Runtime.getRuntime().exec Hangs

Question

I have a java jar file which inturn calls a java program (command).

Command generated works fine when i run it in command promt.

Process proc = Runtime.getRuntime().exec("cmd.exe /C "+ Command);

        BufferedReader stdIn = new BufferedReader(new 
                     InputStreamReader(process.getInputStream()));

                BufferedReader stdErr = new BufferedReader(new 
                     InputStreamReader(process.getErrorStream()));

            String output=null;
            while((output=stdIn.readLine())!=null)
            {
                System.out.println("output is:"+output);
                out.write(output);
                out.newLine();

            }
            while((output=stdErr.readLine())!=null)
            {
                System.out.println("error output is:"+output);
                out.write(output);
                out.newLine();

            } 
            try {
process.waitFor();
....
....
....

I tried ProcessBuilder:

ProcessBuilder proc = new ProcessBuilder("cmd.exe", "/C", Command);         proc.redirectErrorStream(true);
proc.start();

But this throws error as:

java.io.IOException: Cannot run program "java -Xmx1024M ......"
CreateProcess error=2, The system cannot find the file specified

I can run the same command in promt which works absolutely fine.

Answer 1

With

 new ProcessBuilder("cmd.exe", "/C", Command);

you have used the varargs overload of the constructor. This means that the command is assumed to be already parsed into the arguments. However, you are passing the complete Command as a single argument, which means that cmd will interpret the whole command line java -Xmx... as just the command (file name) to run.

Either stick to the single string, relying on the ProcessBuilder class to parse it, or pre-parse everything.

As for the hanging issue you have, there may be several causes:

• maybe your out stream is blocking;
• maybe the program you are starting writes to stderr , which you don't read at all until it is already too late (the program has ended).

Your second approach would fix this by merging stdout and stderr .