spring mvc DispatcherServlet詳解之interceptor和filter的區別

首先咱們看一下spring mvc Interceptor的功能及實現：

http://wenku.baidu.com/link?url=Mw3GaUhCRMhUFjU8iIDhObQpDcbmmRy_IPeumazg0ppnbmwqFUtLp9kSpuPPpeysf6EnHBLYFeWrbjqMq8BlWKQz_7MSDhGQTVl32fpxCMm

SpringMVC 中的Interceptor 攔截器也是至關重要和至關有用的，它的主要做用是攔截用戶的請求並進行相應的處理，其餘的做用好比經過它來進行權限驗證，或者是來判斷用戶是否登錄，日誌記錄，或者限制時間點訪問。

SpringMVC 中的Interceptor 攔截請求是經過HandlerInterceptor 來實現的。在SpringMVC 中定義一個Interceptor 很是簡單，主要有兩種方式，第一種方式是要定義的Interceptor類要實現了Spring 的HandlerInterceptor 接口，或者是這個類繼承實現了HandlerInterceptor 接口的類，好比Spring 已經提供的實現了HandlerInterceptor 接口的抽象類HandlerInterceptorAdapter ；第二種方式是實現Spring的WebRequestInterceptor接口，或者是繼承實現了WebRequestInterceptor的類。

HandlerInterceptor 接口中定義了三個方法，咱們就是經過這三個方法來對用戶的請求進行攔截處理的。

   （1 ）preHandle (HttpServletRequest request, HttpServletResponse response, Object handle) 方法。該方法將在請求處理以前進行調用。SpringMVC 中的Interceptor 是鏈式的調用的，在一個應用中或者說是在一個請求中能夠同時存在多個Interceptor 。每一個Interceptor 的調用會依據它的聲明順序依次執行，並且最早執行的都是Interceptor 中的preHandle 方法，因此能夠在這個方法中進行一些前置初始化操做或者是對當前請求的一個預處理，也能夠在這個方法中進行一些判斷來決定請求是否要繼續進行下去。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。

   （2 ）postHandle (HttpServletRequest request, HttpServletResponse response, Object handle, ModelAndView modelAndView) 方法，由preHandle 方法的解釋咱們知道這個方法包括後面要說到的afterCompletion 方法都只能是在當前所屬的Interceptor 的preHandle 方法的返回值爲true 時才能被調用。postHandle 方法，顧名思義就是在當前請求進行處理以後，也就是Controller 方法調用以後執行，可是它會在DispatcherServlet 進行視圖返回渲染以前被調用，因此咱們能夠在這個方法中對Controller 處理以後的ModelAndView 對象進行操做。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，這和Struts2 裏面的Interceptor 的執行過程有點類型。Struts2 裏面的Interceptor 的執行過程也是鏈式的，只是在Struts2 裏面須要手動調用ActionInvocation 的invoke 方法來觸發對下一個Interceptor 或者是Action 的調用，而後每個Interceptor 中在invoke 方法調用以前的內容都是按照聲明順序執行的，而invoke 方法以後的內容就是反向的。

   （3 ）afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handle, Exception ex) 方法，該方法也是須要當前對應的Interceptor 的preHandle 方法的返回值爲true 時纔會執行。顧名思義，該方法將在整個請求結束以後，也就是在DispatcherServlet 渲染了對應的視圖以後執行。這個方法的主要做用是用於進行資源清理工做的。 咱們的系統日誌的攔截在這個方法中，能夠記錄日誌的相關的參數，檢測方法的執行。

 

在這裏，咱們就有一個疑問了：攔截器和過濾器到底有什麼不一樣呢？

首先，咱們看一下官方是怎麼解釋的：

public interface HandlerInterceptor
Workflow interface that allows for customized handler execution chains. Applications can register any number of existing or custom interceptors for certain groups of handlers, to add common preprocessing behavior without needing to modify each handler implementation.
A HandlerInterceptor gets called before the appropriate HandlerAdapter triggers the execution of the handler itself. This mechanism can be used for a large field of preprocessing aspects, e.g. for authorization checks, or common handler behavior like locale or theme changes. Its main purpose is to allow for factoring out repetitive handler code.

In an async processing scenario, the handler may be executed in a separate thread while the main thread exits without rendering or invoking the postHandle and afterCompletion callbacks. When concurrent handler execution completes, the request is dispatched back in order to proceed with rendering the model and all methods of this contract are invoked again. For further options and details see org.springframework.web.servlet.AsyncHandlerInterceptor

Typically an interceptor chain is defined per HandlerMapping bean, sharing its granularity. To be able to apply a certain interceptor chain to a group of handlers, one needs to map the desired handlers via one HandlerMapping bean. The interceptors themselves are defined as beans in the application context, referenced by the mapping bean definition via its "interceptors" property (in XML: a <list> of <ref>).

HandlerInterceptor is basically similar to a Servlet 2.3 Filter, but in contrast to the latter it just allows custom pre-processing with the option of prohibiting the execution of the handler itself, and custom post-processing. Filters are more powerful, for example they allow for exchanging the request and response objects that are handed down the chain. Note that a filter gets configured in web.xml, a HandlerInterceptor in the application context.

As a basic guideline, fine-grained handler-related preprocessing tasks are candidates for HandlerInterceptor implementations, especially factored-out common handler code and authorization checks. On the other hand, a Filter is well-suited for request content and view content handling, like multipart forms and GZIP compression. This typically shows when one needs to map the filter to certain content types (e.g. images), or to all requests.

 

public interface Filter
A filter is an object that performs filtering tasks on either the request to a resource (a servlet or static content), or on the response from a resource, or both. 

Filters perform filtering in the doFilter method. Every Filter has access to a FilterConfig object from which it can obtain its initialization parameters, a reference to the ServletContext which it can use, for example, to load resources needed for filtering tasks.

Filters are configured in the deployment descriptor of a web application

Examples that have been identified for this design are
1) Authentication Filters 
2) Logging and Auditing Filters 
3) Image conversion Filters 
4) Data compression Filters 
5) Encryption Filters 
6) Tokenizing Filters 
7) Filters that trigger resource access events 
8) XSL/T filters 
9) Mime-type chain Filter 

interceptor 和filter的概念類似，但主要不一樣點有：

web應用的過濾請求，僅使用web應用；

interceptor應用於特定組別的handler，能夠web應用也能夠企業應用；

從google找到的資料：http://www.linkedin.com/groups/what-is-difference-between-interceptor-3983267.S.5844715100472107010

Filter is used only in web applications whereas interceptor can be used with web as well as enterprise applications. Life cycle methods of both, also differs. The Interceptor stack fires on requests in a configured package while filters only apply to their mapped URL's. Example: A Servlet Filter is used in the web layer only, you can't use it outside of a web context. Interceptors can be used anywhere. The interceptor stack fires on every request. Filters only apply to the urls for which they are defined. Filters can be used when you want to modify any request or response parameters like headers. For example you would like to add a response header "Powered By Surya" to each generated response. Instead of adding this header in each resource method you would use a response filter to add this header. There are filters on the server side and the client side. In Summary: Filters: (1)Based on Servlet Specification (2)Executes on the pattern matches on the request. (3) Not configurable method calls. Interceptors: (1)Based on Struts2. (2)Executes for all the request qualifies for a front controller( A Servlet filter ).And can be configured to execute additional interceptor for a particular action execution. (3)Methods in the Interceptors can be configured whether to execute or not by means of excludemethods or includeMethods