I've started a new project using Spring MVC. In doing so, I've had to invert my thinking.
Spring is an inversion of control container, which means that you don't code dependencies of one class directly upon another. Instead you put all of your dependencies into one configuration file and keep your code as loosely coupled as possible. This one configuration file creates a graph of objects, each with references to the others. Since the configuration file specifies the classes and references, the code for one class doesn't need to know the names of other classes.
Why is this important?
The dependency inversion principle tells us that it is better to depend upon something abstract than something concrete. This helps us to change how something is done without breaking the things that need it to be done. If your code depends upon an interface, you can change the thing that implements that interface without changing your code.
There are other reasons which have come to light since Robert Martin published his paper back in 1996. Dependency inversion makes unit testing easier, because you can replace the components that a unit calls with mock objects. Dependency inversion makes distributed computing easier, because you can replace business objects with proxies that call business logic on remote servers. In general, dependency inversion is a good goal.
How does Spring do it?
So if your class depends upon an interface rather than a concrete class, how does it get a reference to an object that implements that interface? It can't use "new" to create an instance, because "new" needs a concrete class name. To invert dependency, you have to move all of your "new"s into one place. That place is an inversion of control (IoC) container.
Spring reads an XML file that contains a bunch of object descriptions. You can think of each of these as a call to "new". Each one specifies a class name, an instance name, and a set of properties. These are write-once properties (what I like to call definitive), and should be initialized and never changed. These properties can include references to other object instances, thus forming a graph.
Spring MVC combines the dependency inversion principle with the model-view-controller pattern to create a pretty compelling web framework. The controllers and URL mappings are all configured through the IoC container. The URL mapper has a reference to each of the controllers, so it knows how to delegate the handling of a request. Because dependency has been inverted, the URL mapper doesn't know about the concrete classes that are the controllers, it only knows about an interface. So you can use the out-of-the-box URL mapper with your own custom controllers.
But it just so happens that Spring has a quaint mechanism for database access. In your XML file, you can configure a data source by providing a driver class name and a connection string to an instance of "org.apache.commons.dbcp.BasicDataSource". Then you can use this data source to execute queries using "org.springframework.jdbc.core.simple.SimpleJdbcTemplate". I wanted to use this technique from my data access layer. However, the XML file that defines the object graph is way up in my web layer. How can I push that object graph, or at least the data source, down through the business logic layer and into the data access layer?
That's when the full realization of dependency inversion hit me. I was thinking about the web layer depending upon the business logic layer, which then depended upon the data access layer. This is not the Spring way. Instead, the web, business logic, and data access layers are all independent. The IoC container depends upon them all. The individual components within these layers only depend upon interfaces.
So the one XML file declares a data source. It then declares a data access component and gives it a reference to the data source. Next comes a business logic component and with a reference to the data access component. Then, the controller with a reference to the business logic component. And finally, the URL mapper comes last with a reference to the controller. As more URLs, controllers, and components are added, this chain widens into a graph.
You can't pick and choose which pieces of your application use dependency inversion. Please don't try. Once you start down the Spring path, all dependencies are inverted. The graph that's defined at the highest layer of your application delves deep into the lowest, and touches all layers in between. Consider dependency inversion for your next project, and think carefully about the consequences.