Filter过滤器的使用
本篇文章是介绍javax.servlet包中Filter的几个功能,一是字符编码过滤,二是拦截指文件夹中的文件,实现登录验证.实现以上两个功能需以下几个步骤:
1、实现javax.servlet.Filter接口，编写两个类，如下：

/**
 * 作用：过滤编码
 */
package filter;
import java.io.IOException;
import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
public class EncodingFilter implements Filter
{
    protected FilterConfig filterConfig;
    private String targetEncoding;
    public EncodingFilter()
    {
        targetEncoding = "gb2312";//直接初始化0912
    }
    public void init(FilterConfig filterconfig) throws ServletException
    {
        filterConfig = filterconfig;
        //targetEncoding = filterconfig.getInitParameter("encoding");//web.xml挂参初始化
    }
    public void doFilter(ServletRequest servletrequest, ServletResponse servletresponse, FilterChain filterchain)
        throws IOException, ServletException
    {
        HttpServletRequest httpservletrequest = (HttpServletRequest)servletrequest;
        httpservletrequest.setCharacterEncoding(targetEncoding);
        System.out.println("使用" targetEncoding "对请求进行编码过滤");
        filterchain.doFilter(servletrequest, servletresponse);
    }
    public void setFilterConfig(FilterConfig filterconfig)
    {
        filterConfig = filterconfig;
    }
    public void destroy()
    {
        filterConfig = null;
    }
}

/**
 * 作用：过滤外界非法的进入页面
 * 注意：使用该功能时，一定要把登录页与被过滤文件夹设为同级，否则会出现循环
 */
package filter;
import java.io.IOException;
import javax.servlet.*;
import javax.servlet.http.*;
public class ManageFilter implements Filter
{
    String LOGIN_PAGE;
    protected FilterConfig filterConfig;
    public ManageFilter()
    {
        LOGIN_PAGE = "/login.jsp";
    }
    public void doFilter(ServletRequest servletrequest, ServletResponse servletresponse, FilterChain filterchain)
        throws IOException, ServletException
    {
        HttpServletRequest request = (HttpServletRequest) servletrequest;
        HttpServletResponse response = (HttpServletResponse) servletresponse;
        HttpSession session = request.getSession();
        String key = "";
        try
        {
            //通过验证时在session中存的一个标志
     key = (String) session.getAttribute("key");
          
           
            if (key!= null&&key.equals("ok"))
            {
              
                filterchain.doFilter(servletrequest, servletresponse);
            }
            else
            {
                response.sendRedirect(LOGIN_PAGE);
                System.out.println("被拦截一个未认证的请求");
            }
        }
        catch (Exception exception)
        {
            exception.printStackTrace();
        }
    }
    public void setFilterConfig(FilterConfig filterconfig)
    {
        filterConfig = filterconfig;
    }
    public void destroy()
    {
        filterConfig = null;
    }
    public void init(FilterConfig filterconfig)
        throws ServletException
    {
        filterConfig = filterconfig;
    }
}

2、在web.xml中做一些设置
 <filter>
     <filter-name>code</filter-name>
     <filter-class>filter.EncodingFilter</filter-class>
 </filter>
 
 <filter>
     <filter-name>manage</filter-name>
     <filter-class>filter.ManageFilter</filter-class>
 </filter>
 <filter-mapping>
     <filter-name>code</filter-name>
     <url-pattern>/*</url-pattern>
 </filter-mapping>
 
 <filter-mapping>
     <filter-name>manage</filter-name>
     <!-- <url-pattern>/ShuiBaoRHPWK_NEW/system/ruhepaiwukou/*</url-pattern>-->
     <url-pattern>/servlets-examples/*</url-pattern>
 </filter-mapping>