From e454fef40e1e1e7a889327d3371fc7b5ff2b68df Mon Sep 17 00:00:00 2001
From: wiicart <wiicart@pedestriamc.com>
Date: Mon, 23 Jun 2025 22:50:59 -0400
Subject: [PATCH] Add support for private constructors in plugin main classes
 (#12652)

---
 .../paper/plugin/provider/util/ProviderUtil.java     | 11 ++++++++++-
 .../org/bukkit/plugin/java/PluginClassLoader.java    | 12 ++++++++++--
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/paper-api/src/main/java/io/papermc/paper/plugin/provider/util/ProviderUtil.java b/paper-api/src/main/java/io/papermc/paper/plugin/provider/util/ProviderUtil.java
index 48a67c1b60..5a1f30f9b5 100644
--- a/paper-api/src/main/java/io/papermc/paper/plugin/provider/util/ProviderUtil.java
+++ b/paper-api/src/main/java/io/papermc/paper/plugin/provider/util/ProviderUtil.java
@@ -4,6 +4,8 @@ import com.destroystokyo.paper.util.SneakyThrow;
 import org.jetbrains.annotations.ApiStatus;
 import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InaccessibleObjectException;
 
 /**
  * An <b>internal</b> utility type that holds logic for loading a provider-like type from a classloaders.
@@ -56,7 +58,14 @@ public final class ProviderUtil {
                     throw new ClassCastException("class '%s' does not extend '%s'".formatted(clazz, classType));
                 }
 
-                clazzInstance = pluginClass.getDeclaredConstructor().newInstance();
+                final Constructor<? extends T> constructor = pluginClass.getDeclaredConstructor();
+                try {
+                    constructor.setAccessible(true); // Allow non-public constructors
+                } catch (final InaccessibleObjectException | SecurityException ex) {
+                    throw new RuntimeException("Inaccessible constructor");
+                }
+
+                clazzInstance = constructor.newInstance();
             } catch (final IllegalAccessException exception) {
                 throw new RuntimeException("No public constructor");
             } catch (final InstantiationException exception) {
diff --git a/paper-api/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java b/paper-api/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
index 37827b19e6..8c5a5043ad 100644
--- a/paper-api/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
+++ b/paper-api/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
@@ -6,6 +6,7 @@ import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.lang.reflect.Constructor;
+import java.lang.reflect.InaccessibleObjectException;
 import java.lang.reflect.InvocationTargetException;
 import java.net.MalformedURLException;
 import java.net.URL;
@@ -91,13 +92,20 @@ public final class PluginClassLoader extends URLClassLoader implements io.paperm
         try {
             pluginConstructor = pluginClass.getDeclaredConstructor();
         } catch (NoSuchMethodException ex) {
-            throw new InvalidPluginException("main class `" + description.getMain() + "' must have a public no-args constructor", ex);
+            throw new InvalidPluginException("main class `" + description.getMain() + "' must have a no-args constructor", ex);
+        }
+
+        try {
+            // Support non-public constructors
+            pluginConstructor.setAccessible(true);
+        } catch (InaccessibleObjectException | SecurityException ex) {
+            throw new InvalidPluginException("main class `" + description.getMain() + "' constructor inaccessible", ex);
         }
 
         try {
             plugin = pluginConstructor.newInstance();
         } catch (IllegalAccessException ex) {
-            throw new InvalidPluginException("main class `" + description.getMain() + "' constructor must be public", ex);
+            throw new InvalidPluginException("main class `" + description.getMain() + "' constructor inaccessible", ex);
         } catch (InstantiationException ex) {
             throw new InvalidPluginException("main class `" + description.getMain() + "' must not be abstract", ex);
         } catch (IllegalArgumentException ex) {