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I am trying to understand basic Spring MVC stuff.

Consider the web.xml: 

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
    <!-- The definition of the Root Spring Container shared by all Servlets and Filters -->
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/spring/root-context.xml</param-value>
    </context-param>
    <!-- Creates the Spring Container shared by all Servlets and Filters -->
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>
    <!-- Processes application requests -->
    <servlet>
        <servlet-name>appServlet</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>/WEB-INF/spring/appServlet/servlet-context.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>  
    <servlet-mapping>
        <servlet-name>appServlet</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>
</web-app>

Also consider below diagram that is given in the official docs:

enter image description here

Doubts:

  1. What is precise types of instances created by DispatcherServlet and ContextLoaderListener? The confusion arise, since some articles online say ContextLoaderListener creates ApplicationContext instance and DispatcherServlet creates WebApplicationContext instance. But, after looking at their source, I feel both create instance of type WebApplicationContext, and since its subtype of ApplicationContext, some articles say ContextLoaderListener creates ApplicationContext instance. Is it also the case that only ApplicationContext-provided functionalities of instance created by ContextLoaderListener are used, and hence the article say ContextLoaderListener creates ApplicationContext, but not WebApplicationContext `

  2. Is it like the WebApplicaionContext created by ContextLoaderListener is what usually online articles refer to as "root" WebApplicationContext, just to differentiate it from WebApplicationContexts created by DispatcherServlet?

  3. Reading this article, I understood that DispatcherServlet loads from [servlet-name]-servlet.xml file which should load "web tier components". On the other hand ContextLoaderListener loads "middle tier components". Given that there can be only one ContextLoaderListener in web.xml (right?), there should be only single single set of "middle tier components", whereas, since there can be multiple DispatcherServlets, there can be multiple sets of "web tier components". Is this right? If so, then why the above diagram shows multiple WebApplicationContexts of middle tier components and single WebApplicationContext of web tier component?

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  • 1) Both WebApplicationContext and ApplicationContext are interfaces, so when documentation says "creates ApplicationContext instance" it means that it creates an object instance of a class that implements ApplicationContext. Whether that class also implements WebApplicationContext is besides the point. It may. It may not. Doesn't invalidate the statement that it "creates ApplicationContext instance". Commented Oct 2, 2018 at 21:01
  • ohh thats bummer...How did I not notice that. When we create normal Spring MVC app with web.xml, it creates XmlWebApplicationContext? Also can you answer the other two questions...or they are senseless? Commented Oct 3, 2018 at 19:11

1 Answer 1

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  1. Both WebApplicationContext and ApplicationContext are interfaces, so when documentation says "creates ApplicationContext instance" it means that it creates an object instance of a class that implements ApplicationContext. Whether that class also implements WebApplicationContext is besides the point. It may. It may not. Doesn't invalidate the statement that it "creates ApplicationContext instance".

  2. what usually online articles refer to as "root"

    No, it's what the javadoc of ContextLoaderListener itself refers to a root:

    Performs the actual initialization work for the root application context.

  3. why the above diagram shows multiple WebApplicationContexts of middle tier components and single WebApplicationContext of web tier component?

    It only shows a single WebApplicationContext of web tier component because the diagram is for a single DispatcherServlet.

    Just because ContextLoaderListener only creates a single root context, doesn't mean that multiple contexts couldn't be created by other means. The diagram accurately shows what is possible.

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2 Comments

Thanks, that clears considerable amount of confusion in my mind. But a new little confusion created by "Just because ContextLoaderListener only creates a single root context, doesn't mean that multiple contexts couldn't be created by other means." Can you elaborate / give small example which "other means"?
@anir E.g. write your own listener and create however many contexts you want, split however you want.

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