@PostConstruct 和 @PreDestroy 注解的实时应用

Question

And also in a spring-boot application, we don't shutdown context anywhere so how web application manages this shutdown context and destroying beans working.

When actually this web application will trigger the shutdown of context and trigger @PreDestroy annotation.

Answer 1

It depends on the scope of the bean .

For the request scoped bean , it is called when finish processing an HTTP request.

For the session scoped bean, it is called when the session is destroyed (eg timeout due to inactivity).

For the prototype bean, it will not be called and it is the developer responsibility to call the destruction logic. (Mentioned in the document in this section ).

For the singleton bean, it is called when you terminate the application normally (ie.by sending a SIGTERM signal such as kill 12345 where 12345 is your application process ID). Please note that it is not called if you "hard kill" the application using kill -9 12345

Answer 2

@PostConstruct is very helpful if you wanted to do anything after all your beans are initialized. I had the following use case where I had used @PostConstruct

I had 11 implementations of customerProcess class and after each operation, I had to call a different process so we needed to autowire all its implementation instead we used postconstruct to get an enum map to which we supplied the class name and we got the corresponding bean, So Post Construct worked here for me

 @PostConstruct
  private void init() {
    CustomerProcessTask.getClassList().forEach( aClass -> {
      final CustomerTask customerTypeBean = applicationContext.getBean(aClass);
      factory.put(aClass,customerTypeBean);
    });
  }

@PreDestroy has various use cases where you use it when you need to shutdown open db connection or rollback something before applicationContext.close is called. But in the case of web application, dispatcherServlet creates applicationContext and it closes the context whenever server stops. We don't need to explicitly invoke applicationContext.close().

@PreDestroy()
public void dataSourceDestroy() throws SQLException {
    rollbackUnCommittedTransaction();
    }
}