在Spring中，filter默认继承OncePerRequestFilter，
OncePerRequestFilter源代码如下：
/** Copyright 2002-2008 the original author or authors.** Licensed under the Apache License, Version 2.0 (the "License");* you may not use this file except in compliance with the License.* You may obtain a copy of the License at** http://www.apache.org/licenses/LICENSE-2.0** Unless required by applicable law or agreed to in writing, software* distributed under the License is distributed on an "AS IS" BASIS,* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.* See the License for the specific language governing permissions and* limitations under the License.*/package org.springframework.web.filter;import java.io.IOException;import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;/*** 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*/ public abstract class OncePerRequestFilter extends GenericFilterBean {/*** Suffix that gets appended to the filter name for the* "already filtered" request attribute.* @see #getAlreadyFilteredAttributeName*/public static final String ALREADY_FILTERED_SUFFIX = ".FILTERED";/*** This <code>doFilter</code> implementation stores a request attribute for* "already filtered", proceeding without filtering again if the* attribute is already there.* @see #getAlreadyFilteredAttributeName* @see #shouldNotFilter* @see #doFilterInternal*/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);}}}/*** Return the name of the request attribute that identifies that a request* is already filtered.* <p>Default implementation takes the configured name of the concrete filter* instance and appends ".FILTERED". If the filter is not fully initialized,* it falls back to its class name.* @see #getFilterName* @see #ALREADY_FILTERED_SUFFIX*/protected String getAlreadyFilteredAttributeName() {String name = getFilterName();if (name == null) {name = getClass().getName();}return name + ALREADY_FILTERED_SUFFIX;}/*** Can be overridden in subclasses for custom filtering control,* returning <code>true</code> to avoid filtering of the given request.* <p>The default implementation always returns <code>false</code>.* @param request current HTTP request* @return whether the given request should <i>not</i> be filtered* @throws ServletException in case of errors*/protected boolean shouldNotFilter(HttpServletRequest request) throws ServletException {return false;}/*** Same contract as for <code>doFilter</code>, but guaranteed to be* just invoked once per request. Provides HttpServletRequest and* HttpServletResponse arguments instead of the default ServletRequest* and ServletResponse ones.*/protected abstract void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)throws ServletException, IOException;}
OncePerRequestFilter，顾名思义，它能够确保在一次请求中只通过一次filter，而需要重复的执行。大家常识上都认为，一次请求本来就只filter一次，为什么还要由此特别限定呢，往往我们的常识和实际的实现并不真的一样，经过一番资料的查阅，此方法是为了兼容不同的web container，也就是说并不是所有的container都入我们期望的只过滤一次，servlet版本不同，执行过程也不同，
请看官方对OncePerRequestFilter的注释：
/*** 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*/
如：servlet2.3与servlet2.4也有一定差异：
在servlet2.3中，Filter会经过一切请求，包括服务器内部使用的forward转发请求和<%@ include file=”/login.jsp”%>的情况servlet2.4中的Filter默认情况下只过滤外部提交的请求，forward和include这些内部转发都不会被过滤，因此，为了兼容各种不同运行环境和版本，默认filter继承OncePerRequestFilter是一个比较稳妥的选择
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