Spring WebApplicationInitializer and ApplicationContextInitializer confusion

╰半夏微凉° 2022-08-10 12:49 169阅读 0赞

These are two concepts that I mix up occasionally - a WebApplicationInitializer and an ApplicationContextInitializer, and wanted to describe each of them to clarify them for myself.   
  
I have previously blogged about [WebApplicationInitializer][][here][] and [here][here 1]. It is relevant purely in a Servlet 3.0+ spec compliant servlet container and provides a hook to programmatically configure the servlet context. How does this help - you can have a web application without potentially any web.xml file, typically used in a Spring based web application to describe the root application context and the Spring web front controller called the[DispatcherServlet][]. An example of using WebApplicationInitializer is the following:

[view source][]

[print][] [?][Link 1]

`01.` `public` `class` `CustomWebAppInitializer` `extends` `AbstractAnnotationConfigDispatcherServletInitializer {`

`02.` ` ``@Override`

`03.` ` ``protected` `Class<?>[] getRootConfigClasses() {`

`04.` `  ``return` `new` `Class<?>[]{RootConfiguration.``class``};`

`05.` ` ``}`

`06.`

`07.` ` ``@Override`

`08.` ` ``protected` `Class<?>[] getServletConfigClasses() {`

`09.` `  ``return` `new` `Class<?>[]{MvcConfiguration.``class``};`

`10.` ` ``}`

`11.`

`12.` ` ``@Override`

`13.` ` ``protected` `String[] getServletMappings() {`

`14.` `  ``return` `new` `String[]{``"/"``};`

`15.` ` ``}`

`16.` `}`

Now, what is an [ApplicationContextInitializer][]. It is essentially code that gets executed before the Spring application context gets completely created. A good use case for using an ApplicationContextInitializer would be to set a Spring environment profile programmatically, along these lines:

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[print][] [?][Link 1]

`01.` `public` `class` `DemoApplicationContextInitializer``implements` `ApplicationContextInitializer<ConfigurableApplicationContext> {`

`02.`

`03.` ` ``@Override`

`04.` ` ``public` `void` `initialize(ConfigurableApplicationContext ac) {`

`05.` `  ``ConfigurableEnvironment appEnvironment = ac.getEnvironment();`

`06.` `  ``appEnvironment.addActiveProfile(``"demo"``);`

`07.`

`08.` ` ``}`

`09.` `}`

If you have a [Spring-Boot][] based application then registering an ApplicationContextInitializer is fairly straightforward:

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`01.` `@Configuration`

`02.` `@EnableAutoConfiguration`

`03.` `@ComponentScan`

`04.` `public` `class` `SampleWebApplication {`

`05.` ` `

`06.` ` ``public` `static` `void` `main(String[] args) {`

`07.` `  ``new` `SpringApplicationBuilder(SampleWebApplication.``class``)`

`08.` `    ``.initializers(``new` `DemoApplicationContextInitializer())`

`09.` `    ``.run(args);`

`10.` ` ``}`

`11.` `}`

For a non Spring-Boot Spring application though, it is a little more tricky, if it is a programmatic configuration of web.xml, then the configuration is along these lines:

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[print][] [?][Link 1]

`01.` `public` `class` `CustomWebAppInitializer` `implements` `WebApplicationInitializer {`

`02.`

`03.` ` ``@Override`

`04.` ` ``public` `void` `onStartup(ServletContext container) {`

`05.` `  ``AnnotationConfigWebApplicationContext rootContext =``new` `AnnotationConfigWebApplicationContext();`

`06.` `  ``rootContext.register(RootConfiguration.``class``);`

`07.` `  ``ContextLoaderListener contextLoaderListener =``new` `ContextLoaderListener(rootContext);`

`08.` `  ``container.addListener(contextLoaderListener);`

`09.` `  ``container.setInitParameter(``"contextInitializerClasses"``,``"mvctest.web.DemoApplicationContextInitializer"``);`

`10.` `  ``AnnotationConfigWebApplicationContext webContext =``new` `AnnotationConfigWebApplicationContext();`

`11.` `  ``webContext.register(MvcConfiguration.``class``);`

`12.` `  ``DispatcherServlet dispatcherServlet =``new` `DispatcherServlet(webContext);`

`13.` `  ``ServletRegistration.Dynamic dispatcher = container.addServlet(``"dispatcher"``, dispatcherServlet);`

`14.` `  ``dispatcher.addMapping(``"/"``);`

`15.` ` ``}`

`16.` `}`

If it a normal web.xml configuration then the initializer can be specified this way:

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`1.` `<``context-param``>`

`2.` `<``param-name``>contextInitializerClasses</``param-name``>`

`3.` `<``param-value``>com.myapp.spring.SpringContextProfileInit</``param-value``>`

`4.` `</``context-param``>`

`5.`

`6.` `<``listener``>`

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

`8.` `</``listener``>`

So to conclude, except for the Initializer suffix, both WebApplicationInitializer and ApplicationContextInitializer serve fairly different purposes. Whereas the WebApplicationInitializer is used by a Servlet Container at startup of the web application and provides a way for programmatic creating a web application(replacement for a web.xml file), ApplicationContextInitializer provides a hook to configure the Spring application context before it gets fully created.

[WebApplicationInitializer]: http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/WebApplicationInitializer.html
[here]: http://biju-allandsundry.blogspot.com/2012/05/spring-webapplicationinitializer-and.html
[here 1]: http://biju-allandsundry.blogspot.com/2014/03/servlet-30-servletcontainerinitializer.html
[DispatcherServlet]: http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/servlet/DispatcherServlet.html
[view source]: http://java.dzone.com/articles/spring-1#viewSource
[print]: http://java.dzone.com/articles/spring-1#printSource
[Link 1]: http://java.dzone.com/articles/spring-1#about
[ApplicationContextInitializer]: http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/context/ApplicationContextInitializer.html
[Spring-Boot]: http://projects.spring.io/spring-boot/