Load and remap libraries at runtime with libby and PaperMC's PluginRemapper

RemappedBukkitLibraryManager.java

import net.byteflux.libby.BukkitLibraryManager;
import net.byteflux.libby.Library;
import org.bukkit.plugin.Plugin;

import java.io.IOException;
import java.io.InputStream;
import java.io.UncheckedIOException;
import java.lang.reflect.Field;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardCopyOption;
import java.util.Collections;
import java.util.Enumeration;
import java.util.List;
import java.util.function.Function;
import java.util.jar.JarEntry;
import java.util.jar.JarFile;
import java.util.jar.JarOutputStream;
import java.util.jar.Manifest;

public class RemappedBukkitLibraryManager extends BukkitLibraryManager {
    private final Function<List<Path>, List<Path>> REMAPPER;

    {
        Function<List<Path>, List<Path>> remapper;
        try {
            Class<?> clazz = Class.forName("org.bukkit.plugin.java.LibraryLoader");
            Field remapperField = clazz.getDeclaredField("REMAPPER");

            //noinspection unchecked
            remapper = (Function<List<Path>, List<Path>>) remapperField.get(null);
        } catch (Exception e) {
            remapper = null;
        }
        REMAPPER = remapper;
    }

    public RemappedBukkitLibraryManager(Plugin plugin) {
        super(plugin);
    }

    public RemappedBukkitLibraryManager(Plugin plugin, String directoryName) {
        super(plugin, directoryName);
    }

    private Path remap(Path file) {
        return REMAPPER != null ? REMAPPER.apply(Collections.singletonList(file)).get(0) : file;
    }

    // This method's only purpose is to add the "paperweight-mappings-namespace" attribute to the MANIFEST.MF file of the library
    // So that it can be remapped by Paper's PluginRemapper
    // Thanks Copilot for this monstrosity of a method that solves the problem
    private void modifyManifest(Path file, Path modifiedFile) {
        try {
            Path tempJarPath = Files.createTempFile("temp-", ".jar");

            try (JarFile jarFile = new JarFile(file.toFile());
                 JarOutputStream tempJarOutputStream = new JarOutputStream(Files.newOutputStream(tempJarPath))) {

                // Copy existing entries
                Enumeration<JarEntry> entries = jarFile.entries();
                while (entries.hasMoreElements()) {
                    JarEntry entry = entries.nextElement();
                    if (JarFile.MANIFEST_NAME.equals(entry.getName())) {
                        continue; // Skip the existing MANIFEST.MF
                    }
                    try (InputStream entryInputStream = jarFile.getInputStream(entry)) {
                        tempJarOutputStream.putNextEntry(new JarEntry(entry.getName()));
                        byte[] buffer = new byte[1024];
                        int bytesRead;
                        while ((bytesRead = entryInputStream.read(buffer)) != -1) {
                            tempJarOutputStream.write(buffer, 0, bytesRead);
                        }
                        tempJarOutputStream.closeEntry();
                    }
                }

                // Modify the MANIFEST.MF
                Manifest manifest = jarFile.getManifest();
                if (manifest == null) {
                    manifest = new Manifest();
                }
                manifest.getMainAttributes().putValue("paperweight-mappings-namespace", "spigot");

                // Write the modified MANIFEST.MF
                tempJarOutputStream.putNextEntry(new JarEntry(JarFile.MANIFEST_NAME));
                manifest.write(tempJarOutputStream);
                tempJarOutputStream.closeEntry();
            }

            Files.move(tempJarPath, modifiedFile, StandardCopyOption.REPLACE_EXISTING);
        } catch (IOException e) {
            throw new UncheckedIOException(e);
        }
    }

    public void remapAndLoadLibrary(Library library) {
        Path path = saveDirectory.resolve(library.getPath());
        Path parent = path.getParent();
        String name = path.getFileName().toString();
        name = name.substring(0, name.lastIndexOf('.'));

        boolean downloaded = false;
        if (!Files.exists(path)) {
            downloadLibrary(library);
            downloaded = true;
        }

        Path modifiedPath = parent.resolve(name + "-modified.jar");
        if (downloaded || !Files.exists(modifiedPath)) {
            modifyManifest(path, modifiedPath);
        }

        Path remappedPath = remap(modifiedPath);
        if (library.isIsolatedLoad()) {
            addToIsolatedClasspath(library, remappedPath);
        } else {
            addToClasspath(remappedPath);
        }
    }
}

TestPlugin.java

import net.byteflux.libby.Library;
import org.bukkit.plugin.java.JavaPlugin;

public final class TestPlugin extends JavaPlugin {
    @Override
    public void onEnable() {
        RemappedBukkitLibraryManager libraryManager = new RemappedBukkitLibraryManager(this);
        Library lib = Library.builder()
                .groupId("net{}wesjd")
                .artifactId("anvilgui")
                .version("1.10.5-SNAPSHOT")
                .repository("https://repo.codemc.io/repository/maven-public/")
                .build();
        libraryManager.remapAndLoadLibrary(lib);
    }
}