@PostConstruct annotation and spring lifecycle

Question

I'm new to Spring, I would like to know:

I have a java class annotated with @Component (spring) and inside I have a method annotated with @PostConstruct. The class is then referenced by @Autowired annotated field in another class. Can I assume that the class is only injected after @PostConstruct is called?

@Component
class AuthenticationMetrics {

    private static final MetricRegistry metrics = new MetricRegistry();

    final Counter requestsTotal;

    final Meter guestLogins;

    final Meter kfUserLogins;

    final Timer guestLoginResponseTime;

    final Timer kfLoginResponseTime;

    @PostConstruct
    public void populateMetricsRegistry() {
        metrics.counter("authentication.requests.totals");
    }
}

Solution 1

If you are asking is injection of given class happening after @PostConstruct in that bean is called, then the answer is yes - @PostConstruct is executed before bean is considered as "injectable"

If you are asking if @PostConstruct on given bean is executed after all injections has been done (on the same bean) - then yes - @PostConstruct is executed after injections are commited to given bean. This is the reason it exists. Normally you could put @PostConstruct actions into the constructor. However, when new object is created (constructor is called) injections are not performed yet - so any initialization that depends on injected objects would fail due to NPE. That is why you need @PostConstruct

Solution 2

The handling of annotations such as @PostConstruct, @Resource, @PreDestroy is done via a BeanPostProcessor, in this case the CommonAnnotationBeanPostProcessor. You can see in the following diagram from Spring that these BPP's are handled after Dependency Injection but before Bean Ready For Use (Which means as much as injectable).

Solution 3

Yes. Bean creation workflow is:

1. constructior call
2. @Autowired fields
3. @Autowired setters
4. BeanPostProcessor's postProcessBeforeInitialization(), i.e. @PostConstruct called by CommonAnnotationBeanPostProcessor
5. InitializingBean.afterPropertiesSet()
6. BeanPostProcessor's postProcessAfterInitialization()
7. Bean is ready and can be injected to other bean

Comments

[Daniele]

Hi, yes I'm asking if @PostConstruct is executed before bean is considered as "injectable", that what I wanted to know. Thanks!

[Daniele]

you can remove this part: "If you are asking if @PostConstruct execution means that bean is injected somewhere - then no." I will accept the answer.

[Sebastiaan van den Broek]

So does this mean that there is absolutely no use case for @PostConstruct when doing constructor injection?

[Kalanka]

what will be the next life cycle step after @PostConstruct? i want to append user details from principal to the request header through a custom interceptor..please help

[rogerdpack]

@Kalanka maybe ask a new question for it? And link back to it here.;. :)

[sjakovac]

You're missing init() or other custom initialization method between steps 5 and 6