Runtime.exec() hangs with 1.4.1

Question

Hello altogether,
I am trying to execute a command with Runtime.getRuntime.exec()
I have already taken care of capturing the output and I observe that depending of the program I try to execute the process hangs.
I am using JRE 1.4.1_02 under Redhat 7.2 with kernel 2.4.18-26
Executing the same program under JRE 1.3.1 the program does not hang.
Executing 'top -bn0q' hangs, executing 'ls -als' it hangs.
Here is my sample code:
<code>
import java.io.*;
public class Exec
/** catches the output in a parallel thread */
class StreamReader extends Thread
String category = null;
InputStream is;
StreamReader(String category, InputStream is)
this.category = category;
this.is = is;
public void run()
try
System.out.println(this.category+": reader runs");
InputStreamReader isr = new InputStreamReader(this.is);
BufferedReader br = new BufferedReader(isr);
String line = null;
while (br.ready() && (line = br.readLine())!=null)
System.out.println(this.category+':'+line);
catch (Exception e)
e.printStackTrace();
public void run(String[] cmd)
StringBuffer outStrBuf = new StringBuffer();
try
Runtime rt = Runtime.getRuntime();
System.out.println("got runtime");
Process process = rt.exec(cmd);
System.out.println("fired cmd");
// any errors
System.out.println("prepare error stream");
StreamReader errSr = new StreamReader("ERR",process.getErrorStream());
// any output
System.out.println("prepare output stream");
StreamReader outSr = new StreamReader("OUT",process.getInputStream());
// start the readers to read
System.out.println("start readers");
errSr.start();
outSr.start();
System.out.println("waiting for process to end");
process.waitFor(); //Waits for the subprocess to complete.
catch (Exception e)
System.err.println("Error while executing cmd: " + cmd);
e.printStackTrace();
System.out.println(outStrBuf);
     public static void main(String[] args)
String [] cmd = {"top","-bn0q"};
if (args.length >= 1)
cmd = args;
System.out.println(args[0]);
Exec exec = new Exec();
exec.run(cmd);
</code>
The output of java Exec is:
[user]$ java Exec
got runtime
fired cmd
prepare error stream
prepare output stream
start readers
waiting for process to end
OUT: reader runs
ERR: reader runs
...and there it hangs. Interesting is, that when I use ls -als as command, I get the directory listing.
Do you have any ideas what I am doing wrong? Is there any difference in the Runtime.exec() between 1.3 and 1.4 version?

Answer 1

Unbelievable and what a shame. I was hacking 2 days on several variations of this problem and the solution and I finally found one difference:
while (br.ready() && (line = br.readLine())!=null)
I assume that when executing the command, the output streams are not ready and my Output gobbler threads end.
...however the command is still executing and starts to write its output. And as we all know this will overflow the buffer and the process hangs.
So the final solution is:
/** catches the output in a parallel thread */
class StreamReader extends Thread
  String category = null;
  InputStream is;
  StreamReader(String category, InputStream is)
    this.category = category;
    this.is = is;
  public void run()
    try
      System.out.println(this.category+": reader runs");
      InputStreamReader isr = new InputStreamReader(this.is);
      BufferedReader br = new BufferedReader(isr);
      String line = null;
      while (/**br.ready() &&*/ (line = br.readLine())!=null)
        System.out.println(this.category+':'+line);
    catch (Exception e)
      e.printStackTrace();
}So the only question that I have open: Why does this makes no problem with 1.3 but with 1.4 ?