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.
i.e., Applying IoC, objects are given their dependencies
at creation time by some external entity that coordinates each object in the
system. That is, dependencies are injected into objects. So, IoC means an
inversion of responsibility with regard to how an object obtains references to
collaborating objects.
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 the benefits of IOC
(Dependency Injection)?
Benefits of IOC (Dependency
Injection) are as follows:
- Minimizes the amount of code in your application. With
IOC containers you do not care about how services are created and how you
get references to the ones you need. You can also easily add additional services
by adding a new constructor or a setter method with little or no extra
configuration.
- Make your application more testable by not requiring
any singletons or JNDI lookup mechanisms in your unit test cases. IOC
containers make unit testing and switching implementations very easy by
manually allowing you to inject your own objects into the object under
test.
- Loose coupling is promoted with minimal effort and
least intrusive mechanism. The factory design pattern is more intrusive
because components or services need to be requested explicitly whereas in
IOC the dependency is injected into requesting piece of code. Also some
containers promote the design to interfaces not to implementations design
concept by encouraging managed objects to implement a well-defined service
interface of your own.
- IOC containers support eager instantiation and lazy
loading of services. Containers also provide support for instantiation of
managed objects, cyclical dependencies, life cycles management, and
dependency resolution between managed objects etc.
4. What is Spring ?
Spring is an open source framework
created to address the complexity of enterprise application development. One of
the chief advantages of the Spring framework is its layered architecture, which
allows you to be selective about which of its components you use while also
providing a cohesive framework for J2EE application development.
5. What are the advantages of
Spring framework?
The advantages of Spring are as
follows:
- Spring has layered architecture. Use what you need and
leave you don't need now.
- Spring Enables POJO Programming. There is no behind the
scene magic here. POJO programming enables continuous integration and
testability.
- Dependency Injection and Inversion of Control
Simplifies JDBC
- Open source and no vendor lock-in.
6. What are features of Spring
?
spring is
lightweight when it comes to size and transparency. The basic version of spring
framework is around 1MB. And 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.
Spring
supports Aspect oriented programming and enables cohesive development by
separating application business logic from system services.
Spring
contains and manages the life cycle and configuration of application objects.
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. But other
frameworks can be easily used instead of Spring MVC Framework.
Spring
framework provides a generic abstraction layer for transaction management. This
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.
The JDBC
abstraction layer of the Spring offers a meaningful exception hierarchy, which
simplifies the error handling strategy. Integration with Hibernate, JDO, and
iBATIS: Spring provides best Integration services with Hibernate, JDO and
iBATIS
7. How many modules are there in
Spring? What are they?
Spring comprises of seven modules.
They are..
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.
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.
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.
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.
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.
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.
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.
8. What are the types of
Dependency Injection Spring supports?
Setter-based
DI is realized by calling setter methods on your beans after invoking a
no-argument constructor or no-argument static factory method to instantiate
your bean.
Constructor-based
DI is realized by invoking a constructor with a number of arguments, each
representing a collaborator.
9. 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.
10. 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.
11. What is the difference
between Bean Factory and Application Context ?
On the surface, an application
context is same as a bean factory. But application context offers much more..
- Application contexts provide a means for resolving text
messages, including support for i18n of those messages.
- Application contexts provide a generic way to load file
resources, such as images.
- Application contexts can publish events to beans that
are registered as listeners.
- Certain operations on the container or beans in the
container, which have to be handled in a programmatic fashion with a bean
factory, can be handled declaratively in an application context.
- ResourceLoader support: Spring’s Resource interface us
a flexible generic abstraction for handling low-level resources. An
application context itself is a ResourceLoader, Hence provides an
application with access to deployment-specific Resource instances.
MessageSource support: The
application context implements MessageSource, an interface used to obtain
localized messages, with the actual implementation being pluggable
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12. 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.
13. How is a typical spring
implementation look like ?
For a typical
Spring Application we need the following files:
- An interface that defines the functions.
- An Implementation that contains properties, its setter
and getter methods, functions etc.,
- Spring AOP (Aspect Oriented Programming)
- A XML file called Spring configuration file.
- Client program that uses the function.
14. 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.
15. What do you mean by Bean wiring ?
The act of creating associations
between application components (beans) within the Spring container is reffered
to as Bean wiring.
16. What do you mean by Auto Wiring?
The Spring
container is able to autowire relationships between collaborating beans. This
means that it is possible to automatically let Spring resolve collaborators
(other beans) for your bean by inspecting the contents of the BeanFactory. The
autowiring functionality has five modes.
- no
- byName
- byType
- constructor
- autodirect
17. What is
DelegatingVariableResolver?
Spring
provides a custom JavaServer Faces VariableResolver implementation that extends
the standard Java Server Faces managed beans mechanism which lets you use JSF
and Spring together. This variable resolver is called as
DelegatingVariableResolver
18. How to integrate Java
Server Faces (JSF) with Spring?
JSF and Spring do
share some of the same features, most noticeably in the area of IOC services.
By declaring JSF managed-beans in the faces-config.xml configuration file, you
allow the FacesServlet to instantiate that bean at startup. Your JSF pages have
access to these beans and all of their properties.We can integrate JSF and
Spring in two ways:
- DelegatingVariableResolver: Spring comes with a JSF variable resolver that lets
you use JSF and Spring together.
<?xml
version="1.0" encoding="UTF-8"?>
<!DOCTYPE
beans PUBLIC "-//SPRING//DTD BEAN//EN"
"http://www.springframework.org/dtd/spring-beans.dtd">
<faces-config>
<application>
<variable-resolver>
org.springframework.web.jsf.DelegatingVariableResolver
</variable-resolver>
</application>
</faces-config>
The
DelegatingVariableResolver will first delegate value lookups to the default
resolver of the underlying JSF implementation, and then to Spring's 'business
context' WebApplicationContext. This allows one to easily inject dependencies
into one's JSF-managed beans.
- FacesContextUtils:custom VariableResolver works well
when mapping one's properties to beans in faces-config.xml, but at times
one may need to grab a bean explicitly. The FacesContextUtils class makes
this easy. It is similar to WebApplicationContextUtils, except that it
takes a FacesContext parameter rather than a ServletContext parameter.
ApplicationContext
ctx =
FacesContextUtils.getWebApplicationContext(FacesContext.getCurrentInstance());
