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);
}
}
}