File is corrupted when I send it to servlet using GZIPOutputStream

Question

I tried changing @BalusC's great tutorial here to send gziped compressed files. This is the java working class:

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.util.zip.GZIPOutputStream;

public final class NetworkService {

    // *** EDIT THOSE AS APROPRIATE
    private static final String FILENAME = "C:/Dropbox/TMP.txt";
    private static final String URL =
            "http://192.168.1.64:8080/DataCollectionServlet/";
    // *** END EDIT
    private static final CharSequence CRLF = "\r\n";
    private static boolean isServerGzip = true; // ***
    private static String charsetForMultipartHeaders = "UTF-8";

    public static void main(String[] args) {
        HttpURLConnection connection = null;
        OutputStream serverOutputStream = null;
        try {
            File file = new File(FILENAME);
            final String boundary = Long
                    .toHexString(System.currentTimeMillis());
            connection = connection(true, boundary);
            serverOutputStream = connection.getOutputStream();
            try {
                flushMultiPartData(file, serverOutputStream, boundary);
            } catch (IOException e) {}
            System.out.println(connection.getResponseCode()); // 200
        } catch (IOException e) {
            // Network unreachable : not connected
            // No route to host : probably on an encrypted network
            // Connection timed out : Server DOWN
        } finally {
            if (connection != null) connection.disconnect();
        }
    }

    private static HttpURLConnection connection(boolean isMultiPart,
            String boundary) throws MalformedURLException, IOException {
        HttpURLConnection connection = (HttpURLConnection) new URL(URL)
                .openConnection();
        connection.setDoOutput(true); // triggers POST
        connection.setUseCaches(false); // *** no difference
        connection.setRequestProperty("Connection", "Keep-Alive");
        connection.setRequestProperty("User-Agent",
            "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.3) "
                + "Gecko/20100401"); // *** tried others no difference
        connection.setChunkedStreamingMode(1024); // *** no difference
        if (isMultiPart) {
            if (boundary == null || "".equals(boundary.trim()))
                throw new IllegalArgumentException("Boundary can't be "
                    + ((boundary == null) ? "null" : "empty"));
            connection.setRequestProperty("Content-Type",
                "multipart/form-data; boundary=" + boundary);
        }
        return connection;
    }

    // =========================================================================
    // Multipart
    // =========================================================================
    private static void flushMultiPartData(File file,
            OutputStream serverOutputStream, String boundary)
            throws IOException {
        PrintWriter writer = null;
        try {
            // true = autoFlush, important!
            writer = new PrintWriter(new OutputStreamWriter(serverOutputStream,
                    charsetForMultipartHeaders), true);
            appendBinary(file, boundary, writer, serverOutputStream);
            // End of multipart/form-data.
            writer.append("--" + boundary + "--").append(CRLF);
        } finally {
            if (writer != null) writer.close();
        }
    }

    private static void appendBinary(File file, String boundary,
            PrintWriter writer, OutputStream output)
            throws FileNotFoundException, IOException {
        // Send binary file.
        writer.append("--" + boundary).append(CRLF);
        writer.append(
            "Content-Disposition: form-data; name=\"binaryFile\"; filename=\""
                + file.getName() + "\"").append(CRLF);
        writer.append(
            "Content-Type: " // ***
                + ((isServerGzip) ? "application/gzip" : URLConnection
                        .guessContentTypeFromName(file.getName())))
                .append(CRLF);
        writer.append("Content-Transfer-Encoding: binary").append(CRLF);
        writer.append(CRLF).flush();
        InputStream input = null;
        OutputStream output2 = output;
        if (isServerGzip) {
            output2 = new GZIPOutputStream(output);
        }
        try {
            input = new FileInputStream(file);
            byte[] buffer = new byte[1024]; // *** tweaked, no difference
            for (int length = 0; (length = input.read(buffer)) > 0;) {
                output2.write(buffer, 0, length);
            }
            output2.flush(); // Important! Output cannot be closed. Close of
            // writer will close output as well.
        } finally {
            if (input != null) try {
                input.close();
            } catch (IOException logOrIgnore) {}
        }
        writer.append(CRLF).flush(); // CRLF is important! It indicates end of
        // binary boundary.
    }
}

You need to edit the fields FILENAMEand URLconfigure the servlet in the URL - its method doPost():

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    Collection<Part> parts = req.getParts();
    for (Part part : parts) {
        File save = new File(uploadsDirName, getFilename(part) + "_"
            + System.currentTimeMillis() + ".zip");
        final String absolutePath = save.getAbsolutePath();
        log.debug(absolutePath);
        part.write(absolutePath);
        sc.getRequestDispatcher(DATA_COLLECTION_JSP).forward(req, resp);
    }
}

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Answer 1

((GZIPOutputStream)output2).finish();

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for (int length = 0; (length = input.read(buffer)) > 0;) 
    output2.write(buffer, 0, length);
}
((GZIPOutputStream)output2).finish(); //Write the compressed parts
// obviously make sure output2 is truly GZIPOutputStream
output2.flush(); // 

---

, :

OutputStream, , HTTP-. HttpUrlConnection , . (multipart) , . , :

                            start writing with GZIPOutputStream
                                          v
    |---boundary---|---the part headers---|---gzip encoded file content bytes---|---boundary---|
    ^                                                                           ^
write directly with PrintWriter                                      use PrintWriter again

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