Waiting for Sub Process to complete

Question

Hi,
Below mentioned is my requirement:
1. Main process begins
2. Copy a tar file from a source directory to target directory.
3. Call a sub-process to untar the file (using MKS tool kit utility from command prompt).
4. sub-process ends
5. Work on the folders (untared) by applying rules.
6. Main process ends.
When I call sub-process like
main() // main method
// calling sub-process
Process p = Runtime.getRuntime().exec(tar -xvf C:\\Source\\ -C C:\\Target);
p.waitFor();
// calling rules
rule("C:\\Target"); // private method to perform validation on the untared files located in Target folder.
//main method ends
The program gets stuck up here.
The file size to untar is small (1MB). But, after some time, when I kill the main process, I could see sub-process completing its task.
Can you any one suggest me to know the sub-process completion, so that I could call my rules()

Answer 1

I f you read the documentation for exec() and Process carefully you will find that when you an exec() a process, pipes are created back to the main process for the stdout and sterr of the sub-process. Those pipes have a finite size and when full the sub-process will block, hence waitFor() will wait forever.
You need to create threads to drain stdout/stderr of the sub-process, or else re-direct the subprocess output (start a shell to run the actual command) so that the pipes won't get full and block.