前言

本文章将介绍SpringBoot注册Servlet的三大组件

一、介绍

由于SpringBoot默认是以jar包的方式运行嵌入式Servlet容器来启动应用，没有web.xml文件，
所以Spring提供以下Bean来注册三大组件。

ServletRegistrationBean 注册自定义Servlet
FilterRegistrationBean 注册自定义Filter
ServletListenerRegistrationBean 注册自定义Listener

二、注入Bean

2.1.ServletRegistrationBean

首先我们需要一个类继承至HttpServlet

public class MyServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("get请求进入servlet");
        resp.getWriter().println("<h1>get:hello world</h1>");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("post请求进入servlet");
        resp.getWriter().println("<h1>post:hello world</h1>");
    }
}

然后在配置类中注入ServletRegistrationBean

@Configuration
public class SpringBootConfig {
@Bean
    public ServletRegistrationBean doRegiste(){
        ServletRegistrationBean bean = new ServletRegistrationBean();
        //将自己创建的继承至HttpServlet的类添加进ServletRegistrationBean 
        bean.setServlet(new MyServlet());
        //设置请求路径
        bean.setUrlMappings(Arrays.asList("/myServlet"));
        //设置优先级
        bean.setLoadOnStartup(1);
        return bean;
    }
}

2.2.FilterRegistrationBean

首先我们需要一个类实现Filter接口，重写它的doFilter方法

public class MyFilter implements Filter {
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        System.out.println("============请求过滤");

        request.setCharacterEncoding("utf-8");

        //分水岭
        chain.doFilter(request, response);


        response.setCharacterEncoding("utf-8");

        System.out.println("============响应过滤");
    }
}

在配置类中将FilterRegistrationBean注入

@Configuration
public class SpringBootConfig {
    //注册Servlet
    //替换：@WebServlet(urlPatterns = "/myServlet")
    @Bean
    public ServletRegistrationBean doRegiste(){
        ServletRegistrationBean bean = new ServletRegistrationBean();
        bean.setServlet(new MyServlet());
        bean.setUrlMappings(Arrays.asList("/myServlet"));
        bean.setLoadOnStartup(1);
        return bean;
    }

    //注册Filter
    //替换//@WebFilter(urlPatterns = {"/*"})
    @Bean
    public FilterRegistrationBean doFilter(){
        FilterRegistrationBean bean = new FilterRegistrationBean();
        bean.setFilter(new MyFilter());
        bean.addUrlPatterns("/*");
        return bean;
    }
}

2.3.ServletListenerRegistrationBean

首先需要一个类实现ServletContextListener接口，重现它的contextInitialized和contextDestroyed方法监听程序初始化和销毁

public class MyListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        ServletContextListener.super.contextInitialized(sce);
        System.out.println("-------------MyListener inited !");
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        ServletContextListener.super.contextDestroyed(sce);
        System.out.println("----------------MyListener Destroy !");
    }
}

在配置类中注入ServletListenerRegistrationBean

@Configuration
public class SpringBootConfig {
    //注册Servlet
    //替换：@WebServlet(urlPatterns = "/myServlet")
    @Bean
    public ServletRegistrationBean doRegiste(){
        ServletRegistrationBean bean = new ServletRegistrationBean();
        bean.setServlet(new MyServlet());
        bean.setUrlMappings(Arrays.asList("/myServlet"));
        bean.setLoadOnStartup(1);
        return bean;
    }

    //注册Filter
    //替换//@WebFilter(urlPatterns = {"/*"})
    @Bean
    public FilterRegistrationBean doFilter(){
        FilterRegistrationBean bean = new FilterRegistrationBean();
        bean.setFilter(new MyFilter());
        bean.addUrlPatterns("/*");
        return bean;
    }

    //注册Listener
    //替换//@WebListener
    @Bean
    public ServletListenerRegistrationBean doListener(){
        ServletListenerRegistrationBean bean = new ServletListenerRegistrationBean();
        bean.setListener(new MyListener());
        return bean;
    }
}