What is the order of Spring Boot bean creation and dependency injection?

Question

I was working on a project on Spring Boot 1.5. It had the following type of configuration class:

@Configurtion
public class Foo{

 @Autowired
 private DependencyA dependencyA;

 @Bean
 public DependencyA getDependency(){
   return new DependencyAImpl();
  }

}

This worked okay in Spring Boot 1.5, but when I upgraded to Spring Boot 2, this no longer worked, the application would not start up with the exception 'No bean of type DependencyA found'. I figured that this might be because the bean wasn't created when Spring tried to inject the dependency, and so, as a 'hack', added @Lazy to the dependencyA injection. This worked.

In light of this, what is the order of execution of a configuration class. Is it: A) First create beans and then inject dependencies B) Try to create instance with all dependencies, and then create any beans in configuration.

Answer 1

Interesting finding;

A configuration class is also a Spring Bean.

Normally Configuration class gets scanned and instantiated first. This has to be the starting point to know about other configurations and beans.

However you have added @Autowire to do Field Injection. As I said, a configuration class is a Spring Bean too. Hence spring needs to resolve its dependencies first. And got into dead lock.

To answer your question. Configuration Class gets Instantiated First, before other Beans.