Filter过滤器和Listener监听器详解

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");    }  }