From 31ecace083bbfb8fee7021bb25be96d0b9b86d18 Mon Sep 17 00:00:00 2001
From: Josiah Glosson <soujournme@gmail.com>
Date: Sun, 6 Jul 2025 14:09:49 -0500
Subject: [PATCH] Fix launching older Forge versions (#3920)

---
 .../java/com/modrinth/theseus/MinecraftLaunch.java   | 12 ++++++++++--
 packages/app-lib/src/launcher/mod.rs                 |  6 ++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/packages/app-lib/java/src/main/java/com/modrinth/theseus/MinecraftLaunch.java b/packages/app-lib/java/src/main/java/com/modrinth/theseus/MinecraftLaunch.java
index f32e25a57..f56ef52c4 100644
--- a/packages/app-lib/java/src/main/java/com/modrinth/theseus/MinecraftLaunch.java
+++ b/packages/app-lib/java/src/main/java/com/modrinth/theseus/MinecraftLaunch.java
@@ -2,6 +2,7 @@ package com.modrinth.theseus;
 
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.lang.reflect.AccessibleObject;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
 import java.util.Arrays;
@@ -76,14 +77,14 @@ public final class MinecraftLaunch {
 
             Object thisObject = null;
             if (!Modifier.isStatic(mainMethod.getModifiers())) {
-                thisObject = mainClass.getDeclaredConstructor().newInstance();
+                thisObject = forceAccessible(mainClass.getDeclaredConstructor()).newInstance();
             }
 
             final Object[] parameters = mainMethod.getParameterCount() > 0 ? new Object[] {args} : new Object[] {};
 
             mainMethod.invoke(thisObject, parameters);
         } else {
-            findSimpleMainMethod(mainClass).invoke(null, new Object[] {args});
+            forceAccessible(findSimpleMainMethod(mainClass)).invoke(null, new Object[] {args});
         }
     }
 
@@ -115,4 +116,11 @@ public final class MinecraftLaunch {
     private static Method findSimpleMainMethod(Class<?> mainClass) throws NoSuchMethodException {
         return mainClass.getMethod("main", String[].class);
     }
+
+    private static <T extends AccessibleObject> T forceAccessible(T object) throws ReflectiveOperationException {
+        final Method setAccessible0 = AccessibleObject.class.getDeclaredMethod("setAccessible0", boolean.class);
+        setAccessible0.setAccessible(true);
+        setAccessible0.invoke(object, true);
+        return object;
+    }
 }
diff --git a/packages/app-lib/src/launcher/mod.rs b/packages/app-lib/src/launcher/mod.rs
index 63912b5b9..d39fd94a9 100644
--- a/packages/app-lib/src/launcher/mod.rs
+++ b/packages/app-lib/src/launcher/mod.rs
@@ -619,6 +619,12 @@ pub async fn launch_minecraft(
         .into_iter(),
     );
 
+    // The java launcher requires access to java.lang.reflect in order to force access in to
+    // whatever module the main class is in
+    if java_version.parsed_version >= 9 {
+        command.arg("--add-opens=java.base/java.lang.reflect=ALL-UNNAMED");
+    }
+
     // The java launcher code requires internal JDK code in Java 25+ in order to support JEP 512
     if java_version.parsed_version >= 25 {
         command.arg("--add-opens=jdk.internal/jdk.internal.misc=ALL-UNNAMED");