Spring autowiring order and @PostConstruct

问题

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?

回答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.

回答2:

Your Answer is Correct as you shown in Your question.

Now Getting the concept of Notation @Autowired. All @Autowired Objects are initialized and loaded in memory just after class Loading is done.

Now here is your SpringBootApplication

@SpringBootApplication
public class Demo1Application {
    @Autowired
    BeanB beanb;   // You are trying to autowire a Bean class Named BeanB.

Here at above Console Application that you have write try to autowire and inject a object of type BeanB.

Now here is your definition of BeanB

@Service
public class BeanB {

    @Autowired
    private BeanA beana ;

In BeanB class you are trying to inject the Object of Class BeanA which is also defined in your console Project.

So, In Your Demo1Application to inject a Object of Class BeanB there must need to inject a Object of class BeanA. Now BeanA Class Object is Created First.

Now if you see the definition of Your Class BeanA

 @Service
public class BeanA {

    @Autowired
    private BeanB b;

    @PostConstruct   // after Creating bean init() will be execute.
    public void init(){
        System.out.println("bean a is called");
        b.printMe();
    }
}

So, After injecting the Object BeanA method bind with @PostContruct annotation is going to execute.

So, execution flow will be..

System.out.println("bean a is called");
System.out.println("print me is called in Bean B");
System.out.println("beanb is called");

来源：https://stackoverflow.com/questions/38068303/spring-autowiring-order-and-postconstruct