Architectural benefits of Spring

Before we get down to specifics, let's look at some of the benefits Spring can bring to a project:

1. Spring can effectively organize your middle tier objects, whether or not you choose to use EJB. Spring takes care of plumbing that would be left up to you if you use only Struts or other frameworks geared to particular J2EE APIs. And while it is perhaps most valuable in the middle tier, Spring's configuration management services can be used in any architectural layer, in whatever runtime environment. 
2. Spring can eliminate the proliferation of Singletons seen on many projects. In my experience, this is a major problem, reducing testability and object orientation. 
3. Spring can eliminate the need to use a variety of custom properties file formats, by handling configuration in a consistent way throughout applications and projects. Ever wondered what magic property keys or system properties a particular class looks for, and had to read the Javadoc or even source code? With Spring you simply look at the class's JavaBean properties or constructor arguments. The use of Inversion of Control and Dependency Injection helps achieve this simplification. 
4. Spring can facilitate good programming practice by reducing the cost of programming to interfaces, rather than classes, almost to zero. 
5. Spring is designed so that applications built with it depend on as few of its APIs as possible. Most business objects in Spring applications have no dependency on Spring. 
6. Applications built using Spring are very easy to unit test. 
7. Spring can make the use of EJB an implementation choice, rather than the determinant of application architecture. You can choose to implement business interfaces as POJOs or local EJBs without affecting calling code. 
8. Spring helps you solve many problems without using EJB. Spring can provide an alternative to EJB that's appropriate for many applications. For example, Spring can use AOP to deliver declarative transaction management without using an EJB container; even without a JTA implementation, if you only need to work with a single database. 
9. Spring provides a consistent framework for data access, whether using JDBC or an O/R mapping product such as TopLink, Hibernate or a JDO implementation. 
10. Spring provides a consistent, simple programming model in many areas, making it an ideal architectural "glue." You can see this consistency in the Spring approach to JDBC, JMS, JavaMail, JNDI and many other important APIs.