source: javavenv/src/main/java/geniusweb/javavenv/Call.java@ 5

package geniusweb.javavenv;

import java.io.IOException;
import java.io.InputStream;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.TimeUnit;

/**
 * A straight python call. Handles all the connection stuff: connecting to the
 * input and output streams so that we can try to follow what python is doing.
 * NOTE generally this is for internal use, instead use {@link PythonVenv}
 */
public class Call {
        private final List<String> allargs = new LinkedList<>();
        // size 2, we expect both stdin and stderr to always post something here.
        private final BlockingQueue<Object> result = new ArrayBlockingQueue<Object>(
                        2);
        private final int deadlinems;

        /**
         * Call python with given arguments. call {@link #run()} to run. We
         * recommend thatn you use {@link PythonVenv} to run commands instead of
         * this direct call.
         * 
         * @param pythonexe  a working python3 exe, typically
         *                   "/usr/local/bin/python3" or something in a venv on temp
         *                   disk.
         * @param args       the other args for the call
         * @param deadlinems the deadline (ms) after which the call should be
         *                   cancelled.
         */

        public Call(String pythonexe, List<String> args, int deadlinems) {
                if (pythonexe == null || args == null)
                        throw new IllegalArgumentException("args must be not null");
                allargs.add(pythonexe);
                allargs.addAll(args);
                this.deadlinems = deadlinems;
        }

        /**
         * 
         * @return the result of the python call (that is, what the program printed
         *         out)
         * @throws InterruptedException if the call does not terminate in time.
         * @throws IOException          if we can not connect properly to the python
         *                              process
         * @throws PythonError          if the python program printed to stderr, eg
         *                              when an error occured. We try to capture all
         *                              the error lines as text in the pythonerror
         *                              object.
         */
        public String run() throws InterruptedException, IOException, PythonError {
                ProcessBuilder processBuilder = new ProcessBuilder(allargs);
                // System.out.println("all args " + allargs);
                Process p;
                p = processBuilder.start();

                Runnable stdinreader = new Runnable() {
                        @Override
                        public void run() {
                                String totalreply = "";
                                byte[] cbuf = new byte[500];
                                InputStream is = p.getInputStream();
                                try {
                                        while (true) {
                                                int n = is.read(cbuf);
                                                if (n == -1)
                                                        break;
                                                totalreply += new String(cbuf, 0, n, "UTF8");
                                        }
                                        result.add(totalreply);
                                } catch (IOException e) {
                                        result.add(e);
                                }
                        }
                };

                Runnable errorlogger = new Runnable() {
                        @Override
                        public void run() {
                                byte[] cbuf = new byte[500];
                                InputStream is = p.getErrorStream();
                                String totalerror = "";
                                try {
                                        while (true) {
                                                int n = is.read(cbuf);
                                                if (n == -1)
                                                        break;
                                                totalerror += new String(cbuf, 0, n, "UTF8");
                                        }
                                        if (!totalerror.isEmpty())
                                                result.add(new PythonError(totalerror));
                                        else
                                                result.add("");
                                } catch (IOException e) {
                                        result.add(e);
                                }
                        }

                };
                new Thread(stdinreader).start();
                new Thread(errorlogger).start();

                // now we wait till both are finished.
                // wait till both are finished. Order is irrelevant.
                Object res1 = result.poll(deadlinems, TimeUnit.MILLISECONDS);
                if (res1 == null)
                        throw new InterruptedException("Python code failed to return");
                Object res2 = result.poll(100, TimeUnit.MILLISECONDS);
                if (res2 == null)
                        throw new InterruptedException(
                                        "Second python channel failed to close");
                // If there is a non-empty error, return it, otherwise return res
                rethrowIfException(res1);
                rethrowIfException(res2);
                if (!((String) res1).isEmpty())
                        return (String) res1;
                return (String) res2;

        }

        @Override
        public String toString() {
                return "python " + allargs;
        }

        private void rethrowIfException(Object obj)
                        throws IOException, PythonError {
                if (obj instanceof Exception) {
                        throw new PythonError("Failed to execute " + this, (Exception) obj);
                }
        }

}