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How Spring Autowiring Works Internally
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Autowiring is one of Spring’s most powerful features — just add @Autowired, and boom — your dependencies are injected. But have you ever wondered how this works under the hood?
In this article, we’ll peel back the layers and show you how Spring autowiring actually works internally — step by step, with examples, real internals, and best practices.
What Is Autowiring?
Autowiring is Spring’s way of automatically resolving and injecting beans into your classes without you manually instantiating or wiring them.
@Service
public class OrderService {
@Autowired
private PaymentService paymentService;
}Here, Spring will scan for a PaymentService bean and inject it into OrderService. But how does this happen?
Step-by-Step: What Happens Internally?
1. Component Scanning and Bean Definitions
When your app starts, Spring performs component scanning (via @ComponentScan or @SpringBootApplication) and creates BeanDefinition objects for each annotated class (@Component, @Service, etc.).
At this point, OrderService and PaymentService are just definitions — they haven’t been created yet.
2. BeanPostProcessors Are Registered
Before beans are created, Spring registers a number of BeanPostProcessor implementations.
One of them is:
org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessorThis class is responsible for:
- Finding fields, constructors, or methods annotated with
@Autowired - Resolving dependencies
