Part 1 - Building a Mini Spring Framework (v0.1)-Dependency Injection Basics

Wed, 10 Dec 2025 23:14:46 +0100

The problem Spring solves

easy right ? let’s see :)

imagine you are building an e-commerce backend, and you have OrderService that needs to process payments

the naive approach is: inside the OrderService you write for example

PaymentProcess process = new PaypalService();

if your boss comes and says “we’re switching the contract from PayPal to Stripe” what do you have to do to your code?

The answer is use an interface

public interface PaymentProcess {
	void pay(double amount);
}

and the OrderService looks like