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?
Below should be possible sequence
beanb
starts to get autowired Beanb
, beana starts to get autowired @PostConstruct
ie init()
of beana gets called init()
, System.out.println("bean a is called");
gets called b.printMe();
gets called causing System.out.println("print me is called in Bean B");
to execute beana
completed the @PostConstruct
ie init()
of beanb
gets called 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.
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");
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