文章：
https://github.com/Snailclimb/springboot-guide/blob/master/docs/basis/springboot-filter.md
https://blog.csdn.net/weixin_39933264/article/details/100181291

demo源码：https://github.com/FDzhang/demo-study/tree/master/demo-springboot/demo-filter

2020-02-19

• 1 pom.xml 依赖

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

• 2 springboot的启动类添加注解 @ServletComponentScan

@SpringBootApplication
@ServletComponentScan
public class DemoFilterApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoFilterApplication.class, args);
    }
}

• 3 简单使用

/**
 * order(number)  : 过滤器的顺序
 * filterName : 过滤器名称
 * urlPatterns : 需要过滤的路径
 * initParams : 初始化参数，存于FilterConfig中
 */
@Order(1)
@WebFilter(filterName = "DemoFilter1", urlPatterns = "/*" , initParams = {})
public class DemoFilter1 implements Filter {
    /**
     * filter对象只会创建一次，init方法也只会执行一次。
     */
    @Override
    public void init(FilterConfig filterConfig) {
    }
    /**
     * 主要的业务代码编写方法
     */
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;

        // 获取请求参数中的cityCode
        String cityCode = request.getParameter("cityCode");

        // 如果 cityCode==001 则通过
        if ("001".equals(cityCode)){
            // 放行
            filterChain.doFilter(servletRequest,servletResponse);
        }else {
            // 返回错误信息
            Map<String,Object> map = new HashMap<>(4);
            map.put("code","301");
            map.put("message","cityCode错误");

            // 设置编码 和 数据格式
            response.setCharacterEncoding("UTF-8");
            response.setContentType("application/json; charset=utf-8");

            Gson gson = new Gson();
            response.getWriter().write(gson.toJson(map));
        }
    }
    /**
     * 在销毁Filter时自动调用。
     */
    @Override
    public void destroy() {
    }
}