1. Spring boot默认使用Tomcat作为嵌入式的servlet容器，只要引入spring-boot-starter-web依赖，就会默认用Tomcat作为servlet容器。

2. Spring boot默认以jar包的方式启动嵌入式的servlet容器，来启动spring boot的web应用，没有web.xml

3. 以往要注册servlet组件都是在web.xml文件中配置，现在没有了web.xml，该如何注册。

4. spring boot提供ServletRegistrationBean，FilterRegistrationBean以及ServletListenerRegistrationBean类，供我们向嵌入式的servlet容器中注册Servlet、Filter、Listener组件，获取完全控制。

🆗回顾一下，以往的web应用程序中，是如何来向servlet容器中注册servlet组件

自定义servlet组件

import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import java.io.IOException;
import java.io.PrintWriter;

public class MyServlet extends HttpServlet {

    @Override
    public void init() throws ServletException {
        System.out.println("配置自动加载后会自动执行init()方法");
    }

    @Override
    public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException {
        System.out.println("正在执行服务...");
        response.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=utf-8");
        PrintWriter out = response.getWriter();
        out.println("<a href='http://baidu.com'>百度一下</a>");
    }

}

在web.xml中注册servlet，并指定映射路径

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <!--注册MyServlet-->
    <servlet>
        <servlet-name>myServlet</servlet-name>
        <servlet-class>com.cd.servlet.MyServlet</servlet-class>
    </servlet>
    <!--指定MyServlet的映射路径-->
    <servlet-mapping>
        <servlet-name>myServlet</servlet-name>
        <url-pattern>/baidu</url-pattern>
    </servlet-mapping>

</web-app>

效果

/cd是项目的根路径(应用上下文路径)，/baidu是MyServlet这个servlet组件的映射路径

🆗接下来看看，spring boot是如何来向servlet容器中注册servlet组件

自定义servlet组件

import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import java.io.IOException;
import java.io.PrintWriter;

public class MyServlet extends HttpServlet {

    @Override
    public void init() throws ServletException {
        System.out.println("配置自动加载后会自动执行init()方法");
    }

    @Override
    public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException {
        System.out.println("正在执行服务...");
        response.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=utf-8");
        PrintWriter out = response.getWriter();
        out.println("<a href='http://baidu.com'>百度一下</a>");
    }

}

通过ServletRegistrationBean类向servlet容器注册servlet，并指定映射路径

import org.springframework.boot.web.servlet.ServletRegistrationBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class MyServletConfig {

    //注册servlet
    @Bean(name = "myServlet")
    public ServletRegistrationBean servletRegistrationBean(){
        //注册MyServlet类到servlet容器中，并指定映射路径
        ServletRegistrationBean servletRegistrationBean = new ServletRegistrationBean(new MyServlet(),"/baidu");
        return servletRegistrationBean;
    }
}

效果

/cd是项目的根路径(应用上下文路径)，/baidu是MyServlet这个servlet组件的映射路径

Servlet容器有三大组件servlet、filter、listener。前面我们讲了spring boot注册servlet组件的方式，下面顺便介绍filter组件的注册

自定义一个编码过滤器

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class MyFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("过滤器初始化. . .");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;
        request.setCharacterEncoding("utf-8");
        response.setCharacterEncoding("utf-8");
        //放行，继续执行过滤器链的下一个组件(下一个过滤器)
        filterChain.doFilter(request,response);
    }

}

通过spring boot提供的FilterRegistrationBean类，向servlet容器注册filter组件

import org.springframework.boot.web.servlet.FilterRegistrationBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import java.util.Arrays;

@Configuration
public class MyFilterConfig {

    //注册Filter
    @Bean(name = "myFilter")
    public FilterRegistrationBean filterRegistrationBean() {
        FilterRegistrationBean filterRegistrationBean = new FilterRegistrationBean();
        //指定要注册的Filter
        filterRegistrationBean.setFilter(new MyFilter());
        //指定映射路径
        filterRegistrationBean.setUrlPatterns(Arrays.asList("/*"));
        return filterRegistrationBean;
    }

}