Reading streams from java Runtime.exec

Question

I have the following snippet of code: Process proc = runtime.exec(command); errorGobbler = new ErrorStreamGobbler(proc.getErrorStream(), logErrors, mdcMap); outputGobbler = new OutputStreamGobbler(proc.getInputStream(), mdcMap); executor.execute(errorGobbler); executor.execute(outputGobbler); processExitCode = proc.waitFor();

where the gobblers are Runnables which use a BufferedReader to read the input and error streams of the executing process. While this works most of the time, I get the occasional window (of about 2 minutes or so) where I get the processExitCode as 0, which indicates normal termination but there is nothing in the input and error streams - nothing to even indicate end-of-stream.

Like I indicated before, this works most of the time but this failure occurs every once in a while - and I am totally puzzled. Any ideas?

Rags

Answer 1

Hi,

I've struggled with the same kind of issues. I can't remember what exactly was wrong (maybe I forgot to flush / close the streams correctly or something ...). Anyway, here is what I came up with.

/**
 *  Handle communication with a process, reading its output/error and feeding its input
 *  @param process The process to execute
 *  @param _in Reader that will feed the input pipe of the process
 *  @param out Writer that will receive the output of the process
 *  @param err Writer that will receive the error pipe of the process
 */
public static void communicate(
        Process process,
        final Reader _in,
        final Writer out,
        final Writer err)
{
    // Buffer the input reader
    final BufferedReader in = new BufferedReader(_in);

    // Final versions of the the params, to be used within the threads
    final BufferedReader stdOut = new BufferedReader(new InputStreamReader(process.getInputStream()));
    final BufferedReader stdErr = new BufferedReader(new InputStreamReader(process.getErrorStream()));
    final BufferedWriter stdIn = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()));

    // Thread that reads std out and feeds the writer given in input
    new Thread() {
        @Override public void run() {
            String line;
            try {
                while ((line = stdOut.readLine()) != null) {
                   out.write(line + newline);
                }
            } catch (Exception e) {throw new Error(e);}
            try {
                out.flush();
                out.close();
            } catch (IOException e) { /* Who cares ?*/ }
        }
    }.start(); // Starts now

    // Thread that reads std err and feeds the writer given in input
    new Thread() {
        @Override public void run() {
            String line;
            try {
                while ((line = stdErr.readLine()) != null) {
                    err.write(line + newline);
                }
            } catch (Exception e) {throw new Error(e);}
            try {
                err.flush();
                err.close();
            } catch (IOException e) { /* Who cares ?*/ }
        }
    }.start(); // Starts now

    // Thread that reads the std in given in input and that feeds the input of the process
    new Thread() {
        @Override public void run() {
            String line;
            try {
                while ((line = in.readLine()) != null) {
                    stdIn.write(line + newline);
                }
            } catch (Exception e) {throw new Error(e);}

            try {
                stdIn.flush();
                stdIn.close();
            } catch (IOException e) { /* Who cares ?*/ }
        }
    }.start(); // Starts now

    // Wait until the end of the process
    try {
         process.waitFor();
    } catch (Exception e) {
        throw new Error(e);
    }

} // End of #communicate

I hope this helps.