一：编写过滤代码，如下

package com.sm.global;


import javax.servlet.*;
import java.io.IOException;

public class EncodingFilter implements Filter {
//    1：默认编码
    private String encoding="UTF-8";

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
//       2:如果配置文件配置了编码格式，则赋值，没有的话，使用默认
        if (filterConfig.getInitParameter("ENCODING")!=null)
        encoding=filterConfig.getInitParameter("ENCODING");

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
//        3：请求和响应分别设置编码格式
        servletRequest.setCharacterEncoding(encoding);
        servletResponse.setCharacterEncoding(encoding);
//        4：继续过滤下一个
        filterChain.doFilter(servletRequest,servletResponse);
    }

    @Override
    public void destroy() {
        encoding=null;
    }
}

二：配置web.xml文件，如下

<?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_3_1.xsd"
         version="3.1">


  <filter>
    <filter-name>Encoding</filter-name>
    <filter-class>com.sm.global.EncodingFilter</filter-class>
    <init-param>
      <param-name>ENCODING</param-name>
      <param-value>UTF-8</param-value>
    </init-param>
  </filter>

  <filter-mapping>
    <filter-name>Encoding</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>


</web-app>