在SpringBoot应用中，嵌入式的 Servlet 3.0+ 容器不会直接使用 ServletContainerInitializer 和 WebApplicationInitializer，即通过以上两个接口实现的 Servlet、Filter、Listener 配置都是无效的，这是为了防止第三方代码的设计损坏应用程序，原文如下

Embedded servlet containers will not directly execute the Servlet 3.0+ javax.servlet.ServletContainerInitializer interface, or Spring’s org.springframework.web.WebApplicationInitializer interface. This is an intentional design decision intended to reduce the risk that 3rd party libraries designed to run inside a war will break Spring Boot applications.

If you need to perform servlet context initialization in a Spring Boot application, you should register a bean that implements the org.springframework.boot.context.embedded.ServletContextInitializer interface. The single onStartup method provides access to the ServletContext, and can easily be used as an adapter to an existing WebApplicationInitializer if necessary.

综上，可以采取以下配置

配置策略一：ServletContextInitializer

由官方原文可知，我们可以使用替代方案：ServletContextInitializer，示例如下

@Configuration
public class GoServletContextInitializer implements ServletContextInitializer {

    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {

        //配置 Log4j Config Listener  
        servletContext.setInitParameter("log4jConfigLocation", "classpath:config/properties/log4j.properties");  
        servletContext.addListener(Log4jConfigListener.class);  
          
          
        //配置 CharacterEncodingFilter
        FilterRegistration.Dynamic characterEncodingFilter =
                servletContext.addFilter("characterEncodingFilter", CharacterEncodingFilter.class);
        characterEncodingFilter.setInitParameter("encoding", "UTF-8");
        characterEncodingFilter.setInitParameter("forceEncoding", "true");
        characterEncodingFilter.addMappingForUrlPatterns(
                EnumSet.of(DispatcherType.REQUEST, DispatcherType.FORWARD, DispatcherType.INCLUDE),
                false, "/*");
          
          
        //配置 statViewServlet  
        StatViewServlet statViewServlet = new StatViewServlet();
        ServletRegistration.Dynamic dynamic = servletContext.addServlet(
                "statViewServlet", statViewServlet);
        dynamic.setLoadOnStartup(2);
        dynamic.addMapping("/druid/*");
    }
}

亲测，即使将 Spring Boot 打包成 war，并部署到 Tomcat 8.5，这份配置也是有效的

配置策略二：ServletContextInitializer 的延伸

请看类继承体系

原理：最下边的三个子类会自动在运行时注册 Servlet、Listener、Filter（一定要将其定义为 Spring 容器的 Bean）

1. ServletRegistrationBean：在Servlet容器初始化时，向 ServletContext 注册一个自定义的 Servlet
2. ServletListenerRegistrationBean：在Servlet容器初始化时，向 ServletContext 注册一个自定义的 ServletContextListener
3. AbstractFilterRegistrationBean：（模板方法模式）通过模板方法 getFilter()，将 Filter 的构建过程延迟到子类，并在Servlet容器初始化时，向 ServletContext 注册该 Filter。开发者可以定义一个子类来重写该模板方法，以配置一个自定义的 Filter。

Servlet 配置示例

@Configuration
public class ServletConfig {

    //配置 StatViewServlet
    @Bean
    public ServletRegistrationBean servletRegistration0() {
        ServletRegistrationBean registration = new ServletRegistrationBean(new StatViewServlet());
        registration.addUrlMappings("/druid/*");
        registration.setLoadOnStartup(0);
        return registration;
    }
}

Filter 配置示例

@Configuration
public class FilterConfig {

    //配置 CharacterEncodingFilter
    @Bean
    public FilterRegistrationBean filterRegistration1() {
        FilterRegistrationBean registration = new FilterRegistrationBean(new CharacterEncodingFilter());
        registration.addUrlPatterns("/*");

        Map<String, String> initParameters = Maps.newHashMap();
        initParameters.put("encoding", "UTF-8");
        initParameters.put("forceEncoding", "true");
        registration.setInitParameters(initParameters);
        
        return registration;
    }
}

Listener 配置示例

@Configuration
public class ListenerConfig {

    //配置 RequestContextListener
    @Bean
    public ServletListenerRegistrationBean<RequestContextListener> listenerRegistration3() {
        return new ServletListenerRegistrationBean<>(
            new RequestContextListener());
    }
}