Spring WebApplicationInitializer and ApplicationContextInitializer confusion


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 WebApplicationInitializerhere and here.
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 theDispatcherServlet.
An example of using WebApplicationInitializer is the following:

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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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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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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.