如何在java Filter 中注入 Service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：
public class WeiXinFilter implements Filter{ @Autowired PRivate Usersservice usersService; public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { HttpServletRequest req = (HttpServletRequest)request; HttpServletResponse resp = (HttpServletResponse)response; Users users = this.usersService.queryByOpenid(openid);
}
上面的 usersService 会报空指针异常。
解决方法：
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { HttpServletRequest req = (HttpServletRequest)request; HttpServletResponse resp = (HttpServletResponse)response; ServletContext sc = req.getsession().getServletContext(); xmlWebapplicationContext cxt = (XmlWebApplicationContext)WebApplicationContextUtils.getWebApplicationContext(sc); if(cxt != null && cxt.getBean("usersService") != null && usersService == null) usersService = (UsersService) cxt.getBean("usersService"); Users users = this.usersService.queryByOpenid(openid);
这样就行了。
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。
Filter 中注入 Service 的示例：
public class WeiXinFilter implements Filter{ private UsersService usersService; public void init(FilterConfig fConfig) throws ServletException {} public WeiXinFilter() {} public void destroy() {} public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { HttpServletRequest req = (HttpServletRequest)request; HttpServletResponse resp = (HttpServletResponse)response; String userAgent = req.getHeader("user-agent"); if(userAgent != null && userAgent.toLowerCase().indexOf("micromessenger") != -1){ // 微信浏览器 String servletPath = req.getServletPath(); String requestURL = req.getRequestURL().toString(); String queryString = req.getQueryString(); if(queryString != null){ if(requestURL.indexOf("mtzs.html") !=-1 && queryString.indexOf("LLFlag")!=-1){ req.getSession().setAttribute("LLFlag", "1"); chain.doFilter(request, response); return; } } String openidDES = CookieUtil.getValueByName("openid", req); String openid = null; if(StringUtils.isNotBlank(openidDES)){ try { openid = DesUtil.decrypt(openidDES, "rxxxxxxxxxde"); // 解密获得openid } catch (Exception e) { e.printStackTrace(); } } // ... ... String[] pathArray = {"/weixin/enterAppFromWeiXin.json", "/weixin/getWeiXinUserInfo.json", "/weixin/getaccessTokenAndOpenid.json", "/sendRegCode.json", "/register.json", "/login.json", "/logon.json", "/dump.json", "/queryInfo.json"}; List<String> pathList = Arrays.asList(pathArray); String loginSuccessUrl = req.getParameter("path"); String fullLoginSuccessUrl = "http://www.axxxxxxx.cn/pc/"; if(requestURL.indexOf("weixin_gate.html") != -1){ req.getSession().setAttribute("loginSuccessUrl", loginSuccessUrl);// ... ... } ServletContext sc = req.getSession().getServletContext();XmlWebApplicationContext cxt = (XmlWebApplicationContext)WebApplicationContextUtils.getWebApplicationContext(sc); if(cxt != null && cxt.getBean("usersService") != null && usersService == null) usersService = (UsersService) cxt.getBean("usersService"); Users users = this.usersService.queryByOpenid(openid); // ... ... if(pathList.contains(servletPath)){ // pathList 中的访问路径直接 pass chain.doFilter(request, response); return; }else{ if(req.getSession().getAttribute(CommonConstants.SESSION_KEY_USER) == null){ // 未登录 String llFlag = (String) req.getSession().getAttribute("LLFlag"); if(llFlag != null && llFlag.equals("1")){ // 处理游客浏览 chain.doFilter(request, response); return; } // ... ...// 3. 从腾讯服务器去获得微信的 openid , req.getRequestDispatcher("/weixin_gate.html").forward(request, response); return; }else{ // 已经登录 // 4. 已经登录时的处理 chain.doFilter(request, response); return; } } }else{ // 非微信浏览器 chain.doFilter(request, response); } }}