Understanding the need for a DI framework

Question

This might be a naive question. I\'m currently learning the Spring framework and dependency injection. While the basic principle of DI is rather easy to grasp, it\'s

Answer 1

I've had the exact same question, and it was answered by this:
Granted, you could do what you've described in "Then you could simply do:..." (let's call that "class A"). However, that would couple class A to HandSaw, or to all dependencies needed from class SawMill. Why should A be coupled to HandSaw - or, if you take a more realistic scenario, why should my business logic be coupled to the JDBC connection implementation needed for the DAO layer?
The solution I proposed then was "then move the dependencies one step further" - ok, so now I've got my view coupled to JDBC connection, where I should only deal with HTML (or Swing, pick your flavor).

The DI framework, configured by an XML (or JavaConfig) solves this by letting you just "get the needed service". You don't care how it's initialized, what it needs to work - you just get the service object and activate it.

Also, you have a misconception there regarding the "plus:" (where you do SawMill springSawMill = (SawMill)context.getBean("sawMill"); springSawMill.run();) - you don't need to get the sawMill bean from the context - the sawMill bean should've been injected into your object (class A) by the DI framework. so instead of ...getBean(...), you'd just go "sawMill.run()", not caring where it came from, who initialized it and how. For all you care, it could go straight to /dev/null, or test output, or real CnC engine... The point is - you don't care. All you care is your tiny little class A which should do what it's contracted to do - activate a saw mill.