BeanPostProcessor confusion

Question

I am trying to understand BeanPostProcessor in Spring and I don't understand what it does. Is it correct that the BeanPostProcessor defines two methods that is called at these points:

• Before initialization (either init method or afterPropertiesSet), but the instance is created.
• After the init method or afterPropertiesSet method is called

Is that correct? Given the example and text on page 118 and further is confusing. I don't think I am allowed to copy more from the text to the question but the annotations and whats happening there is hard to understand.

And are you supposed to implement this interface on the beans you want or are you supposed to use this on a bean that is general for many beans? I see that you get both and object and string argument passed in.

Sometimes, you may find yourself in a position where you need to performsome additional processing immediately before and after Spring instantiates the bean. The processing can be as simple as modifying the bean or as complex as returning a completely different object! The BeanPostProcessor interface has two methods: postProcessBeforeInitialization, which is called before Spring calls any bean initialization hooks (such as InitializingBean.afterPropertiesSet or the init-method), and postProcessAfterInitialization, which Spring calls after the initialization hooks succeed.

Pro Spring 2.5, page 118

Answer 1

Spring gives you a lot of post processors, not only BeanPostProcessor. Also, most of them are used by the Spring itself. The one you mentioned in this question, is used (as its name states) to post process bean after its instantiation. Spring container behavior is as follows:

• Spring instantiates bean calling its constructor
  
• postProcessBeforeInitialization(Object bean, String beanName) is called
  
• bean initialization process: @PostConstruct, afterPropertiesSet() (defined by the InitializingBean callback interface), custom configured init method
  
• postProcessAfterInitialization(Object bean, String beanName) is called

At the first sight, it may look complicated and overwhelming, but when you build complex applications on a top of Spring, all these features are just invaluable.

Possible scenarios, are for example (taken from Spring itself):

• AutowiredAnnotationBeanPostProcessor - scans bean looking for @Autowire annotation in order to perform dependency injection
• RequiredAnnotationBeanPostProcessor - checks if all dependencies marked as @Required has been injected.
• ServletContextAwareProcessor - injects ServletContext to beans implementing ServletContextAware interface
• actually, initialization/desctruction callbacks such as JSR-250 @PostConstruct and @PreDestroy are implemented using post processor: CommonAnnotationBeanPostProcessor

Of course all mentioned post processors must be executed in a specific order, but this is Spring responsibility to ensure it.