Spring autowiring order and @PostConstruct

Question

I have a question about auto-wiring order and @PostConstruct logic in Spring. For example following demo code I have a main Spring Boot class:

@SpringBootApplication
public class Demo1Application {

    @Autowired
    BeanB beanb;

    public static void main(String[] args) {
        SpringApplication.run(Demo1Application.class, args);
    }
}

and 2 @Service Definitions:

@Service
public class BeanB {

    @Autowired
    private BeanA beana ;

    @PostConstruct
    public void init(){
        System.out.println("beanb is called");
    }

    public void printMe(){
        System.out.println("print me is called in Bean B");
    }
}

@Service
public class BeanA {

    @Autowired
    private BeanB b;

    @PostConstruct
    public void init(){
        System.out.println("bean a is called");
        b.printMe();
    }
}

and I have the following output:

bean a is called

print me is called in Bean B

beanb is called

My question is how autowiring takes place step by step like a scenario above?
And how printMe() method of beanb is called without calling its @PostConstruct first?

Answer 1

Below should be possible sequence

1. beanb starts to get autowired
2. During class initialization of Beanb, beana starts to get autowired
3. Once beana gets created the @PostConstruct i.e. init() of beana gets called
4. Inside init(), System.out.println("bean a is called"); gets called
5. Then b.printMe(); gets called causing System.out.println("print me is called in Bean B"); to execute
6. Having the beana completed the @PostConstruct i.e. init() of beanb gets called
7. Then System.out.println("beanb is called"); gets called

Ideally the same can be better observed by a debugger in eclipse.

The Spring reference manual explains how circular dependencies are resolved. The beans are instantiated first, then injected into each other.