2.32 Autowired Principle

1. Class Inheritance Diagram

2. Principle Analysis

1. When the Spring container refreshes, there are two key steps: registerBeanPostProcessors and finishBeanFactoryInitialization.
2. registerBeanPostProcessors injects the AutowiredAnnotationBeanPostProcessor BeanPostProcessor into the Spring container. It has two key methods:

• postProcessMergedBeanDefinition (from MergedBeanDefinitionPostProcessor)

The purpose of this method is to encapsulate information about fields that need dependency injection into the InjectionMetadata class. InjectionMetadata contains which elements need to be injected and into which target class those elements should be injected.

• postProcessPropertyValues (from InstantiationAwareBeanPostProcessor)

The purpose of this method is to complete dependency-property injection.

3. After finishBeanFactoryInitialization creates the Bean instance, it calls AutowiredAnnotationBeanPostProcessor.postProcessMergedBeanDefinition. Then, when assigning values to properties, it calls AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues. Finally, it obtains the Bean instance to inject through BeanFactory.getBean(beanName, beanClass) and completes property injection through reflection.

3. References

• In-Depth Understanding of Spring Series 14: How @Autowired Works - Juejin
• 1. Create Spring Container