SpringServletContainerInitializer实现了Servlet3.0规范中定义的ServletContainerInitializer：

public interface ServletContainerInitializer {void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException;
}

SpringServletContainerInitializer代码如下：

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {@Overridepublic void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)throws ServletException {List<WebApplicationInitializer> initializers = Collections.emptyList();if (webAppInitializerClasses != null) {initializers = new ArrayList<>(webAppInitializerClasses.size());for (Class<?> waiClass : webAppInitializerClasses) {// Be defensive: Some servlet containers provide us with invalid classes,// no matter what @HandlesTypes says...if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&WebApplicationInitializer.class.isAssignableFrom(waiClass)) {try {initializers.add((WebApplicationInitializer)ReflectionUtils.accessibleConstructor(waiClass).newInstance());}catch (Throwable ex) {throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);}}}}if (initializers.isEmpty()) {servletContext.log("No Spring WebApplicationInitializer types detected on classpath");return;}servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");AnnotationAwareOrderComparator.sort(initializers);for (WebApplicationInitializer initializer : initializers) {initializer.onStartup(servletContext);}}}

因为它使用了@HandlesTypes(WebApplicationInitializer.class)注解，所以web容器会寻找类路径上实现了WebApplicationInitializer的类。

它在onStartup方法中会分别调用WebApplicationInitializer的onStartup方法。

SpringServletContainerInitializer的onStartup方法会被支持Servlet3.0规范的web 容器（比如tomcat 9)自动调用

Spring-web模块的META-INFO目录的services目录下有名为javax.servlet.ServletContainerInitializer的文件，内容是

org.springframework.web.SpringServletContainerInitializer

可见，SpringServletContainerInitializer是通过SPI机制加载的。