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在项目中遇到一个问题，在 Filter中注入 Serivce失败，注入的service始终为null。如下所示：
1 public class WeiXinFilter implements Filter{ 2 3 @Autowired 4 private UsersService usersService; 5 6 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { 7 HttpServletRequest req = (HttpServletRequest)request; 8 HttpServletResponse resp = (HttpServletResponse)response; 9 Users users = this.usersService.queryByOpenid(openid); 10 }
上面的 usersService 会报空指针异常。
解决方法一：
1 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { 2 HttpServletRequest req = (HttpServletRequest)request; 3 HttpServletResponse resp = (HttpServletResponse)response; 4 ServletContext sc = req.getSession().getServletContext(); 5 XmlWebApplicationContext cxt = (XmlWebApplicationContext)WebApplicationContextUtils.getWebApplicationContext(sc); 6 7 if(cxt != null && cxt.getBean("usersService") != null && usersService == null) 8 usersService = (UsersService) cxt.getBean("usersService"); 9 10 Users users = this.usersService.queryByOpenid(openid);
解决方法二：
public class WeiXinFilter implements Filter{ private UsersService usersService; public void init(FilterConfig fConfig) throws ServletException { ServletContext sc = fConfig.getServletContext(); XmlWebApplicationContext cxt = (XmlWebApplicationContext)WebApplicationContextUtils.getWebApplicationContext(sc); if(cxt != null && cxt.getBean("usersService") != null && usersService == null) usersService = (UsersService) cxt.getBean("usersService"); }
相关原理：
1. 如何获取 ServletContext：
1）在javax.servlet.Filter中直接获取
ServletContext context = config.getServletContext();
2）在HttpServlet中直接获取
this.getServletContext()
3）在其他方法中，通过HttpServletRequest获得
request.getSession().getServletContext();
2. WebApplicationContext 与 ServletContext (转自：http://blessht.iteye.com/blog/2121845)：
Spring的 ContextLoaderListener是一个实现了ServletContextListener接口的监听器，在启动项目时会触发contextInitialized方法（该方法主要完成ApplicationContext对象的创建），在关闭项目时会触发contextDestroyed方法（该方法会执行ApplicationContext清理操作）。
ConextLoaderListener加载Spring上下文的过程
①启动项目时触发contextInitialized方法，该方法就做一件事：通过父类contextLoader的initWebApplicationContext方法创建Spring上下文对象。
②initWebApplicationContext方法做了三件事：创建 WebApplicationContext；加载对应的Spring文件创建里面的Bean实例；将WebApplicationContext放入 ServletContext（就是Java Web的全局变量）中。
③createWebApplicationContext创建上下文对象，支持用户自定义的上下文对象，但必须继承自ConfigurableWebApplicationContext，而Spring MVC默认使用ConfigurableWebApplicationContext作为ApplicationContext（它仅仅是一个接口）的实 现。
④configureAndRefreshWebApplicationContext方法用 于封装ApplicationContext数据并且初始化所有相关Bean对象。它会从web.xml中读取名为 contextConfigLocation的配置，这就是spring xml数据源设置，然后放到ApplicationContext中，最后调用传说中的refresh方法执行所有Java对象的创建。
⑤完成ApplicationContext创建之后就是将其放入ServletContext中，注意它存储的key值常量。
解决方法三：
直接使用spring mvc中的HandlerInterceptor或者HandlerInterceptorAdapter 来替换Filter:
public class WeiXinInterceptor implements HandlerInterceptor { @Autowired private UsersService usersService; @Override public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { // TODO Auto-generated method stub return false; } @Override public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception { // TODO Auto-generated method stub } @Override public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception { // TODO Auto-generated method stub } }
配置拦截路径：
<mvc:interceptors> <mvc:interceptor> <mvc:mapping path="/**" /> <bean class="net.xxxx.interceptor.WeiXinInterceptor" /> </mvc:interceptor> </mvc:interceptors>
Filter 中注入 Service 的示例：
1 public class WeiXinFilter implements Filter{ 2 private UsersService usersService; 3 public void init(FilterConfig fConfig) throws ServletException {} 4 public WeiXinFilter() {} 5 public void destroy() {} 6 7 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { 8 HttpServletRequest req = (HttpServletRequest)request; 9 HttpServletResponse resp = (HttpServletResponse)response; 10 11 String userAgent = req.getHeader("user-agent"); 12 if(userAgent != null && userAgent.toLowerCase().indexOf("micromessenger") != -1){ // 微信浏览器 13 String servletPath = req.getServletPath(); 14 String requestURL = req.getRequestURL().toString(); 15 String queryString = req.getQueryString(); 16 17 if(queryString != null){ 18 if(requestURL.indexOf("mtzs.html") !=-1 && queryString.indexOf("LLFlag")!=-1){ 19 req.getSession().setAttribute("LLFlag", "1"); 20 chain.doFilter(request, response); 21 return; 22 } 23 } 24 25 String openidDES = CookieUtil.getValueByName("openid", req); 26 String openid = null; 27 if(StringUtils.isNotBlank(openidDES)){ 28 try { 29 openid = DesUtil.decrypt(openidDES, "rxxxxxxxxxde"); // 解密获得openid 30 } catch (Exception e) { 31 e.printStackTrace(); 32 } 33 } 34 // ... ... 35 String[] pathArray = {"/weixin/enterAppFromWeiXin.json", "/weixin/getWeiXinUserInfo.json", 36 "/weixin/getAccessTokenAndOpenid.json", "/sendRegCode.json", "/register.json", 37 "/login.json", "/logon.json", "/dump.json", "/queryInfo.json"}; 38 List<String> pathList = Arrays.asList(pathArray); 39 40 String loginSuccessUrl = req.getParameter("path"); 41 String fullLoginSuccessUrl = "http://www.axxxxxxx.cn/pc/"; 42 if(requestURL.indexOf("weixin_gate.html") != -1){ 43 req.getSession().setAttribute("loginSuccessUrl", loginSuccessUrl); 44 // ... ... 45 } 46 ServletContext sc = req.getSession().getServletContext(); 47 XmlWebApplicationContext cxt = (XmlWebApplicationContext)WebApplicationContextUtils.getWebApplicationContext(sc); 48 49 if(cxt != null && cxt.getBean("usersService") != null && usersService == null) 50 usersService = (UsersService) cxt.getBean("usersService"); 51 52 Users users = this.usersService.queryByOpenid(openid); 53 // ... ... 54 if(pathList.contains(servletPath)){ // pathList 中的访问路径直接 pass 55 chain.doFilter(request, response); 56 return; 57 }else{ 58 if(req.getSession().getAttribute(CommonConstants.SESSION_KEY_USER) == null){ // 未登录 59 String llFlag = (String) req.getSession().getAttribute("LLFlag"); 60 if(llFlag != null && llFlag.equals("1")){ // 处理游客浏览 61 chain.doFilter(request, response); 62 return; 63 } 64 // ... ...// 3. 从腾讯服务器去获得微信的 openid , 65 req.getRequestDispatcher("/weixin_gate.html").forward(request, response); 66 return; 67 }else{ // 已经登录 68 // 4. 已经登录时的处理 69 chain.doFilter(request, response); 70 return; 71 } 72 } 73 }else{ // 非微信浏览器 74 chain.doFilter(request, response); 75 } 76 } 77 78 }
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原文地址：http://www.cnblogs.com/skyer-2013/p/5856280.html
