filter注入service报空指针

来源：互联网发布：手机mac是什么意思啊编辑：程序博客网时间：2024/05/23 19:14

转载自：http://www.cnblogs.com/digdeep/p/4770004.html?tvd

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。

解决方法一：

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。

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：

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);        }    }}

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