Executed program hangs (probably due too much output)

Question

I'm trying to run a program ignoring its output, but it seems to hangs when its output is large. My code is as follows:

Process p = Runtime.getRuntime().exec("program");
p.getOutputStream().write(input.getBytes());
p.getOutputStream().flush();
p.getOutputStream().close();
p.waitFor();

What is the best way to ignore the output?

I tried to redirect the output to /dev/null, but I got a Java IOException 'Broke pipe'.

I'd try checking if the error stream of the Process p is empty. — Bruno Reis, Jun 24 '11 at 17:58

Zéychin · Answer 1 · 2011-06-24T18:20:34.050

1

Did you try:

Process p = Runtime.getRuntime().exec("program >/dev/null 2>&1");

?

I remember having to do something similar in Java before, but I may not have been calling the process the same way.

Edit: I just tested this code and it successfully completes.

class a
{
    public static void main(String[] args) throws Exception
    {
        Process p = Runtime.getRuntime().exec("cat a.java >/dev/null 2>&1");
        p.getOutputStream().write(123123);
        p.getOutputStream().flush();
        p.getOutputStream().close();
        p.waitFor();
    }
}

edited Jun 24 '11 at 18:20

answered Jun 24 '11 at 18:02

Zéychin

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I get `java.io.IOException: Broken pipe` when I send the input. – Alexandru Jun 24 '11 at 18:05
Try "cat >/dev/null 2>&1". I haven't tested myself your code, but the difference is that `cat` doesn't read anything. – Alexandru Jun 24 '11 at 18:38
The 'cat' reads from a file and outputs it to the console. Do you mean that it doesn't read user input? – Zéychin Jun 24 '11 at 18:39

score 0 · Answer 2 · edited May 23 '17 at 11:55

0

You cannot "ignore" the output of a child process in Java, well at least technically. You ought to read the contents of the input stream, for not doing so will result in the output buffer on the other end filling up, resulting in the described hanging behavior. When the child process attempts to write to a full output buffer, it will block until the buffer has been cleared by reading it.

If you do not want the output, at least read it and discard it. ~~Or use a NullInputStream. You do not have to use Apache Commons class; you can build your own NullInputStream class whose read methods have empty bodies, similar to NullOutputStreams.~~

Also, this problem might not be solved by reading the input stream alone. You would have to read the error stream as well, which may be redirect to the input stream.

edited May 23 '17 at 11:55

Community

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answered Jun 24 '11 at 18:00

Vineet Reynolds

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There is `.skip(n)` function just for that, but I haven't yet figured out its correct usage. – Alexandru Jun 24 '11 at 18:01
@Alexandru, you can call `skip`, but it must be called as frequently as the child process writes to the output stream (preferably do this in a loop, in a separate thread). – Vineet Reynolds Jun 24 '11 at 18:14

score 0 · Answer 3 · answered Jun 24 '11 at 18:01

0

I'd use a Stream Gobbler. For more on that, please look here: What to do when Runtime.exec() won't

answered Jun 24 '11 at 18:01

Hovercraft Full Of Eels

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Executed program hangs (probably due too much output)

3 Answers3