spring 攔截器

spring提供了HandlerInterceptor接口，裏面有三個個方法：preHandle，postHandle，afterCompletion

詳細註釋以下：

/**
* Intercept the execution of a handler. Called after HandlerMapping determined
* an appropriate handler object, but before HandlerAdapter invokes the handler.
* <p>DispatcherServlet processes a handler in an execution chain, consisting
* of any number of interceptors, with the handler itself at the end.
* With this method, each interceptor can decide to abort the execution chain,
* typically sending a HTTP error or writing a custom response.
* @param request current HTTP request
* @param response current HTTP response
* @param handler chosen handler to execute, for type and/or instance evaluation
* @return {@code true} if the execution chain should proceed with the
* next interceptor or the handler itself. Else, DispatcherServlet assumes
* that this interceptor has already dealt with the response itself.
* @throws Exception in case of errors
*/
boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception;



/**
* Intercept the execution of a handler. Called after HandlerAdapter actually
* invoked the handler, but before the DispatcherServlet renders the view.
* Can expose additional model objects to the view via the given ModelAndView.
* <p>DispatcherServlet processes a handler in an execution chain, consisting
* of any number of interceptors, with the handler itself at the end.
* With this method, each interceptor can post-process an execution,
* getting applied in inverse order of the execution chain.
* @param request current HTTP request
* @param response current HTTP response
* @param handler handler (or {@link HandlerMethod}) that started async
* execution, for type and/or instance examination
* @param modelAndView the {@code ModelAndView} that the handler returned
* (can also be {@code null})
* @throws Exception in case of errors
*/
void postHandle(
HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)
throws Exception;



/**
* Callback after completion of request processing, that is, after rendering
* the view. Will be called on any outcome of handler execution, thus allows
* for proper resource cleanup.
* <p>Note: Will only be called if this interceptor's {@code preHandle}
* method has successfully completed and returned {@code true}!
* <p>As with the {@code postHandle} method, the method will be invoked on each
* interceptor in the chain in reverse order, so the first interceptor will be
* the last to be invoked.
* @param request current HTTP request
* @param response current HTTP response
* @param handler handler (or {@link HandlerMethod}) that started async
* execution, for type and/or instance examination
* @param ex exception thrown on handler execution, if any
* @throws Exception in case of errors
*/
void afterCompletion(
HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
throws Exception;

　　簡單解釋一下就是 preHandle方法會攔截執行器的執行，具體是在HandlerMapping肯定一個適當的執行器對象以後、HandlerAdapter調用這個執行器以前；DispatcherServlet是在執行鏈中處理執行器的，執行鏈中有許多攔截器，依據他們的聲明順序依次執行，且都先執行prehandle方法，該方法的返回值是布爾值Boolean類型的，當它返回爲false 時，表示請求結束，後續的Interceptor 和Controller 都不會再執行；當返回值爲true 時就會繼續調用下一個Interceptor 的preHandle 方法，若是已是最後一個Interceptor 的時候就會是調用當前請求的Controller 方法。

　　postHandle方法在HandlerAdapter調用這個執行器以後、DispatcherServlet返回view以前調用。經過給定的modelAndView能夠暴露額外的模型對象給view。postHandle 方法被調用的方向跟preHandle 是相反的，也就是說先聲明的Interceptor 的postHandle 方法反而會後執行。

　　afterCompletion方法在請求處理完成以後，即渲染視圖以後執行，這個方法的主要做用是用於進行資源清理工做的。