Spring HandlerInterceptor的使用

标签： spring exception object workflow string bean
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 continues
The target handler handles the action phase of the request (via HandlerAdapter.handleAction(..))
afterActionCompletion(..) is called
preHandleRender(..) is called; if the invocation of this method returns true then this workflow continues
The target handler handles the render phase of the request (via HandlerAdapter.handleRender(..))
postHandleRender(..) is called
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:

 

<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();
    }
}