Filter过滤器原理和登录实现

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;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * HttpFilter
 */
public abstract class HttpFilter implements Filter{

    //保存filterConfig对象
    private FilterConfig filterConfig;
    
    /**
     * 直接返回filterConfig对象
     * @return
     */
    public FilterConfig getFilterConfig() {
        return filterConfig;
    }
    
    /**
     * 不建议子类直接覆盖，若直接失败，将可能导致filterConfig成员变量初始化失败
     */
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {        
        this.filterConfig = filterConfig;    
        init();
    }

    /**
     * 供子类继承的初始化方法，刻通过getFilterConfig()方法获得filterConfig对象
     */
    private void init() {}

    /**
     * 原生的doFilter方法，在方法内部把ServletRequest和ServletResponse转化化为了HttpServletRequest和HttpServletResponse，
     * 并调用了doFilter(HttpServletRequest request, HttpServletResponse response,FilterChain filterChain)方法
     */
    @Override
    public void doFilter(ServletRequest req, ServletResponse res,
            FilterChain filterChain) throws IOException, ServletException {
        
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
        
        doFilter(request, response, filterChain);
    }
    
    /**
     * 抽象方法，为http请求定制，必须实现的方法
     * @param request
     * @param response
     * @param filterChain
     * @throws IOException
     * @throws ServletException
     */
    public abstract void doFilter(HttpServletRequest request, HttpServletResponse response,
            FilterChain filterChain) throws IOException, ServletException;

    @Override
    public void destroy() {}

}

public class RequestUtil {
    private  static  final  Logger  logger = LoggerFactory.getLogger(RequestUtil.class);
    private  static  final  Base64  base64 = new Base64(true);
    public  static  final  String  LAST_PAGE = "lastPage";//未登录时访问的页面
    public  static  final  String  REDIRECT_HOME = "/";//未登录时跳转到首页
    public  static  final  String  LOGIN_HOME = "/index.jsp";//登录成功后进入的页面
       
    
    /**
     * 保存当前请求
     */
    public static void saveRequest(HttpServletRequest request) {
        request.getSession().setAttribute(LAST_PAGE, RequestUtil.hashRequestPage(request));
        logger.debug("被拦截的url的sessionID:{}", request.getSession().getId());
        logger.debug("save request for {}", request.getRequestURI());
    }
    
    /**
     * 加密请求页面
     * @param request
     * @return
     */
    public static  String hashRequestPage(HttpServletRequest request) {
        String reqUri = request.getRequestURI();
        String query = request.getQueryString();
        if (query != null) {
            reqUri += "?" + query;
        }
        String targetPage = null;
        try {
            targetPage = base64.encodeAsString(reqUri.getBytes("UTF-8"));
        } catch (UnsupportedEncodingException ex) {
            //this does not happen
        }
        return targetPage;
    }
    
    /**
     * 取出之前保存的请求
     * @return
     */
    public static String retrieveSavedRequest(HttpServletRequest request) {
        HttpSession session = request.getSession();
        if (session == null) {
            return REDIRECT_HOME;
        }
        String HashedlastPage = (String) session.getAttribute(LAST_PAGE);
        if (HashedlastPage == null) {
            return LOGIN_HOME;
        } else {
            return retrieve(HashedlastPage);
        }
    }

    /**
     * 解密请求的页面
     * @param targetPage
     * @return
     */
    public static String retrieve(String targetPage) {
        byte[] decode = base64.decode(targetPage);
        try {
            String requestUri = new String(decode, "UTF-8");
            int i = requestUri.indexOf("/", 1);
            return requestUri.substring(i);
        } catch (UnsupportedEncodingException ex) {
            //this does not happen
            return null;
        }
    }
}