Importance of SpringBootServletInitializer in the WAR deployment - spring

Question

I have gone through some tutorials and learnt that ApplicationContext is responsible for managing all the beans created in the spring application. When the WAR is to be deployed to a servlet container, it is essential to extend SpringBootServletInitializer and from the documentation it says
An opinionated WebApplicationInitializer to run a SpringApplication from a traditional WAR deployment. Binds Servlet, Filter and ServletContextInitializer beans from the application context to the server.
Can someone help me understand what the above line means ? How ServletContext initializer is related to the SpringBootServletInitializer ? Why can't a servlet container create a ServletContext on it's own ? If this the case how can we get the hold of ServletContext reference of a SpringApplication programatically created by Tomcat after using SpringBootServletInitializer?

Answer 1

The web application uses the static main entry point when you run the embedded application server. The main entry point usually looks like this:
public static void main(final String[] args) {
SpringApplication.run(Application.class, args);
}
or this:
public static void main(final String[] args) {
SpringApplication springApplication = new SpringApplicationBuilder(Application.class)
.properties("spring.main.banner-mode=log")
.build();
springApplication.run(args);
}
Both use the SpringApplication.run method to scan, configure, and run the web application.
The static main doesn't mean anything to an application server. Instead we add a SpringBootServletInitializer implementation that implements in turn the WebApplicationInitializer interface.
Interface to be implemented in Servlet 3.0+ environments in order to configure the ServletContext programmatically -- as opposed to (or possibly in conjunction with) the traditional web.xml-based approach.
Implementations of this SPI will be detected automatically by SpringServletContainerInitializer, which itself is bootstrapped automatically by any Servlet 3.0 container. See its Javadoc for details on this bootstrapping mechanism.
The SpringServletContainerInitializer gets bootstrapped by the Servlet 3.0+ container and executes the WebApplicationInitializer.onStartup(ServletContext context) methods to configure the ServletContext.
We implement the SpringServletContainerInitializer to define the SpringApplication[s] to be run.
The application server can create a ServletContext on its own, it just can't configure the ServletContext on its own.
Why do you want access to the ServletContext? Spring will handle the configuration for you.