Role Purpose of ContextLoaderListener in Spring

Question

I am learning Spring Framework which is being used in my project  I found the  ContextLoaderListener entry in my web xml file  But could not figure out how exactly it helps a developer   In the official documentation of ContextLoaderListener it says  it is to start WebApplicationContext  Regarding WebApplicationContext JavaDocs say      Interface to provide configuration for a web application     But I am not able to understand what I am achieving with ContextLoaderListener which internally initializes the WebApplicationContext    As per my understanding  ContextLoaderListener  reads the Spring configuration file  with value given against contextConfigLocation in web xml   parses it and loads the singleton bean defined in that config file  Similarly when we want to load prototype bean  we will use same webapplication context to load it  So we initialize the webapplication  with ContextLoaderListener so that we read parse validate the config file in advance and whenever we wan to inject dependency we can straightaway do it without any delay  Is this understanding correct

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Your understanding is correct  I wonder why you don t see any advantages in ContextLoaderListener  For example  you need to build a session factory  to manage database   This operation can take some time  so it s better to do it on startup  Of course you can do it with init servlets or something else  but the advantage of Spring s approach is that you make configuration without writing code

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It will give you point of hook to put some code that you wish to be executed on web application deploy time

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Root and child contexts Before reading further  please understand that      Spring can have multiple contexts at a time  One of them will be root context  and all other contexts will be child contexts   All child contexts can access the beans defined in root context  but opposite is not true  Root context cannot access child contexts beans   ApplicationContext    applicationContext xml is the root context configuration for every web application  Spring loads applicationContext xml file and creates the ApplicationContext for the whole application  There will be only one application context per web application  If you are not explicitly declaring the context configuration file name in web xml using the contextConfigLocation param  Spring will search for the applicationContext xml under WEB-INF folder and throw FileNotFoundException if it could not find this file   ContextLoaderListener Performs the actual initialization work for the root application context  Reads a    contextConfigLocation    context-param and passes its value to the context instance  parsing it into potentially multiple file paths which can be separated by any number of commas and spaces  e g     WEB-INF applicationContext1 xml  WEB-INF applicationContext2 xml     ContextLoaderListener is optional  Just to make a point here  you can boot up a Spring application without ever configuring ContextLoaderListener  just a basic minimum web xml with DispatcherServlet   DispatcherServlet  DispatcherServlet is essentially a Servlet  it extends HttpServlet  whose primary purpose is to handle incoming web requests matching the configured URL pattern  It take an incoming URI and find the right combination of controller and view  So it is the front controller   When you define a DispatcherServlet in spring configuration  you provide an XML file with entries of controller classes  views mappings etc  using contextConfigLocation attribute   WebApplicationContext Apart from ApplicationContext  there can be multiple WebApplicationContext in a single web application  In simple words  each DispatcherServlet associated with single WebApplicationContext  xxx-servlet xml file is specific to the DispatcherServlet and a web application can have more than one DispatcherServlet configured to handle the requests  In such scenarios  each DispatcherServlet would have a separate xxx-servlet xml configured  But  applicationContext xml will be common for all the servlet configuration files  Spring will by default load file named    xxx-servlet xml    from your webapps WEB-INF folder where xxx is the servlet name in web xml  If you want to change the name of that file name or change the location  add initi-param with contextConfigLocation as param name   Comparison and relation between them    ContextLoaderListener vs DispatcherServlet  ContextLoaderListener creates root application context  DispatcherServlet entries create one child application context per servlet entry  Child contexts can access beans defined in root context  Beans in root context cannot access beans in child contexts  directly   All contexts are added to ServletContext  You can access root context using WebApplicationContextUtils class   After reading the Spring documentation  following is the understanding   a  Application-Contexts are hierarchial and so are WebApplicationContexts  Refer documentation here   b  ContextLoaderListener creates a root web-application-context for the web-application and puts it in the ServletContext  This context can be used to load and unload the spring-managed beans ir-respective of what technology is being used in the controller layer Struts or Spring MVC    c  DispatcherServlet creates its own WebApplicationContext and the handlers controllers view-resolvers are managed by this context   d  When ContextLoaderListener is used in tandem with DispatcherServlet  a root web-application-context is created first as said earlier and a child-context is also created by DispatcherSerlvet and is attached to the root application-context  Refer documentation here   When we are working with Spring MVC and are also using Spring in the services layer  we provide two application-contexts  The first one is configured using ContextLoaderListener and the other with DispatcherServlet  Generally  you will define all MVC related beans  controller and views etc  in DispatcherServlet context  and all cross-cutting beans such as security  transaction  services etc  at root context by ContextLoaderListener   Refer this for more details    https   siddharthnawani blogspot com 2019 10 contextloaderlistener-vs html

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For a simple Spring application  you don t have to define ContextLoaderListener in your web xml  you can just put all your Spring configuration files in  lt servlet gt     lt servlet gt       lt servlet-name gt hello lt  servlet-name gt       lt servlet-class gt org springframework web servlet DispatcherServlet lt  servlet-class gt       lt init-param gt           lt param-name gt contextConfigLocation lt  param-name gt           lt param-value gt classpath spring mvc-core-config xml  classpath spring business-config xml lt  param-value gt       lt  init-param gt       lt load-on-startup gt 1 lt  load-on-startup gt   lt  servlet gt    For a more complex Spring application  where you have multiple DispatcherServlet defined  you can have the common Spring configuration files that are shared by all the DispatcherServlet defined in the ContextLoaderListener    lt context-param gt       lt param-name gt contextConfigLocation lt  param-name gt       lt param-value gt classpath spring common-config xml lt  param-value gt   lt  context-param gt   lt listener gt       lt listener-class gt org springframework web context ContextLoaderListener lt  listener-class gt   lt  listener gt    lt servlet gt       lt servlet-name gt mvc1 lt  servlet-name gt       lt servlet-class gt org springframework web servlet DispatcherServlet lt  servlet-class gt       lt init-param gt           lt param-name gt contextConfigLocation lt  param-name gt           lt param-value gt classpath spring mvc1-config xml lt  param-value gt       lt  init-param gt       lt load-on-startup gt 1 lt  load-on-startup gt   lt  servlet gt    lt servlet gt       lt servlet-name gt mvc2 lt  servlet-name gt       lt servlet-class gt org springframework web servlet DispatcherServlet lt  servlet-class gt       lt init-param gt           lt param-name gt contextConfigLocation lt  param-name gt           lt param-value gt classpath spring mvc2-config xmll lt  param-value gt       lt  init-param gt       lt load-on-startup gt 1 lt  load-on-startup gt   lt  servlet gt    Just keep in mind  ContextLoaderListener performs the actual initialization work for the root application context   I found this article helps a lot  Spring MVC     Application Context vs Web Application Context

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I believe its real use comes when you want to have more than one config files or you have xyz xml file instead of applicationcontext xml for eg   lt context-param gt  lt param-name gt contextConfigLocation lt  param-name gt   lt param-value gt  WEB-INF training-service xml   WEB-INF training-data xml lt  param-value gt   lt  context-param gt   Another approach to ContextLoaderListener is using ContextLoaderServlet like below   lt servlet gt   lt servlet-name gt context lt  servlet-name gt   lt servlet-class gt org springframework web context ContextLoaderServlet lt  servlet-class gt   lt load-on-startup gt 1 lt  load-on-startup gt   lt  servlet gt

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This Bootstrap listener is to start up and shut down Spring s root WebApplicationContext  As a web application can have multiple dispatcher servlet and each having its own application context containing controllers  view resolver  handler mappings etc But you might want to have service beans  DAO beans in root application context and want to use in all child application context application context created by dispatcher servlets    2nd use of this listener is when you want to use spring security

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Basically you can isolate your root application context and web application context using ContextLoaderListner   The config file mapped with context param will behave as root application context configuration  And config file mapped with dispatcher servlet will behave like web application context   In any web application we may have multiple dispatcher servlets  so multiple web application contexts   But in any web application we may have only one root application context that is shared with all web application contexts   We should define our common services  entities  aspects etc in root application context  And controllers  interceptors etc are in relevant web application context   A sample web xml is   lt  -- language  xml -- gt   lt web-app gt        lt listener gt           lt listener-class gt org springframework web context ContextLoaderListener lt  listener-class gt       lt  listener gt       lt context-param gt           lt param-name gt contextClass lt  param-name gt           lt param-value gt org springframework web context support AnnotationConfigWebApplicationContext lt  param-value gt       lt  context-param gt       lt context-param gt           lt param-name gt contextConfigLocation lt  param-name gt           lt param-value gt example config AppConfig lt  param-value gt       lt  context-param gt       lt servlet gt           lt servlet-name gt restEntryPoint lt  servlet-name gt           lt servlet-class gt org springframework web servlet DispatcherServlet lt  servlet-class gt           lt init-param gt               lt param-name gt contextClass lt  param-name gt               lt param-value gt org springframework web context support AnnotationConfigWebApplicationContext lt  param-value gt           lt  init-param gt           lt init-param gt               lt param-name gt contextConfigLocation lt  param-name gt               lt param-value gt example config RestConfig lt  param-value gt           lt  init-param gt                  lt load-on-startup gt 1 lt  load-on-startup gt       lt  servlet gt       lt servlet-mapping gt           lt servlet-name gt restEntryPoint lt  servlet-name gt           lt url-pattern gt  rest   lt  url-pattern gt       lt  servlet-mapping gt       lt servlet gt           lt servlet-name gt webEntryPoint lt  servlet-name gt           lt servlet-class gt org springframework web servlet DispatcherServlet lt  servlet-class gt           lt init-param gt               lt param-name gt contextClass lt  param-name gt               lt param-value gt org springframework web context support AnnotationConfigWebApplicationContext lt  param-value gt           lt  init-param gt           lt init-param gt               lt param-name gt contextConfigLocation lt  param-name gt               lt param-value gt example config WebConfig lt  param-value gt           lt  init-param gt                  lt load-on-startup gt 1 lt  load-on-startup gt       lt  servlet gt         lt servlet-mapping gt           lt servlet-name gt webEntryPoint lt  servlet-name gt           lt url-pattern gt   lt  url-pattern gt       lt  servlet-mapping gt    lt  web-app gt     Here config class example config AppConfig can be used to configure services  entities  aspects etc in root application context that will be shared with all other web application contexts  for example here we have two web application context config classes RestConfig and WebConfig   PS  Here ContextLoaderListener is completely optional  If we will not mention ContextLoaderListener in web xml here  AppConfig will not work  In that case we need to configure all our services and entities in WebConfig and Rest Config

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ContextLoaderListener is optional  Just to make a point here  you can boot up a Spring application without ever configuring ContextLoaderListener  just a basic minimum web xml with DispatcherServlet   Here is what it would look like   web xml   lt  xml version  1 0  encoding  UTF-8   gt   lt web-app      xmlns xsi  http   www w3 org 2001 XMLSchema-instance       xmlns  http   java sun com xml ns javaee       xmlns web  http   java sun com xml ns javaee web-app 2 5 xsd       xsi schemaLocation           http   java sun com xml ns javaee          http   java sun com xml ns javaee web-app 2 5 xsd       id  WebApp ID       version  2 5  gt     lt display-name gt Some Minimal Webapp lt  display-name gt     lt welcome-file-list gt          lt welcome-file gt index jsp lt  welcome-file gt         lt  welcome-file-list gt      lt servlet gt       lt servlet-name gt dispatcher lt  servlet-name gt       lt servlet-class gt        org springframework web servlet DispatcherServlet      lt  servlet-class gt       lt load-on-startup gt 1 lt  load-on-startup gt     lt  servlet gt      lt servlet-mapping gt       lt servlet-name gt dispatcher lt  servlet-name gt       lt url-pattern gt   do lt  url-pattern gt     lt  servlet-mapping gt   lt  web-app gt    Create a file called dispatcher-servlet xml and store it under WEB-INF  Since we mentioned index jsp in welcome list  add this file under WEB-INF   dispatcher-servlet xml  In the dispatcher-servlet xml define your beans    lt  xml version  1 0  encoding  UTF-8   gt   lt beans      xmlns  http   www springframework org schema beans      xmlns xsi  http   www w3 org 2001 XMLSchema-instance      xmlns p  http   www springframework org schema p      xmlns context  http   www springframework org schema context      xsi schemaLocation           http   www springframework org schema beans         http   www springframework org schema beans spring-beans xsd              http   www springframework org schema context              http   www springframework org schema context spring-context xsd  gt        lt bean id  bean1  gt                 lt  bean gt       lt bean id  bean2  gt                 lt  bean gt                 lt context component-scan base-package  com example    gt       lt  -- Import your other configuration files too -- gt       lt import resource  other-configs xml   gt       lt import resource  some-other-config xml   gt        lt  -- View Resolver -- gt       lt bean          id  viewResolver           class  org springframework web servlet view UrlBasedViewResolver  gt         lt property            name  viewClass             value  org springframework web servlet view JstlView    gt         lt property name  prefix  value   WEB-INF jsp     gt         lt property name  suffix  value   jsp    gt       lt  bean gt   lt  beans gt

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Your understanding is correct   The ApplicationContext is where your Spring beans live   The purpose of the ContextLoaderListener is two-fold    to tie the lifecycle of the ApplicationContext to the lifecycle of the ServletContext and to automate the creation of the ApplicationContext  so you don t have to write explicit code to do create it - it s a convenience function     Another convenient thing about the ContextLoaderListener is that it creates a WebApplicationContext and WebApplicationContext provides access to the ServletContext via ServletContextAware beans and the getServletContext method

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In the context of spring framework purpose of ContextLoaderListener is to load the other beans in your application such as the middle-tier and data-tier components that drive the back end of the application

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Listener class - Listens on an event  Eg   Server startup shutdown     ContextLoaderListener -    Listens during server start up shutdown Takes the Spring configuration files as input and creates the beans as per configuration and make it ready  destroys the bean during shutdown  Configuration files can be provided like this in web xml   lt param-name gt contextConfigLocation lt  param-name gt     lt param-value gt  WEB-INF dispatcher-servlet xml lt  param-value gt

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The blog   Purpose of ContextLoaderListener     Spring MVC  gives a very good explanation   According to it  Application-Contexts are hierarchial and hence DispatcherSerlvet s context becomes the child of ContextLoaderListener s context  Due to which  technology being used in the controller layer  Struts or Spring MVC  can independent of root context created ContextLoaderListener

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ContextLoaderListner is a Servlet listener that loads all the different configuration files  service layer configuration  persistence layer configuration etc  into single spring application context   This helps to split spring configurations across multiple XML files   Once the context files are loaded  Spring creates a WebApplicationContext object based on the bean definition and stores it in the ServletContext of your web application

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If we write web xml without ContextLoaderListener then we cant give the athuntication using customAuthenticationProvider in spring  security  Because DispatcherServelet is the child context of ContextLoaderListener  customAuthenticationProvider is the part of parentContext that is ContextLoaderListener  So parent Context cannot have the dependencies of child context  And so it is best practice to write spring-context xml in contextparam instead of write it in the initparam

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When you want to put your Servlet file in your custom location or with custom name  rather than the default naming convention  servletname -servlet xml and path under Web-INF   then you can use ContextLoaderListener

[java] Role/Purpose of ContextLoaderListener in Spring?

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