Why should I use Dependency Injection?

In what scenarios is dependency injection helpful?

• I hear a lot about dependency injection and and DI frameworks these days.  I don't work at a place that uses it.  I'm having trouble envisioning it's use.  When does dependency injection make your code clearer?  Is it mostly used for testing?

• Answer:

  I think Dependency Injection is one of those things that was done for one reason initially (decoupling implementations of dependencies from their use) but turned out to be really meaningful in another way (code clarity). This is because it turns out that creating and marshalling collaborators is a cross cutting concern that can be abstracted from the business logic that requires the collaborators. so: public void validateAndSave(Thing thing) { Validator validator = new WebThingValidator(); if (validator.validate(thing)) { DatabaseConnectionManager connectionManager = DatabaseConnectionManagerFactory.instance(); ThingDataAccessObject dao = new ThingDataAccessObject(connectionManager); dao.save(thing); } } where collaborators have to be retrieved or instantiated by the same code that is using them compared with: public void validateAndSave(Thing thing) { if (validator.validate(thing)) { dao.save(thing); } } where dependency injection takes care of supplying the collaborators. You can see that the logic is as complete in the second example, but FAR more readable. Bear in mind that code hasn't exactly been eliminated, simply moved, usually to a config file such as xml (I'm not quite sure whether using annotations counts as eliminating code or not). But the separation of concerns leads to much easier to read business logic. You also have decoupled collaboration. This means that writing pure unit tests is much easier in that your resulting business logic classes can have their collaborators set on them, so it's easy to plug in stubs and mocks for testing. One thing to remember is that, even though you've moved your collaborator wiring out of business logic classes, you still have to test it. The wiring logic can go wrong just as much as other code (and, in my experience, usually does more often). Write integration tests that make sure your wiring is correct.