注册Servlet三大组件

由于Spring Boot默认是以jar包的方式启动嵌入式的Servlet容器从而启动web应用的，因此没有web.xml文件。
如果需要注册Spring Boot三大组件（Servlet、Filter、Listener）就必须依赖于Spring Boot为我们提供的对应的配置类。

• ServletRegistrationBean Servlet注册Bean
• FilterRegistrationBean Filter注册Bean
• ServletListenerRegistrationBean Listener注册Bean

注册Servlet

// 创建自定义MyServlet类，实现定制的Servlet功能
public class MyServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws Exception {
        // 定制返回消息
        resp.getWriter().write("Spring Boot, Hello MyServlet...");
    }
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws Exception {
        // get请求直接转发到post
        doPost(req, resp);
    }
}
// 创建配置类，注入ServletRegistrationBean
@Configuration
public class MyServerConfiguration {
    @Bean  // 必须注入到容器才能生效
    public ServletRegistrationBean myServlet(){
        // 引入自定义Servlet，并绑定请求映射uri
        return new ServletRegistrationBean(new MyServlet(), "/helloMyServlet");
    }
}

注册Filter

Filter简介

// 创建自定义的Filter类，实现自定义过滤功能
public class MyFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, 
                         FilterChain filterChain) throws IOException, ServletException {
        System.out.println("Spring Boot, Hello MyFilter...");
        // 处理完成后，继续向下执行
        filterChain.doFilter(servletRequest, servletResponse);
    }
}
// 创建配置类，注入FilterRegistrantionBean
@Configuration
public class MyServerConfiguration {
    @Bean   // 必须注入到容器才能生效
    public FilterRegistrationBean myFiler(){
        FilterRegistrationBean filterRegistrationBean = new FilterRegistrationBean();
        // 引入自定义Filter
        filterRegistrationBean.setFilter(new MyFilter());
        // 指明要拦截过滤的请求
        filterRegistrationBean.setUrlPatterns(Arrays.asList("/myFilter", "/helloMyFilter"));
        return filterRegistrationBean;
    }
}

注册Listener

// 创建自定义的Listener类，实现自定义监听功能
public class MyListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("contextInitialized...  WEB应用启动了！");
    }
    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("contextDestroyed...  当前WEB应用销毁了！");
    }
}
// 创建配置类，注入ServletListenerRegistrationBean
@Configuration
public class MyServerConfiguration {
    @Bean
    public ServletListenerRegistrationBean myListener(){
        ServletListenerRegistrationBean<MyListener> listener = 
            new ServletListenerRegistrationBean<>(new MyListener());
        return listener;
    }
}

Spring Boot帮我们自动配置SpringMVC的时候，也自动的注册SpringMVC的前端控制器DIspatcherServlet。

@ConditionalOnClass({DispatcherServlet.class})
@AutoConfigureAfter({ServletWebServerFactoryAutoConfiguration.class})
public class DispatcherServletAutoConfiguration {
    @Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
    @ConditionalOnBean(value = DispatcherServlet.class, 
                       name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
    public ServletRegistrationBean dispatcherServletRegistration(
                                                DispatcherServlet dispatcherServlet) {
        ServletRegistrationBean registration = new ServletRegistrationBean(
                                dispatcherServlet, this.serverProperties.getServletMapping());
        // 默认拦截 / 所有请求，包静态资源，但是不拦截jsp请求     /* 拦截所有请求，包括jsp
        // 可以通过server.servletPath来修改SpringMVC前端控制器默认拦截的请求路径
        registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
           registration.setLoadOnStartup(this.webMvcProperties.getServlet().getLoadOnStartup());
           if (this.multipartConfig != null) {
              registration.setMultipartConfig(this.multipartConfig);
        }
           return registration;
    }
}