在Java filter中调用service层方法

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);

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：

 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);
         }
     }

 }