Using a thread to capture process output

Question

I am using a thread to capture stream output from a process, and then outputting that stream to the eclipse console. The question I have is when to terminate the thread that is doing the stream output.

Thread t = new Thread(new Runnable(){
  private boolean isProcessDone(Process p)
  {
   //not sure what to do here
  }
  public void run()
  {
   Process p = Runtime.getRuntime().exec("executable with output");
   BufferedReader input = new BufferedReader(new InputStreamReader(p.getInputStream()));
   BufferedReader error = new BufferedReader (new InputStreamReader(p.getErrorStream()));
   while ( !isProcessDone(p) ) {
    String line;
    if( (line = input.readLine()) != null )
    {
     System.out.println(line);
    }
                if( (line = error.readLine()) != null )
    {
     System.out.println(line);
    }
   }
   input.close(); 
            error.close();   
  }
 });
 t.start();

My question is what belongs in the isProcessDone() function. The example I am basing this off of uses the stream's ready() function, but I am unclear whether this will work for programs that either std::err and std::out, but not both. I also tried using

try{
 p.exitValue();
 return true;
}catch(IllegalThreadStateException e){}
return false;

but then the thread finishes before the while loop has a chance to act on the streams, and the output is lost.

Answer 1

You will need to have two threads. One to handle the I/O and another to wait for process completion (Process.waitFor()) and set a flag telling the I/O thread to quit when it exhausts the data.

Answer 2

You need to use Process.waitFor() to wait for process completion.

Additionally, you need to consume stdout and stderr concurrently in order to avoid blocking and a possible process hang. Consequently you need two threads to read these streams, and to continue reading whilst the streams are available.

See this Javaworld article for more info and a StreamGobbler implementation to consume the stdout/err.

Answer 3

You need to process reading the output in a separate thread, theres an example here