GAE

download file

private byte[] downloadFile(String downloadUrl) throws MalformedURLException,
        IOException, ProtocolException {
    URL url = new URL(downloadUrl);
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    connection.setRequestMethod("GET");
    ReadableByteChannel src = Channels.newChannel(connection.getInputStream());
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    WritableByteChannel dest = Channels.newChannel(baos);
    ChannelTools.fastChannelCopy(src, dest);
    
    src.close();
    dest.close();
    
    int statusCode = connection.getResponseCode();
    if (statusCode != HttpURLConnection.HTTP_OK) {
        String msg = String.format("Server returned with error: %s(%d)", HttpStatus.valueOf(statusCode).name(), statusCode);
        log.error(msg);
        throw new IOException(msg);
    }
    
    return baos.toByteArray();
}

public class ChannelTools {

    public static void fastChannelCopy(final ReadableByteChannel src, final WritableByteChannel dest) throws IOException {
        final ByteBuffer buffer = ByteBuffer.allocateDirect(16 * 1024);
        while (src.read(buffer) != -1) {
            // prepare the buffer to be drained
            buffer.flip();
            // write to the channel, may block
            dest.write(buffer);
            // If partial transfer, shift remainder down
            // If buffer is empty, same as doing clear()
            buffer.compact();
        }
        // EOF will leave buffer in fill state
        buffer.flip();
        // make sure the buffer is fully drained.
        while (buffer.hasRemaining()) {
            dest.write(buffer);
        }
    }
}