Allow Bukkit plugin to use Paper PluginLoader API

paper-server/src/main/java/io/papermc/paper/plugin/loader/PaperClasspathBuilder.java

@@ -41,15 +41,7 @@ public class PaperClasspathBuilder implements PluginClasspathBuilder {
     }
 
     public PaperPluginClassLoader buildClassLoader(Logger logger, Path source, JarFile jarFile, PaperPluginMeta configuration) {
-        PaperLibraryStore paperLibraryStore = new PaperLibraryStore();
+        List<Path> paths = this.buildLibraryPaths(true);
-        for (ClassPathLibrary library : this.libraries) {
-            library.register(paperLibraryStore);
-        }
-
-        List<Path> paths = paperLibraryStore.getPaths();
-        if (PluginInitializerManager.instance().pluginRemapper != null) {
-            paths = PluginInitializerManager.instance().pluginRemapper.remapLibraries(paths);
-        }
         URL[] urls = new URL[paths.size()];
         for (int i = 0; i < paths.size(); i++) {
             Path path = paths.get(i);
@@ -69,4 +61,17 @@ public class PaperClasspathBuilder implements PluginClasspathBuilder {
             throw new RuntimeException(exception);
         }
     }
+
+    public List<Path> buildLibraryPaths(final boolean remap) {
+        PaperLibraryStore paperLibraryStore = new PaperLibraryStore();
+        for (ClassPathLibrary library : this.libraries) {
+            library.register(paperLibraryStore);
+        }
+
+        List<Path> paths = paperLibraryStore.getPaths();
+        if (remap && PluginInitializerManager.instance().pluginRemapper != null) {
+            paths = PluginInitializerManager.instance().pluginRemapper.remapLibraries(paths);
+        }
+        return paths;
+    }
 }

paper-server/src/main/java/io/papermc/paper/plugin/provider/type/spigot/SpigotPluginProvider.java

@@ -40,15 +40,17 @@ public class SpigotPluginProvider implements PluginProvider<JavaPlugin>, Provide
     private final PluginDescriptionFile description;
     private final JarFile jarFile;
     private final Logger logger;
+    private final List<Path> paperLibraryPaths;
     private final ComponentLogger componentLogger;
     private ProviderStatus status;
     private DependencyContext dependencyContext;
 
-    SpigotPluginProvider(Path path, JarFile file, PluginDescriptionFile description) {
+    SpigotPluginProvider(Path path, JarFile file, PluginDescriptionFile description, List<Path> paperLibraryPaths) {
         this.path = path;
         this.jarFile = file;
         this.description = description;
         this.logger = PaperPluginLogger.getLogger(description);
+        this.paperLibraryPaths = paperLibraryPaths;
         this.componentLogger = ComponentLogger.logger(this.logger.getName());
     }
 
@@ -120,7 +122,7 @@ public class SpigotPluginProvider implements PluginProvider<JavaPlugin>, Provide
 
             final PluginClassLoader loader;
             try {
-                loader = new PluginClassLoader(this.getClass().getClassLoader(), this.description, dataFolder, this.path.toFile(), LIBRARY_LOADER.createLoader(this.description), this.jarFile, this.dependencyContext); // Paper
+                loader = new PluginClassLoader(this.getClass().getClassLoader(), this.description, dataFolder, this.path.toFile(), LIBRARY_LOADER.createLoader(this.description, this.paperLibraryPaths), this.jarFile, this.dependencyContext); // Paper
             } catch (InvalidPluginException ex) {
                 throw ex;
             } catch (Throwable ex) {

paper-server/src/main/java/io/papermc/paper/plugin/provider/type/spigot/SpigotPluginProviderFactory.java

@@ -1,9 +1,18 @@
 package io.papermc.paper.plugin.provider.type.spigot;
 
+import com.destroystokyo.paper.utils.PaperPluginLogger;
+import io.papermc.paper.plugin.bootstrap.PluginProviderContextImpl;
 import io.papermc.paper.plugin.entrypoint.classloader.BytecodeModifyingURLClassLoader;
+import io.papermc.paper.plugin.entrypoint.classloader.PaperSimplePluginClassLoader;
+import io.papermc.paper.plugin.loader.PaperClasspathBuilder;
+import io.papermc.paper.plugin.loader.PluginLoader;
 import io.papermc.paper.plugin.provider.configuration.serializer.constraints.PluginConfigConstraints;
 import io.papermc.paper.plugin.provider.type.PluginTypeFactory;
+import io.papermc.paper.plugin.provider.util.ProviderUtil;
 import io.papermc.paper.util.MappingEnvironment;
+import java.util.List;
+import java.util.logging.Logger;
+import net.kyori.adventure.text.logger.slf4j.ComponentLogger;
 import org.bukkit.plugin.InvalidDescriptionException;
 import org.bukkit.plugin.PluginDescriptionFile;
 import org.bukkit.plugin.java.LibraryLoader;
@@ -36,7 +45,28 @@ class SpigotPluginProviderFactory implements PluginTypeFactory<SpigotPluginProvi
             throw new InvalidDescriptionException("Restricted name, cannot use 0x20 (space character) in a plugin name.");
         }
 
-        return new SpigotPluginProvider(source, file, configuration);
+        final List<Path> paperLibraryPaths;
+        if (configuration.getPaperPluginLoader() != null) {
+            final Logger logger = PaperPluginLogger.getLogger(configuration);
+            PaperClasspathBuilder builder = new PaperClasspathBuilder(PluginProviderContextImpl.create(
+                configuration, ComponentLogger.logger(logger.getName()), source
+            ));
+
+            try (
+                PaperSimplePluginClassLoader simplePluginClassLoader = new PaperSimplePluginClassLoader(source, file, configuration, this.getClass().getClassLoader())
+            ) {
+                PluginLoader loader = ProviderUtil.loadClass(configuration.getPaperPluginLoader(), PluginLoader.class, simplePluginClassLoader);
+                loader.classloader(builder);
+            } catch (IOException e) {
+                throw new RuntimeException(e);
+            }
+
+            paperLibraryPaths = builder.buildLibraryPaths(false);
+        } else {
+            paperLibraryPaths = null;
+        }
+
+        return new SpigotPluginProvider(source, file, configuration, paperLibraryPaths);
     }
 
     @Override