import javax.servlet.FilterChain;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletException;
import javax.servlet.FilterConfig;
/**
 *
 * @author Dory Doo
 */
public class EncodingFillter implements Filter
{
    protected FilterConfig filterConfig;
    private String targetEncoding="gb2312";
    /**
     *初始化过滤器像一般的Servlet一�它也可以获得初始化参�br />    */
    public void init(FilterConfig config)throws ServletException
    {
        this.filterConfig=config;
        this.targetEncoding=config.getInitParameter("encoding");
    }
    /**
     *˜q›è¡Œ˜q‡æ×o处理åQŒæœ€æœ€è¦çš„地方ž®±æ˜¯˜q™é‡Œäº?br />    */
    public void doFilter(ServletRequest srequest,ServletResponse sresponse,FilterChain chain)
    throws IOException,ServletException
    {
        System.out.println("使用以下æ–ÒŽ(gu¨©)³•˜q›è¡Œ¾~–码:encoding="+targetEncoding);
       
        HttpServletRequest request=(HttpServletRequest)srequest;
        request.setCharacterEncoding(targetEncoding);
        //把处理权发送到下一�br />        chain.doFilter(srequest,sresponse);
    }
   
    public void setFilterConfig(final FilterConfig filterConfig)
    {
        this.filterConfig=filterConfig;
    }
    //销毁过滤器
    public void destroy()
    {
        this.filterConfig=null;
    }
}
然后在web.xml中配¾|®æˆ‘们的应用.
 <web-app>
      .................
        <!--Filter Config-->
       <filter>
            <filter-name>cncoding</filter-name>
            <filter-class>dorydoo.util.EncodingFilter</filter-class>
            <init-param>
                <param-name>encoding</param-name>
                <param-value>gb2312</param-value>
            </init-param>
        </filter>  
        <filter-mapping>
            <filter-name>encoding</filter-name>
            <servlet-name>action</servlet-name>
        </filter-mapping> 
        <filter-mapping>
            <filter-name>encoding</filter-name>
            <url-pattern>/*</url-pattern>
        </filter-mapping>
      ........
 </web-app>


在来看一用户认证的过滤器(SignonFilter)
package dorydoo.util;

import javax.servlet.FilterChain;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletException;
import javax.servlet.FilterConfig;
import javax.servlet.http.HttpSession;
/**
 *
 * @author DuYang
 */
public class SignonFilter implements Filter
{
    protected FilterConfig filterConfig;
    String LOGIN_PAGE="login.jsp";
    /**
     *初始化过滤器像一般的Servlet一�它也可以获得初始化参�br />    */
    public void init(FilterConfig config)throws ServletException
    {
        this.filterConfig=config;
    }
    /**
     *˜q›è¡Œ˜q‡æ×o处理åQŒæœ€æœ€è¦çš„地方ž®±æ˜¯˜q™é‡Œäº?br />    */
    public void doFilter(final ServletRequest req,final ServletResponse res,FilterChain chain)
    throws IOException,ServletException
    {    
        HttpServletRequest hreq=(HttpServletRequest)req;
        HttpServletResponse hres=(HttpServletResponse)res;
        HttpSession session=hreq.getSession();
        String isLogin="";
        try
        {
            isLogin=(String)session.getAttribute("isLogin");
            if(isLogin.equals("true"))
            {
                System.out.println("在SignonFilter中验证通过");
                //验证通过¾l§ç®‹å¤„理
                chain.doFilter(req,res);
            }
            else
            {
                //验证不成功重新登�br />                hres.sendRedirect(LOGIN_PAGE);
                System.out.println("被SignonFilter拦截一个äؓ认证的请æ±?);
            }
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }
   
    public void setFilterConfig(final FilterConfig filterConfig)
    {
        this.filterConfig=filterConfig;
    }
    //销毁过滤器
    public void destroy()
    {
        this.filterConfig=null;
    }
}
在SignonFilterçš„doFilter()æ–ÒŽ(gu¨©)³•ä¸?首先通过isLogin=(String)session.getAttribute("isLogin");判断是否ç™Õd½•ç”¨æˆ·,如果不是则返回到l(f¨¡)ogin™åµé¢.配置web.xmlæ–‡äšgž®±å’Œä¸Šé¢çš„基本上没什么差åˆ?