OncePerRequestFilter

来源：http://ully.iteye.com/blog/1334925

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。

Java代码  

1. public final void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)   
2.             throws ServletException, IOException {   
3.   
4.         if (!(request instanceof HttpServletRequest) || !(response instanceof HttpServletResponse)) {   
5.             throw new ServletException("OncePerRequestFilter just supports HTTP requests");   
6.         }   
7.         HttpServletRequest httpRequest = (HttpServletRequest) request;   
8.         HttpServletResponse httpResponse = (HttpServletResponse) response;   
9.   
10.         String alreadyFilteredAttributeName = getAlreadyFilteredAttributeName();   
11.         if (request.getAttribute(alreadyFilteredAttributeName) != null || shouldNotFilter(httpRequest)) {   
12.             // Proceed without invoking this filter...   
13.             filterChain.doFilter(request, response);   
14.         }   
15.         else {   
16.             // Do invoke this filter...   
17.             request.setAttribute(alreadyFilteredAttributeName, Boolean.TRUE);   
18.             try {   
19.                 doFilterInternal(httpRequest, httpResponse, filterChain);   
20.             }   
21.             finally {   
22.                 // Remove the "already filtered" request attribute for this request.   
23.                 request.removeAttribute(alreadyFilteredAttributeName);   
24.             }   
25.         }   
26.     }  

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);
			}
		}
	}

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。