Can't execute a Java App from PHP

I'm trying to run a Java App from PHP.

system('java -jar /home/dave/web/drupal/java/dist/emp.jar'); fails. nothing prints out, it just returns FALSE.

However, this works:
system('java -jar');

Prints out the help info normally associated with incorrect parameters passed to Java.

This also works:
system("ls -l /home/dave/web/drupal/java/dist/emp.jar");

Prints out a listing of /home/dave/web/drupal/java/dist/emp.jar including permissions.

This has got me foxed. Can anyone suggest anything to try?

The Jar was built using NetBeans and it runs fine from the commend line.

I have posted this on Daniweb.com, too, but nothing from there just yet...