How to pre-buffer an MP3 file completely in Java

Question

I'm working on a music player, which receives a playlist with remote mp3 files (HTTP) and play them subsequently.

I want to have it start streaming the first track, if enough of the song is buffered to play it through, it should already begin to buffer the following song into memory. That is to make up for the unstable internet connection the program is supposed to run on.

How do I tell the BufferedInputStream to just download the whole file?

I'm happy to hear other suggestions on how to solve this, too.

I'm using the JLayer/BasicPlayer library to play audio, this is the code.

String mp3Url = "http://ia600402.us.archive.org/6/items/Stockfinster.-DeadLinesutemos025/01_Push_Push.mp3";
URL url = new URL(mp3Url);
URLConnection conn = url.openConnection();
InputStream is = conn.getInputStream();
BufferedInputStream bis = new BufferedInputStream(is);

BasicPlayer player = new BasicPlayer();
player.open(bis);
player.play();

Answer 1

Ok, to answer my question, here's a working implementation:

/**
 * <code>DownloaderInputStream</code>
 */
public class DownloaderInputStream extends InputStream {

    /**
     * <code>IDownloadNotifier</code> - download listener.
     */
    public static interface IDownloadListener {

        /**
         * Notifies about download completion.
         *
         * @param buf
         * @param offset
         * @param length
         */
        public void onComplete(final byte[] buf, final int offset, final int length);
    }

    /**
     * <code>ByteArrayOutputStreamX</code> - {@link ByteArrayOutputStream}
     * extension that exposes buf variable (to avoid copying).
     */
    private final class ByteArrayOutputStreamX extends ByteArrayOutputStream {

        /**
         * Constructor.
         *
         * @param size
         */
        public ByteArrayOutputStreamX(final int size) {
            super(size);
        }

        /**
         * Returns inner buffer.
         *
         * @return inner buffer
         */
        public byte[] getBuffer() {
            return buf;
        }
    }

    private final class Downloader extends Object implements Runnable {
        // fields

        private final InputStream is;

        /**
         * Constructor.
         *
         * @param is
         */
        public Downloader(final InputStream is) {
            this.is = is;
        }

        // Runnable implementation
        public void run() {
            int read = 0;

            byte[] buf = new byte[16 * 1024];

            try {
                while ((read = is.read(buf)) != -1) {
                    if (read > 0) {
                        content.write(buf, 0, read);

                        downloadedBytes += read;
                    } else {
                        Thread.sleep(50);
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
            }

            listener.onComplete(content.getBuffer(), 0 /*
                     * offset
                     */, downloadedBytes);
        }
    }
    // fields
    private final int contentLength;
    private final IDownloadListener listener;
    // state
    private ByteArrayOutputStreamX content;
    private volatile int downloadedBytes;
    private volatile int readBytes;

    /**
     * Constructor.
     *
     * @param contentLength
     * @param is
     * @param listener
     */
    public DownloaderInputStream(final int contentLength, final InputStream is, final IDownloadListener listener) {
        this.contentLength = contentLength;
        this.listener = listener;

        this.content = new ByteArrayOutputStreamX(contentLength);
        this.downloadedBytes = 0;
        this.readBytes = 0;

        new Thread(new Downloader(is)).start();
    }

    /**
     * Returns number of downloaded bytes.
     *
     * @return number of downloaded bytes
     */
    public int getDownloadedBytes() {
        return downloadedBytes;
    }

    /**
     * Returns number of read bytes.
     *
     * @return number of read bytes
     */
    public int getReadBytes() {
        return readBytes;
    }

    // InputStream implementation
    @Override
    public int available() throws IOException {
        return downloadedBytes - readBytes;
    }

    @Override
    public int read() throws IOException {
        // not implemented (not necessary for BasicPlayer)
        return 0;
    }

    @Override
    public int read(byte[] b, int off, int len) throws IOException {
        if (readBytes == contentLength) {
            return -1;
        }

        int tr = 0;

        while ((tr = Math.min(downloadedBytes - readBytes, len)) == 0) {
            try {
                Thread.sleep(100);
            } catch (Exception e) {/*
                 * ignore
                 */

            }
        }

        byte[] buf = content.getBuffer();

        System.arraycopy(buf, readBytes, b, off, tr);

        readBytes += tr;

        return tr;
    }

    @Override
    public long skip(long n) throws IOException {
        // not implemented (not necessary for BasicPlayer)
        return n;
    }
}