Help running Java runtime.exec() on multiple threads

Question

In my program, I need to run a external command in a Ubuntu environment (ntpdate) using java. Currently my code looks like this:

    Runtime rt = Runtime.getRuntime();

    byte[] readBuffer = new byte[131072];
    // Exec a process to do the query
    Process p = null;
    try {
        p = rt.exec("ntpdate -q " + ip);
    } catch (Exception ex) {
        ex.printStackTrace();
    }

    if(p!= null){
        try {
            Thread.sleep(1000);
        } catch (Exception e) {
        }

        // Read the input stream, copy it to the file
        InputStream in = p.getInputStream();


        try {
            int count = 0, rc;
            while ((rc = in.read(readBuffer, count, readBuffer.length - count)) != -1) {
                count += rc;
                if (count >= readBuffer.length) {
                    p.destroy();
                    break;
                }
            }
            p.destroy();
            result = processOutput(readBuffer, count);

        } catch (IOException ex) {
            ex.printStackTrace();
        }
        p.destroy();

This code need to be ran simultaneously on multiple threads in order to maximize performance (I need to test a list of 1.000.000 addresses using ntpdate). However, it runs very slowly, barely consuming machine processing. What am I doing wrong? How could I make this more efficient?

The same problem arises when trying to execute "dig" using .exec(), so I doubt it is because of the specific program being called. Is there some restriction in using Runtime.exec() in a multi Threaded environment?

Answer 1

Is Java the most appropriate approach here? Perhaps this would be better in a shell script, which calls ntpdate in the background multiple times? I'm not sure what benefit you're getting from this code snippet by doing this in Java.

What are you doing with the InputStream from the process?

---

A bash script could do this like:

for ip in #...IP list
do
  ntpdate -q $ip > $ip.txt &
done