在Spring中，filter默認繼承OncePerRequestFilter，

OncePerRequestFilter源代碼如下：

/*

* 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.wendangku.net/doc/ed56956148d7c1c708a1453c.html/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.

*

*

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 JuergenHoeller

* @since 06.12.2003