最近解決大檔案下載的問題，遇到一個"Out of memory"的exception。建廠controller層的程式碼，發現是用BufferdOutputStream寫入Response中的，緩衝區也只有8m，按理說不應該出現記憶體溢位的。

閱讀程式碼發現問題就出在這個過濾器中，這個過濾器中會將Buffered流轉換成ByteArray流寫入Response。而ByteArrayOutputStream都儲存記憶體中，還需要頻繁的擴容。這在大檔案下載的時候自然會記憶體溢位。

解決方案

考慮到大部分url還是需要該攔截器進行過濾的，只是需要排除掉跟檔案下載相關的url。所以這裡OneCoder決定複寫該Filter，設定一個黑名單，複寫其中的doFilterInternal方法，對於黑名單中的url都直接傳遞給下一個filter，否則super一下，繼續走原來的邏輯。

/**
 * The filter is used for resolving the big file download problem when using
 * {@link ShallowEtagHeaderFilter}. The urls on the black list will be passed
 * directly to the next filter in the chain, the others will be filtered as
 * before.
 * <p>
 * Sample:<br>
 * {@code <filter>}<br>
 * &amp;nbsp&amp;nbsp&amp;nbsp {@code<filter-name>BigFileEtagFilter</filter-name>}<br>
 * &amp;nbsp&amp;nbsp&amp;nbsp
 * {@code<filter-class>com.coderli.filter.BigFileDownloadEtagHeaderFilter</filter-class>}<br>
 * &amp;nbsp&amp;nbsp&amp;nbsp {@code<!-- url sperators includes: blank space ; , and /r/n.
 * Black list is optional.>}<br>
 * &amp;nbsp&amp;nbsp&amp;nbsp {@code<init-param>}<br>
 * &amp;nbsp&amp;nbsp&amp;nbsp&amp;nbsp&amp;nbsp&amp;nbsp {@code<param-name>blackListURL</param-name>}<br>
 * &amp;nbsp&amp;nbsp&amp;nbsp&amp;nbsp&amp;nbsp&amp;nbsp
 * {@code <param-value> /aa /bb/** /cc/* </param-value>}<br>
 * &amp;nbsp&amp;nbsp&amp;nbsp {@code</init-param>}<br>
 * {@code</filter>}<br>
 * {@code<filter-mapping>}<br>
 * &amp;nbsp&amp;nbsp&amp;nbsp {@code<filter-name>BigFileEtagFilter</filter-name>}<br>
 * &amp;nbsp&amp;nbsp&amp;nbsp {@code<url-pattern>/*</url-pattern>}<br>
 * {@code</filter-mapping>}
 * 
 * @author 
 
[email protected]
 * @date 2014-9-12 9:46:38
 */
public class BigFileDownloadEtagHeaderFilter extends ShallowEtagHeaderFilter {

	private final String[] NULL_STRING_ARRAY = new String[0];
	private final String URL_SPLIT_PATTERN = &quot;[, ;\r\n]&quot;;

	private final PathMatcher pathMatcher
 
 = new AntPathMatcher();

	private final Logger logger = LoggerFactory
			.getLogger(BigFileDownloadEtagHeaderFilter.class);
	// url while list
	// private String[] whiteListURLs = null;
	// url black list
	private String[] blackListURLs = null;

	@Override
	public final void initFilterBean() {
		initConfig();
	}

	@Override
	protected void doFilterInternal(HttpServletRequest request,
			HttpServletResponse response, FilterChain filterChain)
			throws ServletException, IOException {
		String reqUrl = request.getPathInfo();
		if (isBlackURL(reqUrl)) {
			logger.debug(&quot;Current url  {} is on the black list.&quot;, reqUrl);
			filterChain.doFilter(request, response);
		} else {
			super.doFilterInternal(request, response, filterChain);
		}
	}

	private void initConfig() {
		// No need white list now.
		// String whiteListURLStr = getFilterConfig().getInitParameter(
		// &quot;whiteListURL&quot;);
		// whiteListURLs = strToArray(whiteListURLStr);
		String blackListURLStr = getFilterConfig().getInitParameter(
				&quot;blackListURL&quot;);
		blackListURLs = strToArray(blackListURLStr);
	}

	// No need white list now.
	// private boolean isWhiteURL(String currentURL) {
	// for (String whiteURL : whiteListURLs) {
	// if (pathMatcher.match(whiteURL, currentURL)) {
	// logger.debug(
	// &quot;url filter : white url list matches : [{}] match [{}] continue&quot;,
	// whiteURL, currentURL);
	// return true;
	// }
	// logger.debug(
	// &quot;url filter : white url list not matches : [{}] match [{}]&quot;,
	// whiteURL, currentURL);
	// }
	// return false;
	// }

	private boolean isBlackURL(String currentURL) {
		for (String blackURL : blackListURLs) {
			if (pathMatcher.match(blackURL, currentURL)) {
				logger.debug(
						&quot;url filter : black url list matches : [{}] match [{}] break&quot;,
						blackURL, currentURL);
				return true;
			}
			logger.debug(
					&quot;url filter : black url list not matches : [{}] match [{}]&quot;,
					blackURL, currentURL);
		}
		return false;
	}

	private String[] strToArray(String urlStr) {
		if (urlStr == null) {
			return NULL_STRING_ARRAY;
		}
		String[] urlArray = urlStr.split(URL_SPLIT_PATTERN);
		List<String> urlList = new ArrayList<String>();

		for (String url : urlArray) {
			url = url.trim();
			if (url.length() == 0) {
				continue;
			}
			urlList.add(url);
		}
		return urlList.toArray(NULL_STRING_ARRAY);
	}
}

關於ShallowEtagHeaderFilter這個“Bug”，OneCoder發現網上也有人向spring反應了，不過好像Spring方面認為這是使用問題，不作為bug來進行處理，那我們就自己解決一下。