package demo;
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 DemoFilter implements Filter
{
  private String name=null;
  @Override
  public void destroy()
  {
  }
  @Override
  public void doFilter(ServletRequest arg0, ServletResponse arg1,
      FilterChain arg2) throws IOException, ServletException
  {
    arg0.setAttribute("name", "init text string : " + name);
    arg2.doFilter(arg0, arg1);
  }
  @Override
  public void init(FilterConfig arg0) throws ServletException
  {
    //获得web.xml中filter init-param 中的配置
    name = arg0.getInitParameter("name");
  }
}

  <!--声明一个filter-->
  <filter>
  <!--filter的名称-->
    <filter-name>demoFilter</filter-name>
  <!--filter 类-->
    <filter-class>demo.DemoFilter</filter-class>
  <!--该filter类需要的配置-->
    <init-param>
      <param-name>name</param-name>
      <param-value>my name is tanyong</param-value>
    </init-param>
  </filter>

  <!--为url-pattern 中相匹配的url链接 映射至相应的filter filter 通过 filter-name来识别-->
  <filter-mapping>
    <filter-name>demoFilter</filter-name>
    <!--匹配映射规则，我这里用的是所有请求都要在demoFilter filter 里面去兜晃一圈-->
    <url-pattern>/*</url-pattern>
  </filter-mapping>