Constructor Injection (Step-by-Step Guide)

What is Constructor Injection?

In Java, when Class A needs Class B to do its work, Class B is called a Dependency. Constructor Injection means passing Class B into Class A through Class A's constructor method.

Why is Constructor Injection Better Than Other Ways?

• 100% Immutable (Safe): Fields can be marked as final so nobody can accidentally overwrite them later.
• Prevents NullPointerExceptions: You can never create an object in an incomplete state.
• Easy Unit Testing: You can easily pass fake (mock) objects into the constructor without starting a slow Spring test server.

Complete Beginner Code Example:

package com.anujsingh.digitalguru.service;

import org.springframework.stereotype.Service;
import com.anujsingh.digitalguru.repository.PaymentRepository;

@Service // Tells Spring to create and manage a single instance of this class
public class OrderService {

    // Step 1: Declare final dependencies
    private final PaymentRepository paymentRepository;

    // Step 2: Pass dependency via Constructor
    // Note: In Spring 4.3+, if a class has 1 constructor, @Autowired is optional!
    public OrderService(PaymentRepository paymentRepository) {
        this.paymentRepository = paymentRepository;
    }

    public void processOrder(double amount) {
        System.out.println("Processing order...");
        paymentRepository.savePayment(amount);
    }
}

Line-by-Line Breakdown:

1. @Service: Tells Spring IoC container: "Hey Spring, please create this bean for me and store it in your container."
2. private final PaymentRepository paymentRepository;: The final keyword guarantees that once set, paymentRepository can never be changed or set to null.
3. public OrderService(PaymentRepository paymentRepository): Spring automatically finds the PaymentRepository bean and passes it here!