io - Java reading standard output from an external program using inputstream

Question

I am trying to develop a class that reads the standard output of an external program(using an instance of Process, Runtime.getRuntime().exec(cmdLine, env, dir)). The program takes user inputs during the process, and would not proceed until a valid input is given; this seems to be causing a problem in the way I am trying to read its output:

    egm.execute(); // run external the program with specified arguments
    BufferedInputStream stdout = new BufferedInputStream(egm.getInputStream());
    BufferedInputStream stderr = new BufferedInputStream(egm.getErrorStream());
    BufferedOutputStream stdin = new BufferedOutputStream(egm.getOutputStream());



    int c; //standard output input stream
    int e; //standadr error input stream

    while((c=stdout.read()) != -1) //<-- the Java class stops here, waiting for input? 
    {
        egm.processStdOutStream((char)c); 
    }
    while((e=stderr.read()) != -1)
    {
        egm.processStdErrStream((char)e);
    }
    //...

How can I fix this so that the program takes in a valid input and proceed? Any help resolving this problem will be great!

Answer 1

You have to consume both the program's stdout and stderr concurrently to avoid blocking scenarios.

See this article for more info, and in particular note the StreamGobbler mechanism that captures stdout/err in separate threads. This is essential to prevent blocking and is the source of numerous errors if you don't do it properly!