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);
        }
    }

}