在spring中，filter都默认继承OncePerRequestFilter，但为什么要这样呢？
OncePerRequestFilter顾名思义，他能够确保在一次请求只通过一次filter，而不需要重复执行。
- 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)) {
-
- filterChain.doFilter(request, response);
- }
- else {
-
- request.setAttribute(alreadyFilteredAttributeName, Boolean.TRUE);
- try {
- doFilterInternal(httpRequest, httpResponse, filterChain);
- }
- finally {
-
- request.removeAttribute(alreadyFilteredAttributeName);
- }
- }
- }
大家常识上都认为，一次请求本来就只过一次，为什么还要由此特别限定呢，呵呵实际上我们常识和实际的实现并不真的一样，经过一番查阅后，此方式是为 了兼容不同的web container，特意而为之（jsr168），也就是说并不是所有的container都像我们期望的只过滤一次，servlet版本不同，表现也不 同：。
写道
/**
* 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
*/
如，servlet2.3与servlet2.4也有一定差异
写道
在servlet-2.3中，Filter会过滤一切请求，包括服务器内部使用forward转发请求和<%@ include file="/index.jsp"%>的情况。
到了servlet-2.4中Filter默认下只拦截外部提交的请求，forward和include这些内部转发都不会被过滤，但是有时候我们需要 forward的时候也用到Filter。
因此，为了兼容各种不同的运行环境和版本，默认filter继承OncePerRequestFilter是一个比较稳妥的选择。
