OncePerRequestFilter的作用

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