在spring中，filter都默認(rèn)繼承OncePerRequestFilter，但為什么要這樣呢？

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OncePerRequestFilter顧名思義，他能夠確保在一次請求只通過一次filter，而不需要重復(fù)執(zhí)行。

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Java代碼??

public?final?void?doFilter(ServletRequest?request,?ServletResponse?response,?FilterChain?filterChain)??

????????????throws?ServletException,?IOException?{??

??

????????if?(!(request?instanceof?HttpServletRequest)?||?!(response?instanceof?HttpServletResponse))?{??

????????????throw?new?ServletException("OncePerRequestFilter?just?supports?HTTP?requests");??

????????}??

????????HttpServletRequest?httpRequest?=?(HttpServletRequest)?request;??

????????HttpServletResponse?httpResponse?=?(HttpServletResponse)?response;??

??

????????String?alreadyFilteredAttributeName?=?getAlreadyFilteredAttributeName();??

????????if?(request.getAttribute(alreadyFilteredAttributeName)?!=?null?||?shouldNotFilter(httpRequest))?{??

????????????//?Proceed?without?invoking?this?filter...??

????????????filterChain.doFilter(request,?response);??

????????}??

????????else?{??

????????????//?Do?invoke?this?filter...??

????????????request.setAttribute(alreadyFilteredAttributeName,?Boolean.TRUE);??

????????????try?{??

????????????????doFilterInternal(httpRequest,?httpResponse,?filterChain);??

????????????}??

????????????finally?{??

????????????????//?Remove?the?"already?filtered"?request?attribute?for?this?request.??

????????????????request.removeAttribute(alreadyFilteredAttributeName);??

????????????}??

????????}??

????}??

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大家常識上都認(rèn)為，一次請求本來就只過一次，為什么還要由此特別限定呢，呵呵實(shí)際上我們常識和實(shí)際的實(shí)現(xiàn)并不真的一樣，經(jīng)過一番查閱后，此方式是為了兼容不同的web container，特意而為之（jsr168），也就是說并不是所有的container都像我們期望的只過濾一次，servlet版本不同，表現(xiàn)也不同：。

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?寫道

/**
* Filter base class that guarantees to be just executed once per request,
* on any servlet container. It provides a {@link #doFilterInternal}
* method with HttpServletRequest and HttpServletResponse arguments.
*
* <p>The {@link #getAlreadyFilteredAttributeName} method determines how
* to identify that a request is already filtered. The default implementation
* is based on the configured name of the concrete filter instance.
*
* @author Juergen Hoeller
* @since 06.12.2003
*/

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如，servlet2.3與servlet2.4也有一定差異

?寫道

在servlet-2.3中，Filter會過濾一切請求，包括服務(wù)器內(nèi)部使用forward轉(zhuǎn)發(fā)請求和<%@ include file="/index.jsp"%>的情況。

到了servlet-2.4中Filter默認(rèn)下只攔截外部提交的請求，forward和include這些內(nèi)部轉(zhuǎn)發(fā)都不會被過濾，但是有時候我們需要 forward的時候也用到Filter。

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因此，為了兼容各種不同的運(yùn)行環(huán)境和版本，默認(rèn)filter繼承OncePerRequestFilter是一個比較穩(wěn)妥的選擇。

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