There's no guarantee that the content length you're provided is actually correct. Try something akin to the following:

ByteArrayOutputStream baos = new ByteArrayOutputStream();
InputStream is = null;
try {
  is = url.openStream ();
  byte[] byteChunk = new byte[4096]; // Or whatever size you want to read in at a time.
  int n;

  while ( (n = is.read(byteChunk)) > 0 ) {
    baos.write(byteChunk, 0, n);
  }
}
catch (IOException e) {
  System.err.printf ("Failed while reading bytes from %s: %s", url.toExternalForm(), e.getMessage());
  e.printStackTrace ();
  // Perform any other exception handling that's appropriate.
}
finally {
  if (is != null) { is.close(); }
}

You'll then have the image data in baos, from which you can get a byte array by calling baos.toByteArray().

This code is untested (I just wrote it in the answer box), but it's a reasonably close approximation to what I think you're after.

try following:

ServletOutputStream outStream = response.getOutputStream();
response.setContentType("application/zip");
response.setHeader("Content-Disposition", "attachment; filename="DATA.ZIP"");
outStream.write(decodedBytes);
outStream.flush();

Your way of doing it seems like absolutely ok.

But if you saying:

"However, this method call tends to take a relatively long time to run"

You can have follow problems :

• Network, connection issue

• Are you sure that download each file in separate thread?

If you are using multithreading for that, be sure that VM args -XmsYYYYM and -XmxYYYYM configured well, because if not you can face problem , that your processor not using all cores. I have faced this problem some time ago.

There are a couple of problems with your code:

• As already noted in a comment, you call openStream() twice.
• If an exception occurs, close() won't be called in your code. Use try-with-resources instead.
• Propagate exceptions to the caller. The caller will generally want to know the exception message.
• Don't ever use printStackTrace(). This is the worst way to report errors.
• After the first printStackTrace(), you continue with a null URI, which will cause a NullPointerException.
• The method should be static.

Here's how I would write this:

    private static byte[] getImageBytes(String imageUrl) throws IOException
    {
        URL url = new URL(imageUrl);

        ByteArrayOutputStream output = new ByteArrayOutputStream();

        try (InputStream stream = url.openStream())
        {
            byte[] buffer = new byte[4096];

            while (true)
            {
                int bytesRead = stream.read(buffer);
                if (bytesRead < 0) { break; }
                output.write(buffer, 0, bytesRead);
            }
        }

        return output.toByteArray();
    }