Spring Q&A

1.  What is IOC (or Dependency Injection)?

The basic concept of the Inversion of Control pattern (also known as dependency injection) is that you do not create your objects but describe how they should be created. You don't directly connect your components and services together in code but describe which services are needed by which components in a configuration file. A container (in the case of the Spring framework, the IOC container) is then responsible for hooking it all up.

2. What are the different types of IOC (dependency injection) ?

There are three types of dependency injection:

• Constructor Injection (e.g. Pico container, Spring etc): Dependencies are provided as constructor parameters.
• Setter Injection (e.g. Spring): Dependencies are assigned through JavaBeans properties (ex: setter methods).
• Interface Injection (e.g. Avalon): Injection is done through an interface.
  Note: Spring supports only Constructor and Setter Injection

3. What are features of Spring ?

• Lightweight:
• Inversion of control (IOC):  
  Loose coupling.
• Aspect oriented (AOP): 
  Separating application business logic from system services.
• Container:
  Spring contains and manages the life cycle and configuration of application objects.
• MVC Framework:
  Spring comes with MVC web application framework, built on core Spring functionality. This framework is highly configurable via strategy interfaces, and accommodates multiple view technologies like JSP, Velocity, Tiles, iText, and POI.
• Transaction Management:
  Allowing the developer to add the pluggable transaction managers, and making it easy to demarcate transactions without dealing with low-level issues. Spring's transaction support is not tied to J2EE environments and it can be also used in container less environments.
• Spring provides best Integration services with Hibernate, JDO and iBATIS.

4. What is Bean Factory ?

A BeanFactory is like a factory class that contains a collection of beans. The BeanFactory holds Bean Definitions of multiple beans within itself and then instantiates the bean whenever asked for by clients.

• BeanFactory is able to create associations between collaborating objects as they are instantiated. This removes the burden of configuration from bean itself and the beans client.
• BeanFactory also takes part in the life cycle of a bean, making calls to custom initialization and destruction methods.

5. What is Application Context?

A bean factory is fine to simple applications, but to take advantage of the full power of the Spring framework, you may want to move up to Springs more advanced container, the application context. On the surface, an application context is same as a bean factory.Both load bean definitions, wire beans together, and dispense beans upon request. But it also provides:

• A means for resolving text messages, including support for internationalization.
• A generic way to load file resources.
• Events to beans that are registered as listeners.

6. What are the common implementations of the Application Context ?

   The three commonly used implementation of 'Application Context' are

• ClassPathXmlApplicationContext : It Loads context definition from an XML file located in the classpath, treating context definitions as classpath resources. The application context is loaded from the application's classpath by using the code .
  ApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");
• FileSystemXmlApplicationContext : It loads context definition from an XML file in the filesystem. The application context is loaded from the file system by using the code .
  ApplicationContext context = new FileSystemXmlApplicationContext("bean.xml");
• XmlWebApplicationContext : It loads context definition from an XML file contained within a web application. 

ApplicationContext.	BeanFactory
Here we can have more than one config files possible	In this only one config file or .xml file
Application contexts can publish events to beans that are registered as listeners	Doesn’t support.
Support internationalization (I18N) messages	It’s not
Support application life-cycle events, and validation.	Doesn’t support.
Support  many enterprise services such JNDI access, EJB integration, remoting	Doesn’t support.

7.  What is the typical Bean life cycle in Spring Bean Factory Container ?

   Bean life cycle in Spring Bean Factory Container is as follows:

• The spring container finds the bean’s definition from the XML file and instantiates the bean.
• Using the dependency injection, spring populates all of the properties as specified in the bean definition
• If the bean implements the BeanNameAware interface, the factory calls setBeanName() passing the bean’s ID.
• If the bean implements the BeanFactoryAware interface, the factory calls setBeanFactory(), passing an instance of itself.
• If there are any BeanPostProcessors associated with the bean, their post- ProcessBeforeInitialization() methods will be called.
• If an init-method is specified for the bean, it will be called.
• Finally, if there are any BeanPostProcessors associated with the bean, their postProcessAfterInitialization() methods will be called. 

8. How to integrate Spring and Hibernate using HibernateDaoSupport?

   Spring and Hibernate can integrate using Spring’s SessionFactory called LocalSessionFactory. The integration process is of 3 steps.

• Configure the Hibernate SessionFactory
• Extend your DAO Implementation from HibernateDaoSupport
• Wire in Transaction Support with AOP 

9. What do you mean by Aspect ?

   Aspects are implemented using regular classes (the schema-based approach) or regular classes annotated with the @Aspect annotation (@AspectJ style).

1) schema-based approach   XML style.
 <aop:config>

<aop:pointcut id="cccdoperation"

         expression="(execution(* com.elleinfo.admin.service.*Service.save*(..))) or (execution(* com.elleinfo.admin.service.*Service.process*(..)))" />

<aop:advisor advice-ref="joeTxAdvice" pointcut-ref="cccdoperation" />

<aop:advisor advice-ref="cccdTxAdvice" pointcut-ref="cccdoperation" />

<aop:advisor advice-ref="appsTxAdvice" pointcut-ref="cccdoperation" />

</aop:config>

<aop:config>
  <aop:aspect id="myAspect" ref="aBean">
    <aop:pointcut id="businessService" 
          expression="execution(* com.xyz.myapp.service.*.*(..)) &amp;&amp; this(service)"/>
    <aop:before pointcut-ref="businessService" method="monitor"/>
    ...
    
  </aop:aspect>

</aop:config>

2)    @Aspect {

               @Pointcut

               @Before/ @Around /@AfterReturning / @AfterThrowing/ @After

        }


The XML style has two disadvantages.

1. It does not fully encapsulate the implementation of the requirement it addresses in a single place.
2. The XML style is slightly more limited in what it can express than the @AspectJ style: only the "singleton" aspect instantiation model is supported, and it is not possible to combine named pointcuts declared in XML.

@Aspect reuqires Java 5.
It is ok to mix both.

10. Explain the similarities and differences between EJB CMT and the Spring Framework's declarative transaction management ?

   The basic approach is similar: it is possible to specify transaction behavior (or lack of it) down to individual method level. It is possible to make a setRollbackOnly() call within a transaction context if necessary. The differences are:

• Unlike EJB CMT, which is tied to JTA, the Spring Framework's declarative transaction management works in any environment. It can work with JDBC, JDO, Hibernate or other transactions under the covers, with configuration changes only.
  
• The Spring Framework enables declarative transaction management to be applied to any class, not merely special classes such as EJBs.
  
• The Spring Framework offers declarative rollback rules: this is a feature with no EJB equivalent. Both programmatic and declarative support for rollback rules is provided.
  
• The Spring Framework gives you an opportunity to customize transactional behavior, using AOP. With EJB CMT, you have no way to influence the container's transaction management other than setRollbackOnly().
  
• The Spring Framework does not support propagation of transaction contexts across remote calls, as do high-end application servers. 

11: What is difference between singleton and prototype bean?

Singleton: means single bean definition to a single object instance per Spring IOC container.
Prototype: means a single bean definition to any number of object instances.

 all the beans in spring framework are by default singleton beans

.

12. How many modules are there in Spring? What are they?

       Spring comprises of seven modules. They are. spring.core, spring.beans, spring.aop, sping.context, spring.jdbc, spring.orm, spring.web, spring.transaction, spring.aspect.

• The core container:
  The core container provides the essential functionality of the Spring framework. A primary component of the core container is the BeanFactory, an implementation of the Factory pattern. The BeanFactory applies the Inversion of Control (IOC) pattern to separate an application's configuration and dependency specification from the actual application code.
  
• Spring context:
  The Spring context is a configuration file that provides context information to the Spring framework. The Spring context includes enterprise services such as JNDI, EJB, e-mail, internalization, validation, and scheduling functionality.
  
• Spring AOP:
  The Spring AOP module integrates aspect-oriented programming functionality directly into the Spring framework, through its configuration management feature. As a result you can easily AOP-enable any object managed by the Spring framework. The Spring AOP module provides transaction management services for objects in any Spring-based application. With Spring AOP you can incorporate declarative transaction management into your applications without relying on EJB components.
  
• Spring DAO:
  The Spring JDBC DAO abstraction layer offers a meaningful exception hierarchy for managing the exception handling and error messages thrown by different database vendors. The exception hierarchy simplifies error handling and greatly reduces the amount of exception code you need to write, such as opening and closing connections. Spring DAO's JDBC-oriented exceptions comply to its generic DAO exception hierarchy.
  
• Spring ORM:
  The Spring framework plugs into several ORM frameworks to provide its Object Relational tool, including JDO, Hibernate, and iBatis SQL Maps. All of these comply to Spring's generic transaction and DAO exception hierarchies.
  
• Spring Web module:
  The Web context module builds on top of the application context module, providing contexts for Web-based applications. As a result, the Spring framework supports integration with Jakarta Struts. The Web module also eases the tasks of handling multi-part requests and binding request parameters to domain objects.
  
• Spring MVC framework:
  The Model-View-Controller (MVC) framework is a full-featured MVC implementation for building Web applications. The MVC framework is highly configurable via strategy interfaces and accommodates numerous view technologies including JSP, Velocity, Tiles, iText, and POI.

13. What are the different types of events related to Listeners?

There are a lot of events related to ApplicationContext of spring framework. All the events are subclasses of org.springframework.context.Application-Event. They are

• ContextClosedEvent – This is fired when the context is closed.
• ContextRefreshedEvent – This is fired when the context is initialized or refreshed.
• RequestHandledEvent – This is fired when the web context handles any request.

14.  What is the Exception class related to all the exceptions that are thrown in spring applications?

DataAccessException - org.springframework.dao.DataAccessException

15.  list of new features for Spring 3.0

• Spring Expression Language.can be used when defining XML and Annotation based bean definitions。 @Value("#{systemProperties.databaseName}")
• IoC enhancements/Java based bean metadata： @Configuration， @Bean， @Value
• General-purpose type conversion system and field formatting system
• Object to XML mapping functionality (OXM) moved from Spring Web Services project
• Comprehensive REST support
• @MVC additions
• Declarative model validation
• Early support for Java EE 6
• Embedded database support

To be Continued.