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）

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：

1. (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.)
2. preHandleAction(..) is called; if the invocation of this method returns true then this workflow continues
3. The target handler handles the action phase of the request (via HandlerAdapter.handleAction(..))
4. afterActionCompletion(..) is called
5. preHandleRender(..) is called; if the invocation of this method returns true then this workflow continues
6. The target handler handles the render phase of the request (via HandlerAdapter.handleRender(..))
7. postHandleRender(..) is called
8. afterRenderCompletion(..) 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: