spring-boot - Which piece of code actually register dispatcher servlet for springMVC?

Question

I am trying to find out in spring-boot, which implementation of WebApplicationInitializer actually register the dispatcher servlet.

I didn\

Answer 1

Below is the description of Spring Boot initialization steps that eventually register DispatcherServlet.

Example Code

@EnableAutoConfiguration
public class TestSpring {

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

Spring Boot Initialization Steps

Here are the steps:

1. SpringApplication.run() creates EmbeddedWebApplicationContext application context;
2. Calls its refresh() method;
3. Refresh process reads annotations of the starting class TestSpring. It looks for import annotations. EnableAutoConfiguration is one of them. For an import annotation the refresh process gets the corresponding class from the annotation value and invokes its selectImports() method;
4. In case of @EnableAutoConfiguration the corresponding class is EnableAutoConfigurationImportSelector whose selectImports() loads tons of other import selectors from the META-INF/spring.factories;
5. This process continues recursively. Also, all bean definitions, that are inside these import selectors, are read. I.e. it includes beans defined by a method with the @Bean annotation, i.e. beans that require the Spring context to call the corresponding method automatically to instantiate them;
6. The resfresh() continues and reaches onRefresh(), the createEmbeddedServletContainer() method is called inside;
7. Among read bean defitions at the previous step, beans implementing ServletContextInitializer are searched for and instantiated. One of them is the bean, defined by the DispatcherServletAutoConfiguration.DispatcherServletRegistrationConfiguration#dispatcherServletRegistration() method of ServletRegistrationBean type that extends ServletContextInitializer. As you can guess from the name of the class, such initializers add a given servlet (in this case DispatcherServlet) to a given ServletContext, when their onStartup() method is invoked;
8. A tomcat embedded server is created (not started completely yet). All found ServletContextInitializers at the previous step are passed to this tomcat initialization - this is where the onStartup() methods of those ServletContextInitializers are called and DispatcherServlet gets created and registered as servlet;
9. End of onRefresh() of application context;
10. The finishRefresh() is called where tomcat is finally started by TomcatEmbeddedServletContainer.start();
11. End of refresh() of application context and other final initialization steps;
12. The app is running.

Answer 2

When you look for something Spring Boot does during auto-configuration, you should look in the *AutoConfiguration classes. In this case, DispatcherServletAutoConfiguration.

Answer 3

If you want to deploy the Spring Boot application as a traditional WAR, then the Servlet 3.0 specification details how service providers can set it up so that the 3.0 compliant servlet container will automatically bootstrap any web assets (Servlet, Filter, ServletContextInitializer's) into the servlet container. The "magic" is accomplished in the spring-web dependency. If you un'jar it, you'll find file "spring-web/META-INF/services/javax.servlet.ServletContainerInitializer". Open the file and you'll see single line "org.springframework.web.SpringServletContainerInitializer". This class delegates to all classes that implement WebApplicationInitializer, more specifically their onStartup(ServletContext servletContext) method. There's one such concrete class I'm aware of in Spring Boot, namely SpringBootServletInitializer.