startup class (extends ServiceMBean) vs load-on-startup servlet

Hi

I am new to jboss and would like to know what are the differences between ServiceMBean and load-on-startup servlet tag in web.xml? Also, I would like to know which one will always get loaded first or they are loaded at the same time? In what situation, I should use MBean and when I should use startup servlet or it doesn't matter?

I need to write a a class/servlet to validate if all the required system properties (e.g -DINSTALL_DIR=blah ) is set. If not, then stop right there. else proceed and start the application.

Thanks in advance

-A

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