java ProcessBuilder run shell script is hanging

Question

I'm trying to run shell script by using ProcessBuilder. The script works but it can't run after the java code. And error stream doesn't output message. I'm running it on centOS 6.9 computer. Please find below my code.

public static ArrayList<String> runCommand(ArrayList<String> command)throws IOException {

        ProcessBuilder processBuilder = new ProcessBuilder();
        processBuilder.command(command);
        Process process = processBuilder.start();
        BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
        BufferedReader errorReader = new BufferedReader(new InputStreamReader(process.getErrorStream()));

        ArrayList<String> commandOutput = new ArrayList<>();
        String str;
        while((str = reader.readLine()) != null) {
            commandOutput.add(str);
        }

        StringBuilder errorBuilder = new StringBuilder();
        while((str = errorReader.readLine()) != null) {
            errorBuilder.append(str);
        }

        String errorMessage = errorBuilder.toString();
        if(!errorMessage.equals("")) {
            String message = LOG_TAG + ",[runCommand] error:" + errorMessage;
            System.out.println(message);
        }

        reader.close();
        errorReader.close();
        process.destroy();

        return commandOutput;
    }

Answer 1

In your case, you are reading something from the output stream of the process, till you consume everything. Then, you try to read error stream.

If the process writes some considerable number of characters on the error stream, the other process will block till they are consumed. To consume both error stream and output stream at the same time, you need to use threads.

You may follow the StreamGobbler technique. You may get some details from that page: https://www.javaworld.com/article/2071275/when-runtime-exec---won-t.html?page=2

This is some code influenced from the page:

public class StreamGobbler extends Thread {
    private static final String EOL = System.lineSeparator();
    private final InputStream inputStream;
    private final StringBuilder output = new StringBuilder();

    public StreamGobbler(InputStream inputStream) {
        this.inputStream = inputStream;
    }

    public void run() {
        try (InputStreamReader inputStreamReader = new InputStreamReader(inputStream);
             BufferedReader reader = new BufferedReader(inputStreamReader);
        ) {
            String line;
            while ((line = reader.readLine()) != null) {
                output.append(line);
                output.append(EOL);
            }
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }
    }

    public String getOutput() {
        return output.toString();
    }
}

In your code, you use StreamGobbler like this:

    StreamGobbler outputGobbler = new StreamGobbler(process.getInputStream());
    StreamGobbler errorGobbler = new StreamGobbler(process.getErrorStream());

    process.waitFor();

    String commandOutput = outputGobbler.getOutput();
    String errorMessage = errorGobbler.getOutput();
    process.destroy();