Abstract template for a static resource servlet

Partly based on this blog from 2007, here's a modernized and highly reusable abstract template for a servlet which properly deals with caching, ETag , If-None-Match and If-Modified-Since (but no Gzip and Range support; just to keep it simple; Gzip could be done with a filter or via container configuration).

public abstract class StaticResourceServlet extends HttpServlet { private static final long serialVersionUID = 1L; private static final long ONE_SECOND_IN_MILLIS = TimeUnit.SECONDS.toMillis(1); private static final String ETAG_HEADER = "W/\"%s-%s\""; private static final String CONTENT_DISPOSITION_HEADER = "inline;filename=\"%1$s\"; filename*=UTF-8''%1$s"; public static final long DEFAULT_EXPIRE_TIME_IN_MILLIS = TimeUnit.DAYS.toMillis(30); public static final int DEFAULT_STREAM_BUFFER_SIZE = 102400; @Override protected void doHead(HttpServletRequest request, HttpServletResponse response) throws ServletException ,IOException { doRequest(request, response, true); } @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doRequest(request, response, false); } private void doRequest(HttpServletRequest request, HttpServletResponse response, boolean head) throws IOException { response.reset(); StaticResource resource; try { resource = getStaticResource(request); } catch (IllegalArgumentException e) { response.sendError(HttpServletResponse.SC_BAD_REQUEST); return; } if (resource == null) { response.sendError(HttpServletResponse.SC_NOT_FOUND); return; } String fileName = URLEncoder.encode(resource.getFileName(), StandardCharsets.UTF_8.name()); boolean notModified = setCacheHeaders(request, response, fileName, resource.getLastModified()); if (notModified) { response.sendError(HttpServletResponse.SC_NOT_MODIFIED); return; } setContentHeaders(response, fileName, resource.getContentLength()); if (head) { return; } writeContent(response, resource); } /** * Returns the static resource associated with the given HTTP servlet request. This returns <code>null</code> when * the resource does actually not exist. The servlet will then return a HTTP 404 error. * @param request The involved HTTP servlet request. * @return The static resource associated with the given HTTP servlet request. * @throws IllegalArgumentException When the request is mangled in such way that it's not recognizable as a valid * static resource request. The servlet will then return a HTTP 400 error. */ protected abstract StaticResource getStaticResource(HttpServletRequest request) throws IllegalArgumentException; private boolean setCacheHeaders(HttpServletRequest request, HttpServletResponse response, String fileName, long lastModified) { String eTag = String.format(ETAG_HEADER, fileName, lastModified); response.setHeader("ETag", eTag); response.setDateHeader("Last-Modified", lastModified); response.setDateHeader("Expires", System.currentTimeMillis() + DEFAULT_EXPIRE_TIME_IN_MILLIS); return notModified(request, eTag, lastModified); } private boolean notModified(HttpServletRequest request, String eTag, long lastModified) { String ifNoneMatch = request.getHeader("If-None-Match"); if (ifNoneMatch != null) { String[] matches = ifNoneMatch.split("\\s*,\\s*"); Arrays.sort(matches); return (Arrays.binarySearch(matches, eTag) > -1 || Arrays.binarySearch(matches, "*") > -1); } else { long ifModifiedSince = request.getDateHeader("If-Modified-Since"); return (ifModifiedSince + ONE_SECOND_IN_MILLIS > lastModified); // That second is because the header is in seconds, not millis. } } private void setContentHeaders(HttpServletResponse response, String fileName, long contentLength) { response.setHeader("Content-Type", getServletContext().getMimeType(fileName)); response.setHeader("Content-Disposition", String.format(CONTENT_DISPOSITION_HEADER, fileName)); if (contentLength != -1) { response.setHeader("Content-Length", String.valueOf(contentLength)); } } private void writeContent(HttpServletResponse response, StaticResource resource) throws IOException { try ( ReadableByteChannel inputChannel = Channels.newChannel(resource.getInputStream()); WritableByteChannel outputChannel = Channels.newChannel(response.getOutputStream()); ) { ByteBuffer buffer = ByteBuffer.allocateDirect(DEFAULT_STREAM_BUFFER_SIZE); long size = 0; while (inputChannel.read(buffer) != -1) { buffer.flip(); size += outputChannel.write(buffer); buffer.clear(); } if (resource.getContentLength() == -1 && !response.isCommitted()) { response.setHeader("Content-Length", String.valueOf(size)); } } } }

Use it together with the below interface representing a static resource.

interface StaticResource { /** * Returns the file name of the resource. This must be unique across all static resources. If any, the file * extension will be used to determine the content type being set. If the container doesn't recognize the * extension, then you can always register it as <code><mime-type></code> in <code>web.xml</code>. * @return The file name of the resource. */ public String getFileName(); /** * Returns the last modified timestamp of the resource in milliseconds. * @return The last modified timestamp of the resource in milliseconds. */ public long getLastModified(); /** * Returns the content length of the resource. This returns <code>-1</code> if the content length is unknown. * In that case, the container will automatically switch to chunked encoding if the response is already * committed after streaming. The file download progress may be unknown. * @return The content length of the resource. */ public long getContentLength(); /** * Returns the input stream with the content of the resource. This method will be called only once by the * servlet, and only when the resource actually needs to be streamed, so lazy loading is not necessary. * @return The input stream with the content of the resource. * @throws IOException When something fails at I/O level. */ public InputStream getInputStream() throws IOException; }

All you need is just extending from the given abstract servlet and implementing the getStaticResource() method according the javadoc.

Concrete example serving from file system:

Here's a concrete example which serves it via an URL like /files/foo.ext from the local disk file system:

@WebServlet("/files/*") public class FileSystemResourceServlet extends StaticResourceServlet { private File folder; @Override public void init() throws ServletException { folder = new File("/path/to/the/folder"); } @Override protected StaticResource getStaticResource(HttpServletRequest request) throws IllegalArgumentException { String pathInfo = request.getPathInfo(); if (pathInfo == null || pathInfo.isEmpty() || "/".equals(pathInfo)) { throw new IllegalArgumentException(); } String name = URLDecoder.decode(pathInfo.substring(1), StandardCharsets.UTF_8.name()); final File file = new File(folder, Paths.get(name).getFileName().toString()); return !file.exists() ? null : new StaticResource() { @Override public long getLastModified() { return file.lastModified(); } @Override public InputStream getInputStream() throws IOException { return new FileInputStream(file); } @Override public String getFileName() { return file.getName(); } @Override public long getContentLength() { return file.length(); } }; } }

Concrete example serving from database:

Here's a concrete example which serves it via an URL like /files/foo.ext from the database via an EJB service call which returns your entity having a byte[] content property: