Spring DI with different styles

Spring as a dependency injection (DI) framework allows us to define our application components' dependency tree in three different ways:

• XML
• Annotation
• Java Config

I've written a simple app, bookstore, with the three styles and they're available in the following repository. You can take a look and see how each style would look like. It also has a version that uses no Spring beans for comparison.

https://github.com/ryu1kn/spring--working-with-beans

Different styles have different pros/cons. Spring lets you mix the styles; so you can change it as you go.

Here is my observations on each style.

XML-based configuration

Pros

• Weak/loose coupling with Spring framework in your app code.
  • Good for keeping the option of removing Spring later
• Class dependency information is centralised.
• Fine-grained control on the dependency definition.
• Changing only the dependency information doesn't require the recompilation of your app code.

Cons

• Unless you have a good IDE support (assisting by looking both XML and Java code), development feedback cycle is slow as you may not notice. an error until you run the app.
• XML can be harder to read for its verbosity.

Annotation-based configuration

Pros

• Less boilerplate code to define components dependency.
• Dependency rules are written on each component.
• Adding a bean is as easy as adding @Component (as long as it's in the scan range)
  • Useful if you don't have ownership for the entire code base?

Cons

• Tighter coupling with Spring framework. Your app becomes a Spring app.
• Class dependency information is decentralised.
• Harder to comprehend if the reader is not familiar with Spring.
• Finer grained control over the dependency definition can become tricky and brittle.
• Can accidentally break the app by your component implicitly picked up (like component in test)

Java Config based configuration

Pros

• Weak/loose coupling with Spring framework in your app code.
• Bean can be defined without a concrete class.
• Bean definition can get good editor support thanks to static typing.
• Class dependency information is centralised.
• Fine-grained control on the dependency definition.

Cons

• Compared to XML-based configuration, test setup may be more coupled with Spring.