What is ApplicationContext? What are its implementations?

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ApplicationContext, like BeanFactory, is also used to represent Spring Container. It is built upon BeanFactory interface.

BeanFactory provides basic functionality while ApplicationContext provides advance features to our spring applications which make them enterprise level applications, like i18n, event publishing, JNDI access, EJB integration, Remoting etc.

ApplicationContext is always preferred over BeanFactory and is suitable for J2EE Applications. ApplicationContext includes all functionality of the BeanFactory.

ApplicationContext and Singleton beans:

While using BeanFactory, beans get instantiated when they get requested first time, like in getBean("bean_id") method, not when object of BeanFactory itself gets created. This is known as lazy-instantiation.

But while using ApplicationContext, singleton beans does not get created lazily. By default, ApplicationContext immediately instantiates the singleton beans and wire/set its properties as it's object itself gets created. So ApplicationContext loads singleton beans eagerly (pre-instantiated).

Example:

ApplicationContext acObj=new ClassPathXmlApplicationContext("beanconfig.xml");

MyBean beanObj=(MyBean)acObj.getBean("mybean");

beanObj.someMethod();

In above example we have created an ApplicationContext object acObj using one if its implementations ClassPathXmlApplicationContext which loads the configuration from source beanconfig.xml under classpath.

If we had used BeanFactory here, mybean would get instantiated when the method getBean(mybean) would  be called.

But here with ApplicationContext, instantiation of bean with id mybean does not get delayed until getBean() method is called. If scope of the bean mybean  is declared in configuration file as singleton, it will be immediately instantiated when we create ApplicationContext object acObj. So when getBean() would be called, mybean would already have got loaded and its dependencies set.

We can change this default behavior so that ApplicationContext does not load singleton beans eagerly by using  lazy-init attribute as:

<bean id="mybean" class="x.y.z..MyBean" lazy-init="true"/>

A lazy-initialized bean tells the Spring to create a bean instance when it is first requested, rather than at the time of creation of ApplicationContext object.

Implementations of ApplicationContext:

There are many implementations of ApplicationContext interface. Important ones are:

1)  ClassPathXmlApplicationContext:

It loads bean definitions from XML files located in the classpath.

           

Example1:

ApplicationContext context = new ClassPathXmlApplicationContext("myconfig.xml");

Example2:  Loading configuration from multiple files under classpath.

ApplicationContext context = new ClassPathXmlApplicationContext(

newString[]{"servicesconfig.xml","daoconfig.xml"});

2) FileSystemXmlApplicationContext:

            It loads bean definitions from XML files in the file system.

           

Example:        

           

ApplicationContext context = new FileSystemXmlApplicationContext("c:/myconfig.xml");

3)XmlWebApplicationContext:

XmlWebApplicationContext is used to represent Spring Container for web applications.By defalut Spring creates object of XmlWebApplicationContext class to represent application context/spring container for web applications.

It loads bean definitions from an XML file contained within a web application. By default it loads the configuration from file "/WEB-INF/applicationContext.xml".

If we want to load bean definitions from more than one xml files we can specify their locations in  contextConfigLocation  parameter of ContextLoaderListener or DispatcherServlet in web.xml.Read more about this here.

XmlWebApplicationContext is an implementation of WebApplicationContext interface which in turn extends ApplicationContext interface.

4)AnnotationConfigApplicationContext:

AnnotationConfigApplicationContext class is used when we are using Java-based configuration for the bean definitions instead of Xml files.

In above ApplicationContext implementations (ClassPathXmlApplicationContext, FileSystemXmlApplicationContext) we have supplied bean configuration from xml configuration files. AnnotationConfigApplicationContext class is used to create Spring container which takes bean definitions from java classes annotated with @Configuration, instead of xml files.

It is introduced in Spring 3.0.

Example:

public static void main(String[]args){

/* Creating Spring IoC Container Without XML configuration file*/

ApplicationContext context= new AnnotationConfigApplicationContext(MyConfig.class);

MyBean beanObj = context.getBean(MyBean.class);

beanObj.someMethod();

}

In above code, AnnotationConfigApplicationContext is accepting MyConfig class as input. Here we are obtaining bean definitions from a java class named MyConfig annotated with @Configuration, instead of a Xml file. MyConfig class is described as:

@Configuration

public class MyConfig{

            @Bean

            public MyBean myBeanId(){

            return new MyBean();

            }

}

By giving @Configuration annotation we are treating Myconfig class as  <beans></beans>  tag of xml file.

By giving @Bean annotation we are treating myBean() method as  <bean id="..." class="..."/>

Name of myBeanId() method will be treated as bean id.

Both @Configuration and @Bean are also introduced in Spring 3.0.

5)AnnotationConfigWebApplicationContext:

Like XmlWebApplicationContext is web counterpart for the ClassPathXmlApplicationContext and FileSystemXmlApplicationContext and is used to create application context for web  applications, similarly, AnnotationConfigWebApplicationContext is web counterpart for AnnotationConfigApplicationContext.

AnnotationConfigWebApplicationContext is used to create application context for web applications by using java clases as input for bean definitions instead of xml files.

By default Spring use XmlWebApplicationContext (an implementation of WebApplicationContext) for creating spring container in web applications. But we can change this default value to AnnotationConfigWebApplicationContext by changing the value of contextClass parameter of ContextLoaderListener or DispatcherServlet in web.xml as shown below:

For ContextLoaderListener:

<web-app>

<context-param>

<param-name>contextClass</param-name> 

<param-value>   

org.springframework.web.context.support.AnnotationConfigWebApplicationContext

</param-value> 

</context-param> 

<context-param>

<param-name>contextConfigLocation</param-name> 

<!--MyConfig must be annotated with @Configuration-->

<param-value> MyConfig</param-value>

</context-param>

<listener>

<listener-class> org.springframework.web.context.ContextLoaderListener </listener-class>

</listener> 

</web-app> 

For  DispatcherServlet:

<web-app>

<servlet>

<servlet-name>mydispatcher</servlet-name>

<servlet-class > org.springframework.web.servlet.DispatcherServlet

</servlet-class>

<init-param> 

<param-name>contextClass</param-name> 

<param-value>            

 org.springframework.web.context.support.AnnotationConfigWebApplicationContext     

</param-value>

</init-param> 

<init-param>   

<param-name>contextConfigLocation</param-name>    

<!--MyConfig must be class annotated with @Configuration-->

<param-value> MyConfig </param-value>

</init-param>

</servlet>

<servlet-mapping> 

<servlet-name>mydispatcher</servlet-name>    

<url-pattern>*.htm</url-pattern>

</servlet-mapping>

</web-app>

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