Question

I want to generate a .torrent file in Java, but I don’t want a big API that does anything like scraping trackers, seeding, etc. This is just for a client that generates meta data. What lightweight solutions exist? I am only generating a .torrent of a single .zip file.

Thanks!

Answer 1

I have put together this self-contained piece of Java code to prepare a .torrent file with a single file.

The .torrent file is created by calling createTorrent() passing the name of the .torrent file, the name of the shared file and the tracker URL.

createTorrent() uses hashPieces() to hash the file pieces using Java’s MessageDigest class. Then createTorrent() prepares a meta info dictionary containing the torrent meta-data. This dictionary is then serialized in the proper bencode format using the encode*() methods and saved in a .torrent file.

See the BitTorrent spec for details.

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.HashMap;
import java.util.Map;
import java.util.SortedMap;
import java.util.TreeMap;

public class Torrent {
    private static void encodeObject(Object o, OutputStream out) throws IOException {
        if (o instanceof String)
            encodeString((String) o, out);
        else if (o instanceof Map)
            encodeMap((Map) o, out);
        else if (o instanceof byte[])
            encodeBytes((byte[]) o, out);
        else if (o instanceof Number)
            encodeLong(((Number) o).longValue(), out);
        else
            throw new Error("Unencodable type");
    }

    private static void encodeLong(long value, OutputStream out) throws IOException {
        out.write('i');
        out.write(Long.toString(value).getBytes("US-ASCII"));
        out.write('e');
    }

    private static void encodeBytes(byte[] bytes, OutputStream out) throws IOException {
        out.write(Integer.toString(bytes.length).getBytes("US-ASCII"));
        out.write(':');
        out.write(bytes);
    }

    private static void encodeString(String str, OutputStream out) throws IOException {
        encodeBytes(str.getBytes("UTF-8"), out);
    }

    private static void encodeMap(Map<String, Object> map, OutputStream out) throws IOException {
        // Sort the map. A generic encoder should sort by key bytes
        SortedMap<String, Object> sortedMap = new TreeMap<String, Object>(map);
        out.write('d');
        for (Map.Entry<String, Object> e : sortedMap.entrySet()) {
            encodeString(e.getKey(), out);
            encodeObject(e.getValue(), out);
        }
        out.write('e');
    }

    private static byte[] hashPieces(File file, int pieceLength) throws IOException {
        MessageDigest sha1;
        try {
            sha1 = MessageDigest.getInstance("SHA");
        } catch (NoSuchAlgorithmException e) {
            throw new Error("SHA1 not supported");
        }
        InputStream in = new FileInputStream(file);
        ByteArrayOutputStream pieces = new ByteArrayOutputStream();
        byte[] bytes = new byte[pieceLength];
        int pieceByteCount = 0, readCount = in.read(bytes, 0, pieceLength);
        while (readCount != -1) {
            pieceByteCount += readCount;
            sha1.update(bytes, 0, readCount);
            if (pieceByteCount == pieceLength) {
                pieceByteCount = 0;
                pieces.write(sha1.digest());
            }
            readCount = in.read(bytes, 0, pieceLength - pieceByteCount);
        }
        in.close();
        if (pieceByteCount > 0)
            pieces.write(sha1.digest());
        return pieces.toByteArray();
    }

    public static void createTorrent(File file, File sharedFile, String announceURL) throws IOException {
        final int pieceLength = 512 * 1024;
        Map<String, Object> info = new HashMap<>();
        info.put("name", sharedFile.getName());
        info.put("length", sharedFile.length());
        info.put("piece length", pieceLength);
        info.put("pieces", hashPieces(sharedFile, pieceLength));
        Map<String, Object> metainfo = new HashMap<String, Object>();
        metainfo.put("announce", announceURL);
        metainfo.put("info", info);
        OutputStream out = new FileOutputStream(file);
        encodeMap(metainfo, out);
        out.close();
    }

    public static void main(String[] args) throws Exception {
        createTorrent(new File("C:/x.torrent"), new File("C:/file"), "http://example.com/announce");
    }
}

Code edits: Make this a bit more compact, fix methods visibility, use character literals where appropriate, use instanceof Number. And more recently read the file using block I/O because I ‘m trying to use it for real and byte I/O is just slow,