package com.jyh.filter;
  import java.io.IOException;
  import javax.servlet.Filter;
  import javax.servlet.FilterChain;
  import javax.servlet.FilterConfig;
  import javax.servlet.ServletException;
  import javax.servlet.ServletRequest;
  import javax.servlet.ServletResponse;


  public class FilterDemo1 implements Filter {

    private FilterConfig filterConfig;

    public FilterDemo1(){
      System.out.println("实例化了");
    }

    //初始化
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
      //通过这个获取过滤器在web.xml里面的配置信息
      this.filterConfig = filterConfig;
      System.out.println("过滤器初始化了");
    }

    //执行
    @Override
    public void doFilter(ServletRequest req, ServletResponse res,
        FilterChain chain) throws IOException, ServletException {

      //获取在web.xml中本过滤器配置的名为encoding的参数
      String encoding = filterConfig.getInitParameter("encoding");
      if(encoding == null){
        encoding = "UTF-8";
      }
      //设置编码格式
      req.setCharacterEncoding(encoding);
      res.setContentType("text/html;charset=" + encoding);
      System.out.println("过滤前");
      //放行
      chain.doFilter(req, res);
      System.out.println("过滤后");
    }

    //销毁
    @Override
    public void destroy() {
      System.out.println("过滤器销毁了");
    }

  }

<filter>
  <filter-name>FilterDemo1</filter-name>
  <filter-class>com.jyh.filter.FilterDemo1</filter-class>
  <!-- 配置参数 -->
  <init-param>
    <param-name>encoding</param-name>
    <param-value>UTF-8</param-value>
  </init-param>
 </filter>
 <filter-mapping>
  <filter-name>FilterDemo1</filter-name>
  <url-pattern>/*</url-pattern>
 </filter-mapping>

ServletContextListener: 
HttpSessionListener: 
SerlvetRequestListener:

ServletContextAttributeListener 
HttpSessionAttributeListener 
SerlvetRequestAttributeListener

<listener>
   <!-- 指向自己缩写的监听器类 -->
  <listener-class>com.jyh.listener.MyContextListener</listener-class>
 </listener>