Spring is a framework for developing applications. Spring simplifies configuration and provides a multitude of functionality out of the box and with very little coding (inversion of control, aspect oriented programming, data access abstraction, jdbc simplification, declarative transaction management, simplified jndi, jta, rmi, jms, jmx, security, testing etc) You will need to have a look at the spring resources on the web of which there are many. There are also some excellent books and courses (not a plug but I have attended one by Interface 21 and found it very good)
Put simply - most task are done in a few lines of code and a few lines of configuration. It makes programming a lot faster and the code is much easier to test, maintain and enhance.
Spring doesn't care about the environment. You can use Spring in any application server or any java application. Glassfish is a *great* app server and works brilliantly with Spring.
Hope this helps.
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