Question

The JavaEE API comes with the HttpServletResponseWrapper which, to quote the javadoc, “provides a convenient implementation of the HttpServletResponse interface that can be subclassed by developers wishing to adapt the response from a Servlet.” Without subclassing, this class just passes all calls through to the wrapped response object. There’s a similar wrapper for requests.

Can anyone point me at any utility libraries that provide useful subclass implementations of these classes? Particularly, I’m looking for a subclass of the response wrapper that captures the written response (as a String, byte[], whatever’s appropriate) and exposes it via an API method.

I’ve written a rather half-baked version myself, but frankly, I shouldn’t have to, and I’d rather throw it away and use one off the shelf.

Answer 1

I am not aware of any implementation, even though the gzip example can be adapted easily by just writing to a ByteArrayOutputStream. You can also take ideas by looking at other response wrapper implementations at:

• SiteMesh
• DWR also uses it

Original answer:

There is the classic article in JavaWorld Filter code with Servlet 2.3 model. You can find examples for wrapped request and response:

• Compression the response

  public class CompressionResponseWrapper extends HttpServletResponseWrapper {
    protected ServletOutputStream stream = null;
    protected PrintWriter writer = null;
    protected int threshold = 0;
    protected HttpServletResponse origResponse = null;
    public CompressionResponseWrapper(HttpServletResponse response) {
  super(response);
  origResponse = response;
    }
    public void setCompressionThreshold(int threshold) {
  this.threshold = threshold;
    }
    public ServletOutputStream createOutputStream() throws IOException {
  return (new CompressionResponseStream(origResponse));
    }
    public ServletOutputStream getOutputStream() throws IOException {
  if (writer != null) {
    throw new IllegalStateException("getWriter() has already been " +
                                    "called for this response");
  }
  if (stream == null) {
    stream = createOutputStream();
  }
  ((CompressionResponseStream) stream).setCommit(true);
  ((CompressionResponseStream) stream).setBuffer(threshold);
  return stream;
    }
    public PrintWriter getWriter() throws IOException {
  if (writer != null) {
    return writer;
  }
  if (stream != null) {
    throw new IllegalStateException("getOutputStream() has already " +
                                    "been called for this response");
  }
  stream = createOutputStream();
  ((CompressionResponseStream) stream).setCommit(true);
  ((CompressionResponseStream) stream).setBuffer(threshold);
  writer = new PrintWriter(stream);
  return writer;
    }
  }
  

• Handling file upload

  public class MultipartWrapper extends HttpServletRequestWrapper {
    MultipartRequest mreq = null;
    public MultipartWrapper(HttpServletRequest req, String dir)
                                   throws IOException {
  super(req);
  mreq = new MultipartRequest(req, dir);
    }
    // Methods to replace HSR methods
    public Enumeration getParameterNames() {
  return mreq.getParameterNames();
    }
    public String getParameter(String name) {
  return mreq.getParameter(name);
    }
    public String[] getParameterValues(String name) {
  return mreq.getParameterValues(name);
    }
    public Map getParameterMap() {
  Map map = new HashMap();
  Enumeration enum = getParameterNames();
  while (enum.hasMoreElements()) {
    String name = (String) enum.nextElement();
    map.put(name, mreq.getParameterValues(name));
  }
  return map;
    }
    // Methods only in MultipartRequest
    public Enumeration getFileNames() {
  return mreq.getFileNames();
    }
    public String getFilesystemName(String name) {
  return mreq.getFilesystemName(name);
    }
    public String getContentType(String name) {
  return mreq.getContentType(name);
    }
    public File getFile(String name) {
  return mreq.getFile(name);
    }
  }
  

The code is a bit old (june 2001!), but it demonstrate the usage well.