Top Answers to Spring Interview Questions
Spring is an open-source development framework for Enterprise Java. The core features of the Spring Framework can be used in developing any Java application, but there are extensions for building web applications on top of the Java EE platform. Spring Framework targets to make Java EE development easier to use and promotes good programming practice by enabling a POJO-based programming model.
- Lightweight: Spring is lightweight when it comes to size and transparency. The basic version of the Spring Framework is around 1 MB. Besides, the processing overhead is also very negligible.
- Inversion of Control (IoC): Loose coupling is achieved in Spring using the technique, inversion of control. The objects give their dependencies instead of creating or looking for dependent objects.
- Aspect-oriented Programming (AOP): Spring supports aspect-oriented programming and enables cohesive development by separating the application business logic from system services.
- Container: Spring contains and manages the life cycle and configuration of application objects.
- MVC Framework: Spring comes with an MVC web application framework built on the core Spring functionality. This framework is highly configurable via strategy interfaces and accommodates multiple view technologies such as JSP, Velocity, Tiles, iText, and POI. However, other frameworks can be easily used instead of Spring MVC Framework.
- JDBC Exception Handling: The JDBC abstraction layer of Spring offers a meaningful exception hierarchy, which simplifies the error-handling strategy.
- Integration: Spring provides the best integration services with Hibernate, JDO, and iBATIS.
- Transaction Management: Spring Framework provides a generic abstraction layer for transaction management. This allows the developer to add the pluggable transaction managers and makes it easy to demarcate transactions without dealing with low-level issues. Spring’s transaction support is not tied to J2EE environments, and it can also be used in container-less environments.
The Spring Framework provides a comprehensive programming and configuration model for modern Java-based enterprise applications—on any kind of deployment platforms. A key element of Spring is infrastructural support at the application level: Spring focuses on the ‘plumbing’ of enterprise applications so that teams can focus on the application-level business logic, without having unnecessary ties to specific deployment environments.
Java-based configuration option enables users to write most of their Spring configuration without XML but with the help of few Java-based annotations.
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.
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. Users can poll their database or they 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. The users can do the required housekeeping work after receiving this event.
- ContextClosedEvent: This event is published when the ApplicationContext is closed using the close() method on the ConfigurableApplicationContext interface. As the 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.
The basic modules of the Spring Framework are:
- Core module
- Bean module
- Context module
- Expression Language module
- JDBC module
- ORM module
- OXM module
- Java Message Service (JMS) module
- Transaction module
- Web module
- Web-Servlet module
- Web-Struts module
- Web-Portlet module
Spring supports two types of dependency injections:
Setter-based DI is realized by calling setter methods on the user’s beans after invoking a no-argument constructor or no-argument static factory method to instantiate their bean.
Constructor-based DI is realized by invoking a constructor with a number of arguments, each representing a collaborator.
Spring IoC creates objects, wires them together, configures them, and manages their complete life cycle from creation to destruction. The Spring container uses dependency injection (DI) to manage the components that make up an application.
The objects that form the backbone of the users’ application and that are managed by the Spring IoC container are called beans. A bean is an object that is instantiated, assembled, and otherwise managed by a Spring IoC container. These beans are created with the configuration metadata that the users supply to the container.
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The Spring Framework supports five scopes, three of which are available only if users use a web-aware ApplicationContext.
- Singleton: This scopes the bean definition to a single instance per Spring IoC container.
- Prototype: This scopes a single bean definition to have any number of object instances.
- Request: This scopes a bean definition to an HTTP request, only valid in the context of a web-aware Spring ApplicationContext.
- Session: This scopes a bean definition to an HTTP session, only valid in the context of a web-aware Spring ApplicationContext.
- Global-session: This scopes a bean definition to a global HTTP session, only valid in the context of a web-aware Spring ApplicationContext.
The following is the sequence of a bean life cycle in Spring:
- Instantiate: First, the Spring container finds the bean’s definition from the XML file and instantiates the bean.
- Populate Properties: Using the dependency injection, Spring populates all of the properties as specified in the bean definition.
- Set Bean Name: If the bean implements the BeanNameAware interface, then Spring passes the bean’s ID to the setBeanName() method.
- Set Bean Factory: If the bean implements the BeanFactoryAware interface, then Spring passes the bean factory to the setBeanFactory() method.
- Pre-initialization: It is also called the post-process of the bean. If there are any BeanPostProcessors associated with the bean, then Spring calls the postProcesserBeforeInitialization() method.
- Initialize Beans: If the bean implements IntializingBean, its afterPropertySet() method is called. If the bean has the init method declaration, the specified initialization method is called.
- Post Initialization: If there are any BeanPostProcessors associated with the bean, their postProcessAfterInitialization() methods will be called.
- Ready to Use: Now, the bean is ready to use by the application.
- Destroy: If the bean implements DisposableBean, it will call the destroy() method.