@PostConstruct 注释和 spring 生命周期

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");
    }
}

Answer 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

Answer 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).

Answer 3

Yes. Bean creation workflow is:

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