How can I generate a .torrent in Java?

api, bittorrent, java

Solution

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,

Problem

Original source