SpringBoot 配置 Servlet、Filter、Listener

SpringBoot 配置 Servlet、Filter、Listener

在SpringBoot應用中，嵌入式的 Servlet 3.0+ 容器不會直接使用 ServletContainerInitializer 和 WebApplicationInitializer，即通過以上兩個接口實現的 Servlet、Filter、Listener 配置都是無效的，這是為了防止第三方代碼的設計損壞應用程序，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.

綜上，可以采取以下配置

配置策略一：ServletContextInitializer

由官方原文可知，我們可以使用替代方案：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/*");
    }
}

親測，即使將 Spring Boot 打包成 war，並部署到 Tomcat 8.5，這份配置也是有效的

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配置策略二：ServletContextInitializer 的延伸

請看類繼承體系

原理：最下邊的三個子類會自動在運行時注冊 Servlet、Listener、Filter（一定要將其定義為 Spring 容器的 Bean）

1. ServletRegistrationBean：在Servlet容器初始化時，向 ServletContext 注冊一個自定義的 Servlet
2. ServletListenerRegistrationBean：在Servlet容器初始化時，向 ServletContext 注冊一個自定義的 ServletContextListener
3. AbstractFilterRegistrationBean：（模板方法模式）通過模板方法 getFilter()，將 Filter 的構建過程延遲到子類，並在Servlet容器初始化時，向 ServletContext 注冊該 Filter。開發者可以定義一個子類來重寫該模板方法，以配置一個自定義的 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());
    }
}