Why it is necessary to extend`SpringBootServletInitializer` while deploying it to an external tomcat

Question

Why should we extend SpringBootServletInitializer in order to run a SpringBoot application to a external tomcat?

If without extending SpringBootServletInitializer it runs on embedded tomcat then why it is necessary to extendSpringBootServletInitializer while deploying it to an external tomcat?

Answer 1

Older Servlet containers don’t have support for the ServletContextInitializer bootstrap process used in Servlet 3.0. You can still use Spring and Spring Boot in these containers but you are going to need to add a web.xml to your application and configure it to load an ApplicationContext via a DispatcherServlet.

Inorder to create deployable war file is to provide a SpringBootServletInitializer subclass and override its configure method. This makes use of Spring Framework’s Servlet 3.0 support and allows you to configure your application when it’s launched by the servlet container. Typically, you update your application’s main class to extend SpringBootServletInitializer.

@SpringBootApplication
public class Application extends SpringBootServletInitializer {
@Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
        return application.sources(Application.class);
    }

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

You can refer below link

https://docs.spring.io/spring-boot/docs/current/reference/html/howto-traditional-deployment.html

Answer 2

Here is some more information :

In order to deploy a Servlet based web application(like Spring) you actually need to provide traditional web.xml .

We can also do the same thing programatically using WebApplicationInitializer interface. As per the docs

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.

As SpringBoot suggests to use JavaConfiguration over xml configuration.

It uses JavaConfiguration instead of web.xml.

It has SpringBootServletInitializer class which eventually implement the WebApplicationInitializer interface and override its onStartup to configure things.