OncePerRequestFilter有什么作用

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。

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);  
            }  
        }  
    }  

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。