Java过滤器

 

一个过滤器是一个可以传送请求或者修改相应的对象。

过滤器并不是Servlet,它们并不实际创建一个请求。它们是请求到达一个Servlet前的预处理程序，在相应离开Servlet后的后处理程序。

一个过滤器能够做如下的工作：

（1）在一个servlet被调用前截获该调用

（2）在一个servlet被调用前检查请求

（3）修改在实际请求中提供了可定制请求对象的请求头和请求数据

（4）修改在实际响应中提供了可定制响应对象的响应头和响应数据

（5）在一个servlet被调用之后截获该调用

一个过滤器实现java.servlet.Filter接口

init:           在过滤器执行service前被调用，以设置过滤器的配置对象

destroy:    在过滤器执行service后被调用

doFilter:    执行实际的过滤工作

下面给个例子，使用这个过滤器来计算执行某个servlet所用时间并记录在日志内：

import java.io.*; import javax.servlet.*; import.servlet.http.*; //实现一个过滤器 public class TimerFilter implements Filter{ private FilterConfig config = null; public void init(FilterConfig config) throws ServletException{ this.config = config; } public void destroy(){ config = null; } public void doFilter(ServletRequest request,ServletResponse response,FilterChain chain)throws IOException,ServletException{ long before = System.currentTimeMillis(); chain.doFilter(request,response); long after = System.currentTimeMillis(); String name = ""; if(request instanceof HttpServletRequset){ name = ((HttpServletRequest)request).getRequestURI(); } //记录时间 config.getServletContext().log(name+": "+(after-before) + "ms"); } }

 

使用此过滤器，还需要在web.xml文件中用<filter>标签部署它

 

<filter>

  <filter-name>timerFilter</filter-name>

  <filter-class>TimerFilter</filter-class>

</filter>

 

 

一、使浏览器不缓存页面的过滤器 import javax.servlet.*; import javax.servlet.http.HttpServletResponse; import java.io.IOException; /** * 用于的使 Browser 不缓存页面的过滤器 */ public class ForceNoCacheFilter implements Filter { public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException, ServletException { ((HttpServletResponse) response).setHeader("Cache-Control","no-cache"); ((HttpServletResponse) response).setHeader("Pragma","no-cache"); ((HttpServletResponse) response).setDateHeader ("Expires", -1); filterChain.doFilter(request, response); } public void destroy() { } public void init(FilterConfig filterConfig) throws ServletException { } } 二、检测用户是否登陆的过滤器 import javax.servlet.*; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpSession; import java.util.List; import java.util.ArrayList; import java.util.StringTokenizer; import java.io.IOException; /** * 用于检测用户是否登陆的过滤器，如果未登录，则重定向到指的登录页面 * 配置参数 * checkSessionKey 需检查的在 Session 中保存的关键字 * redirectURL 如果用户未登录，则重定向到指定的页面，URL不包括 ContextPath * notCheckURLList 不做检查的URL列表，以分号分开，并且 URL 中不包括 ContextPath */ public class CheckLoginFilter implements Filter { protected FilterConfig filterConfig = null; private String redirectURL = null; private List notCheckURLList = new ArrayList(); private String sessionKey = null; public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { HttpServletRequest request = (HttpServletRequest) servletRequest; HttpServletResponse response = (HttpServletResponse) servletResponse; HttpSession session = request.getSession(); if(sessionKey == null) { filterChain.doFilter(request, response); return; } if((!checkRequestURIIntNotFilterList(request)) && session.getAttribute(sessionKey) == null) { response.sendRedirect(request.getContextPath() + redirectURL); return; } filterChain.doFilter(servletRequest, servletResponse); } public void destroy() { notCheckURLList.clear(); } private boolean checkRequestURIIntNotFilterList(HttpServletRequest request) { String uri = request.getServletPath() + (request.getPathInfo() == null ? "" : request.getPathInfo()); return notCheckURLList.contains(uri); } public void init(FilterConfig filterConfig) throws ServletException { this.filterConfig = filterConfig; redirectURL = filterConfig.getInitParameter("redirectURL"); sessionKey = filterConfig.getInitParameter("checkSessionKey"); String notCheckURLListStr = filterConfig.getInitParameter("notCheckURLList"); if(notCheckURLListStr != null) { StringTokenizer st = new StringTokenizer(notCheckURLListStr, ";"); notCheckURLList.clear(); while(st.hasMoreTokens()) { notCheckURLList.add(st.nextToken()); } } } } 三、字符编码的过滤器 import javax.servlet.*; import java.io.IOException; /** * 用于设置 HTTP 请求字符编码的过滤器，通过过滤器参数encoding指明使用何种字符编码,用于处理Html Form请求参数的中文问题 */ public class CharacterEncodingFilter implements Filter { protected FilterConfig filterConfig = null; protected String encoding = ""; public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { if(encoding != null) servletRequest.setCharacterEncoding(encoding); filterChain.doFilter(servletRequest, servletResponse); } public void destroy() { filterConfig = null; encoding = null; } public void init(FilterConfig filterConfig) throws ServletException { this.filterConfig = filterConfig; this.encoding = filterConfig.getInitParameter("encoding"); } } 四、资源保护过滤器 package catalog.view.util; import javax.servlet.Filter; import javax.servlet.FilterConfig; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.http.HttpServletRequest; import java.io.IOException; import java.util.Iterator; import java.util.Set; import java.util.HashSet; // import org.apache.commons.logging.Log; import org.apache.commons.logging.LogFactory; /** * This Filter class handle the security of the application. * * It should be configured inside the web.xml. * * @author Derek Y. Shen */ public class SecurityFilter implements Filter { //the login page uri private static final String LOGIN_PAGE_URI = "login.jsf"; //the logger object private Log logger = LogFactory.getLog(this.getClass()); //a set of restricted resources private Set restrictedResources; /** * Initializes the Filter. */ public void init(FilterConfig filterConfig) throws ServletException { this.restrictedResources = new HashSet(); this.restrictedResources.add("/createProduct.jsf"); this.restrictedResources.add("/editProduct.jsf"); this.restrictedResources.add("/productList.jsf"); } /** * Standard doFilter object. */ public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException { this.logger.debug("doFilter"); String contextPath = ((HttpServletRequest)req).getContextPath(); String requestUri = ((HttpServletRequest)req).getRequestURI(); this.logger.debug("contextPath = " + contextPath); this.logger.debug("requestUri = " + requestUri); if (this.contains(requestUri, contextPath) && !this.authorize((HttpServletRequest)req)) { this.logger.debug("authorization failed"); ((HttpServletRequest)req).getRequestDispatcher(LOGIN_PAGE_URI).forward(req, res); } else { this.logger.debug("authorization succeeded"); chain.doFilter(req, res); } } public void destroy() {} private boolean contains(String value, String contextPath) { Iterator ite = this.restrictedResources.iterator(); while (ite.hasNext()) { String restrictedResource = (String)ite.next(); if ((contextPath + restrictedResource).equalsIgnoreCase(value)) { return true; } } return false; } private boolean authorize(HttpServletRequest req) { //处理用户登录 /* UserBean user = (UserBean)req.getSession().getAttribute(BeanNames.USER_BEAN); if (user != null && user.getLoggedIn()) { //user logged in return true; } else { return false; }*/ } }