
1

If I wanted this solution, I could simply put the code in a python file, and execute that "python mycode.py"

Tobiq
– Tobiq

2020-02-11 20:18:39 +00:00
Commented Feb 11, 2020 at 20:18
1

Then I would suggest you use Jython or one of the other integration packages (rolling your own REPL while trying to integrate with CPython is not my idea of fun).

Elliott Frisch
– Elliott Frisch

2020-02-11 20:20:11 +00:00
Commented Feb 11, 2020 at 20:20
1

I've checked out jython, but its a big dependency, and also doesn't make use of the system python version

Tobiq
– Tobiq

2020-02-11 20:22:05 +00:00
Commented Feb 11, 2020 at 20:22
2

Thanks for the fix. Question: what's wrong with writing on the main thread?

Tobiq
– Tobiq

2020-02-11 21:18:13 +00:00
Commented Feb 11, 2020 at 21:18

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Run python script from java as subprocess

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