accessing files created by java runtime process

Question

I'm running a windows program from within java:

            String command = "cmd /C start "+fileName+".bat";
    Runtime rt = Runtime.getRuntime();
    Process pr = rt.exec(command, null, new File(currWD));
    int exitValue = pr.waitFor();

The program completes successfully (exitValue == 0) and creates a file "fileName" in the working directory. I am trying in the same routine to find the size of this file:

    xmlFileSize = (new File(fileName)).length(); 

Java finds the file yet it appear to be empty (xmlFileSize == 0). Once Java finishes I can see, however, that the file is non-empty.

How can I resolve this? All I want is that Java can correctly assesses the size of the file created by the windows program that Java has executed.

Answer 1

A zero-length file indicates that the file may not exist. From the docs:

The length, in bytes, of the file denoted by this abstract pathname, or 0L if the file does not exist.

Note that you use currWD as working directory for your bat-file. You could try to do:

new File(currWD, fileName).length()

to make sure you look for the file in the right directory.

Answer 2

It probably has to do with executing the bat file from a command shell. What does the bat file do? Is it launching a program?

I'm guessing that the script calls or executes another program and returns which allows the shell to die. This in turn let's the java process continue while the process from the script continues executing asynchronously. According to the Java API for Process, that's allowable which it most definitely should be (link java.lang.Process)

Answer 3

I credit this answer to aioobe and John. As John suggests, the external program started by the batch file spawns a process that seems to be running for a while (50-300 millisec) after the Java sub-process running the batch file has returned. I resolved the problem by introducing a pause (as suggested by aioobe) :

int exitValue = pr.waitFor();

try {Thread.currentThread().sleep(300);} catch (InterruptedException e) {e.printStackTrace();}

After the pause Java seems to be able to see the files created by the external program. Thanks again to both contributors who helped me resolve this issue!

If anyone finds a more elegant solution, please, feel welcome to post.