Dependency Injection: The Fun-sized Version

Dependency Injection: The Fun-sized Version

Injecting humor into dependency injection, let's talk about DAOs (Data Access Objects) or Repositories. Imagine creating a UserDAO class for a Java program to access a users table in your database. To perform SQL queries, it needs a DataSource.

The naive approach: Create a new DataSource every time you need one. But there's a catch - it's expensive and time-consuming. Plus, if you create another DAO, you'll need a DataSource there too.

Enter the global Application class: It stores a DataSource singleton, so your DAOs can share. Great, but not perfect. Your DAOs still need to know where to find the Application class, and as your program grows, so does the Application class.

Introducing Inversion of Control: What if your DAOs didn't have to worry about finding their DataSource? Instead of actively searching, they just shout (somehow) that they need one? Cue Inversion of Control. Your DAO now just announces, "Hey, if you want to use me, give me a DataSource!"

Now your UserDAO looks cleaner and doesn't know about the Application class or how to construct DataSources. The downside? You still need to make sure to call new UserDao(dataSource) when running your application.

But wait, there's more: Dependency Injection Containers! Wouldn't it be magical if someone knew your UserDAO has a DataSource constructor dependency and knew how to construct one? That someone is a dependency injection container - and that's where the Spring framework shines.

And that's dependency injection, all wrapped up in a fun-sized blog post. Enjoy!"