Container, Dependency, and IOC

• What is dependency injection and what are the advantages?

  The direct relationship between client and service is decoupled in the code. Client doesn't have to know the concrete type of the service it uses. It only needs to know the abstract type, usually an interface the service implements. This enables the configurability of dependency injection. Different service implementations can be changed without modifying code.

• What is an interface and what are the advantages of making use of them in Java?

  interface is an abstract type that is used to specify a behavior that classes must implement.

  • Why are they recommended for Spring beans?

    Any interface can have multiple implementations registered as Spring beans. We can switch them in runtime without changing code.

• What is meant by “application-context?

  ApplicationContext is a Spring IoC container, which contains all the bean definitions. It is an interface with various implementations able to retrieve bean definitons from different sources.

• How are you going to create a new instance of an ApplicationContext?

  Several implementations of the ApplicationContext interface are supplied with Spring. In stand-alone applications, it is common to create an instance of ClassPathXmlApplicationContext or FileSystemXmlApplicationContext. While XML has been the traditional format for defining configuration metadata, you can instruct the container to use Java annotations or code as the metadata format by providing a small amount of XML configuration to declaratively enable support for these additional metadata formats.

• Can you describe the lifecycle of a Spring Bean in an ApplicationContext?

• How are you going to create an ApplicationContext in an integration test?

• What is the preferred way to close an application context? Does Spring Boot do this for you?

• Can you describe:

  • Dependency injection using Java configuration?
  • Dependency injection using annotations (@Autowired)?
  • Component scanning, Stereotypes?
  • Scopes for Spring beans? What is the default scope?

• Are beans lazily or eagerly instantiated by default? How do you alter this behavior?

• What is a property source? How would you use @PropertySource?

• What is a BeanFactoryPostProcessor and what is it used for? When is it invoked?

  • Why would you define a static @Bean method?
  • What is a ProperySourcesPlaceholderConfigurer used for?

• What is a BeanPostProcessor and how is it different to a BeanFactoryPostProcessor? What do they do? When are they called?

  • What is an initialization method and how is it declared on a Spring bean?
  • What is a destroy method, how is it declared and when is it called?
    • Consider how you enable JSR-250 annotations like @PostConstruct and @PreDestroy? When/how will they get called?
    • How else can you define an initialization or destruction method for a Spring bean?

• What does component-scanning do?

• What is the behavior of the annotation @Autowired with regards to field injection, constructor injection and method injection?

• What do you have to do, if you would like to inject something into a private field? How does this impact testing?

• How does the @Qualifier annotation complement the use of @Autowired?

• What is a proxy object and what are the two different types of proxies Spring can create?

  • What are the limitations of these proxies (per type)?
  • What is the power of a proxy object and where are the disadvantages?

• What does the @Bean annotation do?

• What is the default bean id if you only use @Bean? How can you override this?

• Why are you not allowed to annotate a final class with @Configuration

  • How do @Configuration annotated classes support singleton beans?
  • Why can’t @Bean methods be final either?

• How do you configure profiles? What are possible use cases where they might be useful?

• Can you use @Bean together with @Profile?

• Can you use @Component together with @Profile?

• How many profiles can you have?

• How do you inject scalar/literal values into Spring beans?

• What is @Value used for?

• What is Spring Expression Language (SpEL for short)?

• What is the Environment abstraction in Spring?

• Where can properties in the environment come from – there are many sources for properties – check the documentation if not sure. Spring Boot adds even more.

• What can you reference using SpEL?

• What is the difference between $ and # in @Value expressions?