A 2.3 Servlet Filter that rewrites URI's pointing to .js and .css files that run through HttpServletResponse#encodeURL(String) to contain hashes of the target files. This serves the purpose of assuring that caching headers set on those resources don't stop newer/changed versions of the resources from being downloaded by browsers.

CacheBustFilter

/*
 * Copyright 2014 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.apache.org/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 net.webapp.util;

import java.io.IOException;
import java.io.InputStream;
import java.util.*;
import java.util.zip.CRC32;
import java.util.zip.CheckedInputStream;

import javax.servlet.*;
import javax.servlet.http.*;

import org.apache.commons.io.IOUtils;
import org.springframework.util.AntPathMatcher;

/**
 * A 2.3 Servlet Filter that rewrites URI's pointing to .js and .css files that
 * run through {@link HttpServletResponse#encodeURL(String)} to point contain
 * hashes of the target files. This serves the purpose of assuring that caching
 * headers set on those resources don't stop newer/changed versions of the
 * resources from being downloaded by browsers.
 * 
 * We use a {@link CheckedInputStream} with a {@link CRC32} checksum to
 * calculate the hash. This limits memory usage. The rewritten filenames are
 * cached.
 * 
 * Depends on {@link AntPathMatcher} and {@link IOUtils}.
 * 
 * @author Arien Kock
 * @since 17.11.2014
 */
public class CacheBustFilter implements Filter {
	private final AntPathMatcher pathMatcher = new AntPathMatcher();
	private final String pattern = "/**/*-{hash:[\\w]+}.{ext:(js|css)}";
	private final Map<String, String> cachedNamesMap = new Hashtable<>();
	private ServletContext servletContext;
	private boolean enabled;
	private int contextPathEndIndex;

	/**
	 * The init method reads active Spring Profiles to see if a profile named
	 * dev is active and disables rewriting if there is. It's very important to
	 * disable the filter during development, because this filter does NOT watch
	 * the files for changes.
	 */
	@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		servletContext = filterConfig.getServletContext();
		enabled = !Arrays.asList(
				System.getProperty("spring.profiles.active", "").split(","))
				.contains("dev");
		contextPathEndIndex = servletContext.getContextPath().length();
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		if (enabled
				&& HttpServletRequest.class
						.isAssignableFrom(request.getClass())) {
			final String requestURI = ((HttpServletRequest) request)
					.getRequestURI();
			if (pathMatcher.match(pattern, requestURI)) {
				/**
				 * This request contains a hashed resource URI, we must remove
				 * the hash.
				 */
				String newUri = requestURI.substring(contextPathEndIndex,
						requestURI.lastIndexOf('-'))
						+ requestURI.substring(requestURI.lastIndexOf('.'));
				/**
				 * Forward this request to the default handling mechanism for
				 * the unhashed URI
				 */
				request.getRequestDispatcher(newUri).forward(request, response);
			} else {
				/**
				 * Any HTTP request may call
				 * {@link HttpServletResponse#encodeURL(String)} so we wrap the
				 * response object with our hashing logic.
				 */
				chain.doFilter(request, new HttpServletResponseWrapper(
						(HttpServletResponse) response) {
					@Override
					public String encodeURL(String url) {
						/**
						 * we obviously don't want to do double I/O whenever we
						 * request a resource, so we use a simple cache.
						 */
						String cachedMappedUrl = cachedNamesMap.get(url);
						if (cachedMappedUrl == null && isResource(url)) {
							/**
							 * No cache found and the URI actually points to a
							 * JS/CSS file
							 */
							InputStream stream = servletContext
									.getResourceAsStream(url.substring(contextPathEndIndex));
							if (stream != null) {
								try (CheckedInputStream checkedInputStream = new CheckedInputStream(
										stream, new CRC32())) {
									IOUtils.skip(checkedInputStream,
											Long.MAX_VALUE);
									String hexString = Long
											.toHexString(checkedInputStream
													.getChecksum().getValue());
									String hashedUrl = url.substring(0,
											url.lastIndexOf('.'))
											+ "-"
											+ hexString
											+ url.substring(url
													.lastIndexOf('.'));
									cachedNamesMap.put(url, hashedUrl);
									url = hashedUrl;
								} catch (IOException e) {
									throw new RuntimeException(e);
								}
							}
						} else if (cachedMappedUrl != null) {
							url = cachedMappedUrl;
						}
						return super.encodeURL(url);
					}
				});
			}
		} else {
			chain.doFilter(request, response);
		}
	}

	private boolean isResource(String requestUri) {
		return requestUri.endsWith(".css") || requestUri.endsWith(".js");
	}

	@Override
	public void destroy() {
	}

}

Dude, you're instantiating your wrapper for every request.