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getting binary data after you set StringDecoder as pipeline decoder
pipeline.addLast("decoder", new StringDecoder());
    pipeline.addLast("encoder", new StringEncoder());

public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
      // Save data received from the server.
      Object msg = e.getMessage();
      byte[] rawdata = new byte[((ChannelBuffer)msg).readableBytes()];
      ((ChannelBuffer)msg).getBytes(0, rawdata);
      tmpTarFile.write(rawdata);
    }

ChannelBuffer databuffer = ChannelBuffers.buffer(blobstream.size());
        databuffer.writeBytes(blobstream.toByteArray());
        e.getChannel().write(databuffer);

public class FlexibleStringDecoder extends StringDecoder {

    @Override
    protected Object decode(ChannelHandlerContext ctx, Channel channel, Object msg) throws Exception {
        if (decodeToString(msg)) {
            return super.decode(ctx, channel, msg);
        }
        return msg;
    }

    public boolean decodeToString(Object msg) {
        // Add some logic here....
        return true;
    }

}