Fix extension loading causing whole plugin to error

common/src/main/java/me/lucko/luckperms/common/extension/SimpleExtensionManager.java

@@ -28,6 +28,7 @@ package me.lucko.luckperms.common.extension;
 import com.google.gson.JsonObject;
 
 import me.lucko.luckperms.common.plugin.LuckPermsPlugin;
+import me.lucko.luckperms.common.plugin.classpath.ReflectionClassPathAppender;
 import me.lucko.luckperms.common.util.gson.GsonProvider;
 
 import net.luckperms.api.LuckPerms;
@@ -42,8 +43,6 @@ import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
-import java.lang.reflect.Method;
-import java.net.URL;
 import java.net.URLClassLoader;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
@@ -98,7 +97,7 @@ public class SimpleExtensionManager implements ExtensionManager, AutoCloseable {
                 if (path.getFileName().toString().endsWith(".jar")) {
                     try {
                         loadExtension(path);
-                    } catch (IOException e) {
+                    } catch (Exception e) {
                         this.plugin.getLogger().warn("Exception loading extension from " + path, e);
                     }
                 }
@@ -146,7 +145,7 @@ public class SimpleExtensionManager implements ExtensionManager, AutoCloseable {
         if (useParentClassLoader && isJarInJar()) {
             try {
                 addJarToParentClasspath(path);
-            } catch (Exception e) {
+            } catch (Throwable e) {
                 throw new RuntimeException("Exception whilst classloading extension", e);
             }
         } else {
@@ -200,16 +199,14 @@ public class SimpleExtensionManager implements ExtensionManager, AutoCloseable {
         return thisClassLoaderName.equals("me.lucko.luckperms.common.loader.JarInJarClassLoader");
     }
 
+    @Deprecated
     private static void addJarToParentClasspath(Path path) throws Exception {
         ClassLoader parentClassLoader = SimpleExtensionManager.class.getClassLoader().getParent();
         if (!(parentClassLoader instanceof URLClassLoader)) {
             throw new RuntimeException("useParentClassLoader is true but parent is not a URLClassLoader");
         }
 
-        Method addUrlMethod = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
+        ReflectionClassPathAppender.addUrl(((URLClassLoader) parentClassLoader), path.toUri().toURL());
-        addUrlMethod.setAccessible(true);
-
-        addUrlMethod.invoke(parentClassLoader, path.toUri().toURL());
     }
 
     private static final class LoadedExtension {

common/src/main/java/me/lucko/luckperms/common/plugin/classpath/ReflectionClassPathAppender.java

@@ -43,16 +43,32 @@ public class ReflectionClassPathAppender implements ClassPathAppender {
         try {
             openUrlClassLoaderModule();
         } catch (Throwable e) {
-            // ignore exception - will throw on Java 8 since the Module classes don't exist
+            // ignore
         }
 
-        // Get the protected 'addURL' method on URLClassLoader and set it to accessible.
         try {
             ADD_URL_METHOD = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
-            ADD_URL_METHOD.setAccessible(true);
         } catch (NoSuchMethodException e) {
             throw new RuntimeException(e);
         }
+
+        try {
+            ADD_URL_METHOD.setAccessible(true);
+        } catch (Throwable e) {
+            new RuntimeException("LuckPerms is unable to access the URLClassLoader#addURL method using reflection. \n" +
+                    "You may be able to fix this problem by adding the following command-line argument " +
+                    "directly after the 'java' command in your start script: \n'--add-opens java.base/java.lang=ALL-UNNAMED'", e).printStackTrace();
+        }
+    }
+
+    /**
+     * Adds the given {@link URL} to the class loader.
+     *
+     * @param classLoader the class loader
+     * @param url the url to add
+     */
+    public static void addUrl(URLClassLoader classLoader, URL url) throws ReflectiveOperationException {
+        ADD_URL_METHOD.invoke(classLoader, url);
     }
 
     private final URLClassLoader classLoader;
@@ -69,13 +85,12 @@ public class ReflectionClassPathAppender implements ClassPathAppender {
     @Override
     public void addJarToClasspath(Path file) {
         try {
-            ADD_URL_METHOD.invoke(this.classLoader, file.toUri().toURL());
+            addUrl(this.classLoader, file.toUri().toURL());
         } catch (ReflectiveOperationException | MalformedURLException e) {
             throw new RuntimeException(e);
         }
     }
 
-    @SuppressWarnings("JavaReflectionMemberAccess")
     private static void openUrlClassLoaderModule() throws Exception {
         // This is effectively calling:
         //