ApplicationContext and ServletContext

Question

I get confused between the two ApplicationContext and ServletContext when it comes to Spring MVC Application. I know that There is just only One ApplicationContext per Spring Web Application and there is also just only One ServletContext per web application. To initiate the value for both ApplicationContext and ServletContext, in web.xml, we will add something in context-param tag.

That is the point that makes me confused. What are the differences between these two (i know ApplicationContext has some methods to work with beans)? and When we would use ApplicationContext and When we would use ServletContext?

Answer 1

Servlet Context:

It is initialized when a Servlet application is deployed. Servlet Context holds all the configurations (init-param, context-params, etc) of the whole servlet application.

Application Context:

It is a Spring specific thing. It is initialized by Spring. It holds all the bean definitions and life-cycle of the beans that are defined inside the spring configuration files. Servlet-Context has no idea about these things.

There are two types of contexts in Spring parent and child.

Spring Parent Context (Application Context / Root Context )

  <listener>
        <listener-lass> 
            org.springframework.web.context.ContextLoaderListener
        </listener-class>
  </listener>
  <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>
            /WEB-INF/service-context.xml,
            /WEB-INF/dao-context.xml,
            /WEB-INF/was-context.xml,
            /WEB-INF/jndi-context.xml,
            /WEB-INF/json-context.xml
        </param-value>
  </context-param>

role-purpose-of-contextloaderlistener-in-spring
Spring-ContextLoaderListener-And-DispatcherServlet-Concepts
When spring container starts up, it reads all the bean definitions from the configuration files and creates beans objects, and manages the life cycle of the bean objects. This configuration is totally optional.

DispatcherServlet vs ContextLoaderListener
/declaring-spring-bean-in-parent-context-vs-child-context

Spring Child Context ( WebApplicationContext / Child Context )

<servlet>
    <servlet-name>myWebApplication</servlet-name>
    <servlet-class>
         org.springframework.web.servlet.DispatcherServlet
    </servlet-class>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>myWebApplication</servlet-name>
    <url-pattern>/app/*</url-pattern>
</servlet-mapping>

When spring web application starts it will look for spring bean configuration file myWebApplication-servlet.xml. It will read all the bean definitions and create and manages the bean objects' life cycle. If the parent spring context is available it will merge the child spring context with the parent spring context. If there is no Spring parent context available the application will only have the child spring context.