Java: Dependency Injection (DI)

When writing a complex Java application, application classes should be as independent as possible of other Java classes to increase the possibility to reuse these classes and to test them independently of other classes while unit testing. Dependency Injection (or sometime called wiring) helps in gluing these classes together and at the same time keeping them independent.

Consider you have an application which has a text editor component and you want to provide a spell check. Your standard code would look something like this −

public class TextEditor {
   private SpellChecker spellChecker;
   
   public TextEditor() {
      spellChecker = new SpellChecker();
   }
}

What we’ve done here is, create a dependency between the TextEditor and the SpellChecker. In an inversion of control scenario, we would instead do something like this −

public class TextEditor {
   private SpellChecker spellChecker;
   
   @Autowired
   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
}

Here, the TextEditor should not worry about SpellChecker implementation. The SpellChecker will be implemented independently and will be provided to the TextEditor at the time of TextEditor instantiation. This entire procedure is controlled by the Spring Framework.

Here, we have removed total control from the TextEditor and kept it somewhere else (i.e. XML configuration file) and the dependency (i.e. class SpellChecker) is being injected into the class TextEditor through a Class Constructor. Thus the flow of control has been “inverted” by Dependency Injection (DI) because you have effectively delegated dependances to some external system.