Runtime exec freezes: exec target is a executable .jar that requires stdin

Question

I am having a problem with the Runtime.getRuntime().exec methodology in the following scenerio:
-- Exec.class is a program that uses a StreamGobbler to display stdout to the screen
-- ReadInput.class is a program that requires the user enter information via the console, using stdin
When I try to do this, the ReadInput.class locks up and freezes, presumably because its trying to use stdin which already has been gobbled up by the Exec.class ???
Here is my code, for example, that I run with the following command (on Windows XP) :
start.bat
@echo off
echo.
javac *.java
echo First, test the Exec.class to see if it works
echo.
java Exec "dir *.java"
echo.
echo Then, try to run a program that wants the stdin (click a key to continue)
echo.
pause>nul
java Exec "java -cp . ReadInput"
echo.
echo (click a key to end)
pause>nul
Exec.java
import java.util.*;
import java.io.*;
class StreamGobbler extends Thread
    InputStream is;
    String type;
    StreamGobbler(InputStream is, String type)
        this.is = is;
        this.type = type;
    public void run()
        try
            InputStreamReader isr = new InputStreamReader(is);
            BufferedReader br = new BufferedReader(isr);
            String line=null;
            while ( (line = br.readLine()) != null)
                System.out.println(type + ">" + line);   
            } catch (IOException ioe)
                ioe.printStackTrace(); 
public class Exec
    public static void main(String args[])
        if (args.length < 1)
            System.out.println("USAGE: java Exec <cmd>");
            System.exit(1);
        try
            String osName = System.getProperty("os.name" );
               System.out.println(osName);
            String[] cmd = new String[3];
            if( osName.equals( "Windows XP" ) )
                cmd[0] = "cmd.exe" ;
                cmd[1] = "/C" ;
                cmd[2] = args[0];
            else if( osName.equals( "Linux" ) )
                cmd[0] = "command.com" ;
                cmd[1] = "/C" ;
                cmd[2] = args[0];
            Runtime rt = Runtime.getRuntime();
            System.out.println("Execing " + cmd[0] + " " + cmd[1] + " " + cmd[2]);
            Process proc = rt.exec(cmd);
            // any error message?
            StreamGobbler errorGobbler = new
                StreamGobbler(proc.getErrorStream(), "ERR");           
            // any output?
            StreamGobbler outputGobbler = new
                StreamGobbler(proc.getInputStream(), "OUT");
            // kick them off
            errorGobbler.start();
            outputGobbler.start();
            // any error???
            int exitVal = proc.waitFor();
            System.out.println("ExitValue: " + exitVal);       
        } catch (Throwable t)
            t.printStackTrace();
public class ReadInput {
   public static void main (String[] args) {
      //  prompt the user to enter their name
      System.out.print("Enter your name: ");
      //  open up standard input
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
      String userName = null;
      //  read the username from the command-line; need to use try/catch with the
      //  readLine() method
      try {
         userName = br.readLine();
      } catch (IOException ioe) {
         System.out.println("IO error trying to read your name!");
         System.exit(1);
      System.out.println("Thanks for the name, " + userName);
}  // end of ReadInput classEdited by: sabre150 on 21-Jul-2011 11:26
Added [ code] tags to make the code readable.

Answer 1

When you call proc.getInputStream(), you are opening an InputStream in your app to read from the process's stdout.
If that process "needs stdin", that is, if it normally runs by a user typing stuff at a terminal after it launches, then you have to call proc.getOutputStream() to get an OutputStream that your app can use to feed the process's stdin. So if you're trying to have your app read from the terminal (your app's stdin/System.in) and push that to the process you're launching, then after reading a line from System.in, your app would need to write that line to the OutputStream that you get from proc.getOutputStream().