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Spring Interview Questions and Answers For Freshers Part-4

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spring31.How do you turn on annotation wiring?
Annotation wiring is not turned on in the Spring container by default. So, before we can use annotation-based wiring, we will need to enable it in our Spring configuration file by configuring <context:annotation-config/>.

32.What does @Required annotation mean?
This annotation simply indicates that the affected bean property must be populated at configuration time, through an explicit property value in a bean definition or through autowiring. The container throws BeanInitializationException if the affected bean property has not been populated.

33.What does @Autowired annotation mean?
This annotation provides more fine-grained control over where and how autowiring should be accomplished. The @Autowired annotation can be used to autowire bean on the setter method just like @Required annotation, constructor, a property or methods with arbitrary names and/or multiple arguments.

34.What does @Qualifier annotation mean?
There may be a situation when you create more than one bean of the same type and want to wire only one of them with a property, in such case you can use @Qualifier annotation along with @Autowired to remove the confusion by specifying which exact bean will be wired.

35.What are the JSR-250 Annotations? Explain them.
Spring has JSR-250 based annotations which include @PostConstruct, @PreDestroy and @Resource annotations.

@PostConstruct: This annotation can be used as an alternate of initialization callback.

@PreDestroy: This annotation can be used as an alternate of destruction callback.

@Resource : This annotation can be used on fields or setter methods. The @Resource annotation takes a ‘name’ attribute which will be interpreted as the bean name to be injected. You can say, it follows by-name autowiring semantics.

36.What is Spring Java Based Configuration? Give some annotation example.
Java based configuration option enables you to write most of your Spring configuration without XML but with the help of few Java-based annotations.

For example: Annotation @Configuration indicates that the class can be used by the Spring IoC container as a source of bean definitions. The @Bean annotation tells Spring that a method annotated with @Bean will return an object that should be registered as a bean in the Spring application context.

37.How is event handling done in Spring?
Event handling in the ApplicationContext is provided through the ApplicationEvent class and ApplicationListener interface. So if a bean implements the ApplicationListener, then every time an ApplicationEvent gets published to the ApplicationContext, that bean is notified.

38.Describe some of the standard Spring events.
Spring provides the following standard events:

ContextRefreshedEvent: This event is published when the ApplicationContext is either initialized or refreshed. This can also be raised using the refresh() method on the ConfigurableApplicationContext interface.

ContextStartedEvent: This event is published when the ApplicationContext is started using the start() method on the ConfigurableApplicationContext interface. You can poll your database or you can re/start any stopped application after receiving this event.

ContextStoppedEvent: This event is published when the ApplicationContext is stopped using the stop() method on the ConfigurableApplicationContext interface. You can do required housekeep work after receiving this event.

ContextClosedEvent: This event is published when the ApplicationContext is closed using the close() method on the ConfigurableApplicationContext interface. A closed context reaches its end of life; it cannot be refreshed or restarted.

RequestHandledEvent: This is a web-specific event telling all beans that an HTTP request has been serviced.

39.What is Aspect?
A module which has a set of APIs providing cross-cutting requirements. For example, a logging module would be called AOP aspect for logging. An application can have any number of aspects depending on the requirement. In Spring AOP, aspects are implemented using regular classes (the schema-based approach) or regular classes annotated with the @Aspect annotation (@AspectJ style).

40.What is the difference between concern and cross-cutting concern in Spring AOP?
Concern: Concern is behavior which we want to have in a module of an application. Concern may be defined as a functionality we want to implement. Issues in which we are interested define our concerns.

Cross-cutting concern: It’s a concern which is applicable throughout the application and it affects the entire application. e.g. logging , security and data transfer are the concerns which are needed in almost every module of an application, hence are cross-cutting concerns.

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