servletfilterlistener">

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

servletcontextinitializer">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

---

servletcontextinitializer-的延伸">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());
    }
}

SpringBoot 中配置 Servlet、Filter、Listener源码  OnRefresh()中 调用createWeb()....调用ServletWebServerApplicationContext.selfInitialize方法中getServletContextInitializerBeans中完成从spring容器中把Servlet、Filter、Listener封装成ServletContextInitializer接口实例，后再调用ServletContextInitializer的实例方法onStartUp，完成Servlet、Filter、Listener配置。