How do I fix my race condition with my subprocess?

Question

I have a process that creates another process. The first process waits for the second process to print "done" then continues. However, I am getting a race condition where the first process gets stuck sometimes and can't continue after the second process prints. My code is:

First Process (process is created with ProcessBuilder):

if((new BufferedReader(new InputStreamReader(process.getInputStream())).readLine()).contains("Process done"))
        {

        } 
System.out.println("Second process completed.");
process.destroy();

Second Process:

public static void main(String[] args) 
{
    /*does some action*/
    System.out.println("Process done");
    Thread.sleep(Integer.MAX_VALUE);
}

Sometimes this works, but other times the first process will not get the input stream and the program will hang. What is the problem?

Is the problem that the second process prints `"done"` before the first process reads it, or that the first process gets the `inputStream` before it exists? — durron597, Jul 24 '15 at 21:53
I believe it's the latter. I am unfamiliar with how to handle this in Java. The first process is supposed to wait for the string from the second process, though I am unsure of how synchronization should work in this case. — WinterStar, Jul 25 '15 at 01:05

score 0 · Answer 1 · answered Jul 25 '15 at 01:13

I see two options:

Put a Thread.sleep(1000) in front of the print statement to guarantee you've polled the InputStream late enough.
Have the subprocess write to a file (with ProcessBuilder.redirectOutput(File), and then call .readLine() on the file, which will guarantee that you are not late. I would probably delete the temp file before calling ProcessBuilder.start().

Also I would probably not ever call Thread.sleep(Integer.MAX_VALUE) for any reason.

How do I fix my race condition with my subprocess?

1 Answers1