Spring doc explains the BPPs under Customizing beans using BeanPostProcessor. BPP beans are a special kind of beans that get created before any other beans and interact with newly created beans. With this construct, Spring gives you means to hook-up to and customize the lifecycle behavior simply by implementing a BeanPostProcessor
yourself.
Having a custom BPP like
public class CustomBeanPostProcessor implements BeanPostProcessor {
public CustomBeanPostProcessor() {
System.out.println("0. Spring calls constructor");
}
@Override
public Object postProcessBeforeInitialization(Object bean, String beanName)
throws BeansException {
System.out.println(bean.getClass() + " " + beanName);
return bean;
}
@Override
public Object postProcessAfterInitialization(Object bean, String beanName)
throws BeansException {
System.out.println(bean.getClass() + " " + beanName);
return bean;
}
}
would be called and print out the class and bean name for every created bean.
To undersand how the method fit the bean’s lifecycle, and when exactly the method’s get called check the docs
postProcessBeforeInitialization(Object bean, String beanName) Apply
this BeanPostProcessor to the given new bean instance before any bean
initialization callbacks (like InitializingBean’s afterPropertiesSet
or a custom init-method).postProcessAfterInitialization(Object bean, String beanName) Apply
this BeanPostProcessor to the given new bean instance after any bean
initialization callbacks (like InitializingBean’s afterPropertiesSet
or a custom init-method).
The important bit is also that
The bean will already be populated with property values.
For what concerns the relation with the @PostConstruct
note that this annotation is a convenient way of declaring a postProcessAfterInitialization
method, and Spring becomes aware of it when you either by registerCommonAnnotationBeanPostProcessor
or specify the <context:annotation-config />
in bean configuration file. Whether the @PostConstruct
method will execute before or after any other postProcessAfterInitialization
depends on the order
property
You can configure multiple BeanPostProcessor instances, and you can
control the order in which these BeanPostProcessors execute by setting
the order property.