How can I download a single file from a large remote zip file in Java?

Question

I'm trying to download a small file (0.3 KB) from a given zip file that's around 3-5 GB in size.

I have currently been using the native library libfragmentzip using JNA, which is very fast, but has issues of its own that come with using native libraries (like not being cross-platform).

I have tried this solution, but it is much slower and ends up taking minutes compared to using libfragmentzip, which only seems to take seconds.

This is a URL to a test zip file (the extension is .ipsw but it is really a zip). The file I am trying to download is BuildManifest.plist, in the root of the zip.

Is there a fast way to download a single file from a remote zip file without using a native library?

Answer 1

Using Apache Commons Compress and a custom ByteChannel implementation backed by HTTP:

var url = new URL(...);
var fileName = "file.txt";
var dest = Path.of(fileName);

try (var zip = new ZipFile(new HttpChannel(url), "zip", "UTF8", true, true);
    var stream = zip.getInputStream(zip.getEntry(fileName))) {
    Files.copy(stream, dest, StandardCopyOption.REPLACE_EXISTING);
}

HttpChannel (modified from JCodec):

public class HttpChannel implements SeekableByteChannel {

    private final URL url;
    private ReadableByteChannel ch;
    private long pos;
    private long length;

    public HttpChannel(URL url) {
        this.url = url;
    }

    @Override
    public long position() {
        return pos;
    }

    @Override
    public SeekableByteChannel position(long newPosition) throws IOException {
        if (newPosition == pos) {
            return this;
        } else if (ch != null) {
            ch.close();
            ch = null;
        }
        pos = newPosition;
        return this;
    }

    @Override
    public long size() throws IOException {
        ensureOpen();
        return length;
    }

    @Override
    public SeekableByteChannel truncate(long size) {
        throw new UnsupportedOperationException("Truncate on HTTP is not supported.");
    }

    @Override
    public int read(ByteBuffer buffer) throws IOException {
        ensureOpen();
        int read = ch.read(buffer);
        if (read != -1)
            pos += read;
        return read;
    }

    @Override
    public int write(ByteBuffer buffer) {
        throw new UnsupportedOperationException("Write to HTTP is not supported.");
    }

    @Override
    public boolean isOpen() {
        return ch != null && ch.isOpen();
    }

    @Override
    public void close() throws IOException {
        ch.close();
    }

    private void ensureOpen() throws IOException {
        if (ch == null) {
            URLConnection connection = url.openConnection();
            if (pos > 0)
                connection.addRequestProperty("Range", "bytes=" + pos + "-");
            ch = Channels.newChannel(connection.getInputStream());
            String resp = connection.getHeaderField("Content-Range");
            if (resp != null) {
                length = Long.parseLong(resp.split("/")[1]);
            } else {
                resp = connection.getHeaderField("Content-Length");
                length = Long.parseLong(resp);
            }
        }
    }
}