Java: Redirecting inner process output when running from command line

Question

I use the following code, for redirecting the output of a process I launch from my Java app:

ProcessBuilder builder = new ProcessBuilder("MyProcess.exe");
builder.redirectOutput(Redirect.INHERIT);
builder.redirectErrorStream(true);

Now, this works fine when I run the code from eclipse - I can see the output in Eclipse's console.
Yet when I create a jar file and run it from a cmd window, e.g. java -jar MyJar.jar, it doesn't print the output of the process. What could be the reason for this?

Kevin Workman · Accepted Answer · 2014-02-11 23:15:22Z

2

I know I'm late in answering, but I came across this question before coming across the answer, and wanted to save anybody else in the same boat some searching.

This is actually a known bug for Windows: https://bugs.openjdk.java.net/browse/JDK-8023130

You can get around it by redirecting the streams yourself:

Process p = pb.start();
BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()));
String line = null;
while ((line = br.readLine()) != null) {
   System.out.println(line);
}

p.waitFor();

br.close();

answered Feb 11, 2014 at 23:15

Kevin Workman

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1 Comment

Alexey Over a year ago

I didn't manage to test it yet, as this is code from my previous job... :-) But it seems indeed like the issue in question. Thanks!

Jk1 · Accepted Answer · 2013-10-31 16:28:25Z

0

It may be, that process is printing an error and exiting for some reason. So, the actual output goes into Err stream and not into the Out stream. Your code redirects Out stream only, so important process error information may be lost. I would suggest to inherit both Out and Err streams using this code:

ProcessBuilder builder = new ProcessBuilder("MyProcess.exe");
builder.inheritIO();

One more reason to redirect both streams is related to the output buffering for child process. If parent process (your java application) is not reading or redirecting standard streams (Out and Err) of the child process, then the latter may be blocked after a while, unable to make any further progress.

It definitely wouldn't hurt to have possible errors in the output anyway.

answered Oct 31, 2013 at 16:28

Jk1

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4 Comments

Alexey Over a year ago

Thanks, I actually forgot to add that I also have the line builder.redirectErrorStream(true);. I also tried replacing those two methods with the one you suggested, but the result is the same.

Alexey Over a year ago

Also, the inner process seems to run without errors (according to what I can see through other metrics, e.g. run time).

Jk1 Over a year ago

Are you able to see System.out.println() output in cmd window? Chanses are System.out itself is redirected somewhere, e.g. in log file.

Alexey Over a year ago

Yes, I am able to see System.out output. I have the following code: Process proc = builder.start(); int result = proc.waitFor(); System.out.println(result); This code prints 0, as in the process exited with code 0.

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