Java – Non-blocking thread that runs an external process

javamultithreadingnonblocking

I have created a Java GUI application which functions as a wrapper for many low level external processes. The utility works as is, but is in desperate need of one major improvement.

I want my external process run in a non-blocking manner which would permit me to service additional requests in parallel. In a nutshell I want to be able to process data from the external process as the data is being generated. But it appears my basic attempt to check and see if the external process is still running is blocking.

Below is an excerpt from my ExternalProcess class. Please see inline comments for specific Java functionality questions about threading and blocking.

public void Execute()
{
    System.out.println("Starting thread ...\n");
    Runner = new Thread(this, "ExternalProcessTest");
    Runner.run();
    System.out.println("Ending thread ...\n");
}

public void run() 
{
    System.out.println("In run method ...\n");  // Debug purposes only. 
        // Show that we are in the run loop.
    try
    {
        // Execute string command SomeCommand as background process ...
        Process = Runtime.getRuntime().exec(SomeCommand);
        while(IsRunning())
        {
            // External process generates file IO.  I want to process these
            // files inside this loop.  For the purpose of this demo I have
            // removed all file processing to eliminate it as the cause
            // of blocking.  THIS ROUTINE STILL BLOCKS!
            Thread.sleep(1000);
        }
    }
    catch(Exception e)
    {
        System.out.println(e);
    }
    System.out.println("Exiting run method ...\n");  // Debug purposes only.
        // Show that we are exiting the run loop.
}

// Process (instantiated from Runtime.getRuntime().execute doesn't supports
// either fire-and-forget backgrounding (non-blocking) or you can wait for 
// the process to finish using the waitFor() method (blocking).  I want to
// be able to execute a non-blocking external process that I monitor via
// threading allowing me to process the external process file IO as it is
// created.  To facilitate this goal, I have created an isRunning() method
// that uses the exitValue() method.  If the process is still running, a 
// call to exitValue() will throw an IllegalThreadStateException exception.
// So I simply catch this execption to test if the background process is
// finished -- at which point I can stop processing file IO from the 
// process.  Is this the source of the blocking?  If so, is there another
// way to do this?
public boolean IsRunning()
{
    boolean isRunning = false;
    try
    {
        int exitVal = Process.exitValue();
    }
    catch(IllegalThreadStateException e)
    {
        isRunning = true;
    }
    return isRunning;
}

Best Solution

The run() method on Thread doesn't actually start a new thread, try using Thread.start() instead.