HandlerInterceptor翻译过来就是spring拦截器，它在某些功能应用上特别有用：
1. 用户是否登陆以及用户权限管理 （见http://www.ideawu.net/ideablog/category4/article174.html）
2. 根据用户的选择来决定是用HTML还是用Excel来作为View （该应用后面会讲解）
3. blackboard building block的应用：在每一个controller之前都要生成context，在render view之后都要release context
HandlerInterceptor接口有几个重要的方法：
preHandleAction(ActionRequest request, ActionResponse response, Object handler):
该方法会在Controller的方法执行前会被调用，可以使用这个方法来中断或者继续执行链的处理，当返回true时，处理执行链会继续，当返回false时，则不会去执行Controller的方法。（验证用户是否登陆就是使用preHandleAction方法的最好例子）
afterActionCompletion(ActionRequest request, ActionResponse response, Object handler, Exception ex)
preHandleRender(RenderRequest request, RenderResponse response, Object handler)
postHandleRender(RenderRequest request, RenderResponse response, Object handler, ModelAndView modelAndView)
这3个方法会在在controller的方法执行之后，在DispatcherServlet类导向到view进行render之前依次执行。最有用的是使用postHandleRender方法，因为它有ModelAndView 传进来，那么我们就可以在render view之前往view中添加额外的model对象，或者对view的去处进行修改（例如下面的“用HTML还是用Excel来作为View ”例子就是对view进行了更改）
afterRenderCompletion(RenderRequest request, RenderResponse response, Object handler, Exception ex)
该方法会在render view完成后执行，也可以说在请求过程（request processing）完成之后执行。该方法可以用来清理资源（例如象blackboard building block release context)
总结一下HandlerInterceptor的流程：
(DispatcherServlet maps a request to particular handler and assembles a handler execution chain consisting of the handler that is to be invoked and all of the HandlerInterceptor instances that apply to the request.)preHandleAction(..) is called; if the invocation of this method returns true then this workflow continuesThe target handler handles the action phase of the request (via HandlerAdapter.handleAction(..))afterActionCompletion(..) is calledpreHandleRender(..) is called; if the invocation of this method returns true then this workflow continuesThe target handler handles the render phase of the request (via HandlerAdapter.handleRender(..))postHandleRender(..) is calledafterRenderCompletion(..) is called
我们自己写的拦截器可以不直接实现HandlerInterceptor，而是扩展实现了HandlerInterceptor接口的具体类HandlerInterceptorAdapter，这样的话我们不需要把上面5个方法都实现，而只需要override我们需要的方法就可以了！
下面举一个使用拦截器来实现“用HTML还是用Excel来作为View ”功能的例子(只列出关键代码)：
search.html:
Search for: <input type="text" name="query"><br> Output in: <input type="radio name="format" value="xls"/>Excel or <input type="radio" name="format" value="html"/>Html<br> <input type="submit"/> SearchController.Java:
protected ModelAndView handleRequestInternal( HttpServletRequest req, HttpServletResponse response { String query = RequestUtils.getRequiredStringParameter("query"); List shows; // retrieve shows matching the query ModelAndView mav = new ModelAndView("listshows", "shows", shows); return mav; }OutputFormatModificationInterceptor.java:
public class OutputFormatModificationInterceptor extends HandlerInterceptorAdapter { private String parameter; private String defaultSuffix = ""; private Properties suffices; // setters and getters omitted public void postHandler( HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView mav) throws Exception { String format = request.getParameter("format"); String suffix = suffices.getProperty(format); // modify view name if exists if (suffix != null) { mav.setViewName(mav.getViewName() + suffix); //修改view达到使用不同的output格式的目的 } else { mav.setViewName(mav.getViewName() + defaultSuffix); } }configure xml file:
<bean class="org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping"> <property name="interceptors"> <list> <ref bean="formatInterceptor"/> </list> </property> </bean> <bean id="formatInterceptor" class="org.springframework.prospring.web.OutputFormatModificationInterceptor"> <property name="suffices"> <props> <entry key="xls">-xls</entry> <entry key="html">-html</entry> </props> </property> <property name="parameter"><value>format</value></property> <property name="defaultSuffix"><value>-html</value></property> </bean> <bean name="/listShows.action" class="org.springframework.prospring.web.controller.ListShowsController"> ... </bean> 那么在blackboard building block里可以使用下面的代码： public class BbContextInterceptor extends HandlerInterceptorAdapter { public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { Context ctx = BbServiceManager.getContextManager().setContext(request); request.setAttribute(requestAttributeKeyForBbContext, ctx); //check user is login or not is admin or not. if not, return false return true ; } public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception { request.removeAttribute(requestAttributeKeyForBbContext); BbServiceManager.getContextManager().purgeContext(); } }
