角色：应用方(spring) + servlet + web容器(tomcat)

• 规范提供ServletContainerInitializer接口，应用方提供实现类。
• 规范提供"根目录/resources/META-INF/services/javax.servlet.ServletContainerInitializer"路径，应用方把ServletContainerInitializer接口实现类全类名写在该文件内。
• 规范提供@HandlesTypes注解，由应用方配置，web容器解析，解析得到的实现类集合将通过参数传给ServletContainerInitializer.onStartup方法。
• web容器启动完成回调ServletContainerInitializer.onStartup方法。

spring提供WebApplicationInitializer接口，让程序员方便地进行web容器初始化回调。
spring把WebApplicationInitializer接口配置到@HandlesTypes，让web容器找到所有WebApplicationInitializer接口的实现类。
spring提供SpringServletContainerInitializer类实现ServletContainerInitializer接口，把web容器找到的实现类实例化出来，并执行。
spring把SpringServletContainerInitializer全类名配置到servlet规定的文件内，由web容器调用。

public interface WebApplicationInitializer {
	void onStartup(ServletContext servletContext) throws ServletException;
}

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
	// webAppInitializerClasses：web容器找到的WebApplicationInitializer的实现类集合
	@Override
	public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException {
		// 执行集合
		List<WebApplicationInitializer> initializers = new LinkedList<>();
		if (webAppInitializerClasses != null) {
			for (Class<?> waiClass : webAppInitializerClasses) {
				// 验证 添加到执行集合
				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) {
			// 遍历执行
			initializer.onStartup(servletContext);
		}
	}
}

org.springframework.web.SpringServletContainerInitializer

// 参考spring的WebApplicationInitializer接口
public interface CustomWebApplicationInitializer {
    void onStartup(ServletContext servletContext) throws ServletException;
}

// 提供ServletContainerInitializer接口实现类，加上@HandlesTypes注解，传入自己提供的CustomWebApplicationInitializer接口
@HandlesTypes(CustomWebApplicationInitializer.class)
public class CustomServletContainerInitializer implements ServletContainerInitializer {
    @Override
    public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException {
        List<CustomWebApplicationInitializer> initializers = new LinkedList<>();
        if (c != null) {
            for (Class<?> waiClass : c) {
                if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && CustomWebApplicationInitializer.class.isAssignableFrom(waiClass)) {
                    try {
                        initializers.add((CustomWebApplicationInitializer) ReflectionUtils.accessibleConstructor(waiClass).newInstance());
                    } catch (Throwable ex) {
                        throw new ServletException("Failed to instantiate CustomWebApplicationInitializer class", ex);
                    }
                }
            }
        }
        if (initializers.isEmpty()) {
            ctx.log("No Spring CustomWebApplicationInitializer types detected on classpath");
            return;
        }
        ctx.log(initializers.size() + " Spring CustomWebApplicationInitializers detected on classpath");
        AnnotationAwareOrderComparator.sort(initializers);
        for (CustomWebApplicationInitializer initializer : initializers) {
            initializer.onStartup(ctx);
        }
    }
}

com.tbryant.springmvctest.customspringmvc.core.CustomServletContainerInitializer

public class MyCustomWebApplicationInitializer implements CustomWebApplicationInitializer {
    @Override
    public void onStartup(ServletContext servletContext) {
        System.out.println("web容器初始化回调，可在此添加spring容器启动代码");
    }
}