In the primordial days of crusty old J2EE, Spring Framework came along and made it possible to inject JNDI registered services into EJBs. Gee, you could actually begin to design an EJB to where it could be unit tested without having to fire up a J2EE app server - just mock the JNDI services that it collaborated with via Spring dependency injection.
Well, for it's day, that was a not so minor miracle.
These days, if you want to know why Spring continues to rock, check out this book and learn about this development stack, and how Spring is instrumental as the core bean factory mechanism to everything that surrounds it - from BlazeDS services to iBATIS or Hibernate to ActiveMQ messaging beans:
Pro Flex on Spring
And check out this article:
Integrating Flex and Spring based JMS applications
