Re: IOException thrown by process: avoidable?
- From: "John B. Matthews" <nospam@xxxxxxxxxxxxxx>
- Date: Thu, 02 Jul 2009 16:33:11 -0400
In article
<d9e0655e-0eb5-4a9e-90bd-c7291188cdf7@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Ron <bnoronb@xxxxxxxxx> wrote:
On Jun 30, 8:56 pm, "John B. Matthews" <nos...@xxxxxxxxxxxxxx> wrote:
In article
<705241e2-649a-45df-b1e3-d8ad69f3e...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Ron <bnor...@xxxxxxxxx> wrote:
[...]
I think maybe the process finishes, the stream gets closed, my
java program attempts to read the next line of output and gets
the IOException. I think the timing has to be just right though,
because given the same command, I sometimes see the exception and
sometimes not. I also thought that maybe the process is not
outputing an EOF before exiting as a possible cause, but that
doesn't really explain why, for the same command, I sometimes get
the exception, and sometimes do not.
Looking closer at your initial example, I see that you are creating
a new ProcessBuilder and starting it to yield Process p. Then you
start a new Thread from which you read p's stdout and wait for p to
complete. Instead, shouldn't you do something more like this:
import java.io.BufferedReader;
import java.io.InputStreamReader;
public class PBTest {
public static void main(String[] args) {
ProcessBuilder pb = new ProcessBuilder("ls", ".");
pb.redirectErrorStream(true);
try {
Process p = pb.start();
String s;
// read from the process's combined stdout & stderr
BufferedReader stdout = new BufferedReader (
new InputStreamReader(p.getInputStream()));
while ((s = stdout.readLine()) != null) {
System.out.println(s);
}
System.out.println("Exit value: " + p.waitFor());
p.getInputStream().close();
p.getOutputStream().close();
p.getErrorStream().close();
} catch (Exception ex) {
ex.printStackTrace(System.out);
}
}
}
The way I did it is recommended in a number of places, including a
paper on Sun's website, and also Mr. Green's java glossary. Having
dedicated threads to service the standard out, standard error and
standard input lets you do stuff with all three simultaneously, and
you can treat stdout and stderr separately, if desired. The
waitFor() method also gives you the exit value of the process, if
that interests you.
But if you don't need any of that, I don't see why your example
program would not work. But comparing the two, I don't immediately
see a reason why your example and my example would behave differently
with regard to the problem I am seeing.
I was puzzled by the variability of the problem, which makes me wonder
about a threading issue. I see that ProcessBuilder is not synchronized.
In particular, "If multiple threads access a ProcessBuilder instance
concurrently, and at least one of the threads modifies one of the
attributes structurally, it must be synchronized externally." Roedy's
example doesn't appear to modify any process attributes, but your code
might do so.
<http://java.sun.com/j2se/1.5.0/docs/api/java/lang/ProcessBuilder.html>
<http://mindprod.com/jgloss/exec.html>
However, I will try it and see whether I actually do get the same
issue with your example.
I look forward to your results.
--
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>
.
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