If you have not looked into Spring yet, it is time. Here is why you should!

Grady Booch once said that the great thing about objects is that they can be replaced. The great thing about Spring is it helps you replace them. With Spring, you simply inject collaborating objects called dependencies using JavaBeans properties and configuration files. Then it's easy enough to switch out collaborating objects when you need to. Spring allows you to dynamically add services to objects called aspects. This is similar to the Decorator Design pattern, but does not require you to recompile your code base to apply these services. This allows you to replace objects with objects that enhance the originals.

The ability to inject collaborating objects is often called IoC (inversion of control). Thus, Spring is an IoC container. If you follow the latest developer buzz then you've likely heard of IoC (Inversion of Control) containers and AOP (aspect-oriented programming). Like many developers, however, you may not see where these technologies fit into your development efforts. As the word inversion implies, IoC is like JNDI turned inside out. Instead of using a tangle of abstract factories, service locators, singletons, and straight construction, each object is constructed with its collaborating objects. Thus, the container manages the collaborators. (Collaborators are objects that an object needs to fullfill its role.)

The ability to dynamically add services to objects is called AOP. AOP allows developers to create non-domain concerns, called crosscutting concerns, and insert them in their application code. With AOP, common services like logging, persistence, transactions, and the like can be factored into aspects and applied to domain objects without complicating the object model of the domain objects.

Thus, Spring is an IoC/AOP container. There are many IoC containers. There are also many AOP frameworks. If Spring was only an IoC/AOP container, it would be worth your attention and interest since it seems to be the most mature. Spring is a lot more than an IoC/AOP container.

What makes Spring different than the other frameworks and containers, is Spring goes beyond just being an IoC container or an AOP framework. The other containers are academic interests, and some are quite good. They provide good support of IoC and AOP.

Spring goes one step further by eating its own dog food. It uses IoC and AOP to provide a comprehensive library for simplifying J2EE development. This comprehensive library is written with aspects, dependency injection and OOP best practices.

Spring makes J2EE development easier. It does this with a variety of mechanisms. One common mechanism is its use of templates. A template is a cross between a utility class and execution environment. Spring Templates are an embellishment and extention of the Template design pattern (GOF).

At first glance templates appear to be well written utility classes. However, templates provide a lot more than just utility functions. Templates provide and execution environment. When using a template you first endeavor to use one of its it utility method. If the template doesn't have a utility method you need, you implement a callback object. The callback object has a method that executes a method in the environment of the template. The template therefore takes care of things like exception handling and resource management in a consistent manner. This means your code base will not be littered with try/catch/finally blocks, and it is easier to ensure that resources and exception are handled correctly.

Spring promotes good programming practices. It does this by providing great examples how to use IoC and AOP in a consistent manner. It also does this by showing how to build things like templates to manage resources and exceptions in the consistent manner. It goes even further by building sets of frameworks on top of  IoC, AOP and templates that are the embodiment of good OO programming.

The IoC capabilities allowing injection of dependent objects turns out to be a great mechanism for testing your code. It is easy now to inject mock objects (object for testing), and test your classes in an isolated manner. For example, you can test you business delegates without relying on the DAO (Data access objects) objects talking to the database. Essentially Spring took back development from the design pattern hacks deemed necessary to program J2EE. As Rod Johnson once put it: Spring puts the OO back in J2EE development.

Spring provides portability through abstraction of common services. For example, Spring provides a common interface object relational management (ORM) systems like Hibernate, JDO, Cayenne, Spring JDBC and iBatis. It provides a mechanism for building DAO objects that divorces the client code from the underlying implementation. it does this by providing a common set of exceptions like object not found exception, and making these exception runtime exceptions.

Spring provides a easy on-ramp for many industry-standard projects. And not just industry-standard projects but the de facto industry-standard projects. The projects that people actually use to get their daily work done. For example, Spring provides support for JDO, Hibernate, Quartz, Tapestry, JavaServer faces, and many more frameworks. Its been said that Spring simplifies J2EE development, and that is a primary focus of Spring. However,Spring provides utilities to work with all tiers of an n-tier application. For an MVC application, there are utilities for working with View technologies (Struts, Spring, Rich Client etc.), Model (EJB, AOP based transaction, AOP based security, etc.), etc. You can use Spring to build Swing and SWT applications.

If you have not looked into Spring yet, it is time.

------------------------------------------------
The above started as a reply to Cameron Purdy's prediction list, which I really liked. Then I got carried away and it became a full blown plug for Spring. (Darn Caffiene!)

What is Spring?

Spring is a popular AOP/IoC framework that was developed by Rod Johnson, Juergen Hoeller et al. Spring simplifies J2EE and Java development. Rod Johnson is the famed author who wrote Expert One-on-One J2EE Design and Development.

Spring makes J2EE development easier. Spring is a J2EE framework that simplifies commons tasks and encourages good design based on programming to interfaces. Springs makes your application easier to configure and reduces the need for many J2EE design patterns (quite a few J2EE design patterns are really glorified hacks that clutter your code base). Spring puts the OO design back into your J2EE application.