From 8a38c5f4b72cf610c49bff5a3fef5f8752921c6b Mon Sep 17 00:00:00 2001
From: Luck <git@lucko.me>
Date: Thu, 26 Nov 2020 15:52:39 +0000
Subject: [PATCH] Properly open URLClassLoader module on Java 9+ to allow
 reflection and avoid access warning (#952)

---
 .../classloader/ReflectionClassLoader.java    | 72 ++++++++++---------
 1 file changed, 40 insertions(+), 32 deletions(-)

diff --git a/common/src/main/java/me/lucko/luckperms/common/dependencies/classloader/ReflectionClassLoader.java b/common/src/main/java/me/lucko/luckperms/common/dependencies/classloader/ReflectionClassLoader.java
index c5b5ab41a..e68cc43d7 100644
--- a/common/src/main/java/me/lucko/luckperms/common/dependencies/classloader/ReflectionClassLoader.java
+++ b/common/src/main/java/me/lucko/luckperms/common/dependencies/classloader/ReflectionClassLoader.java
@@ -25,9 +25,6 @@
 
 package me.lucko.luckperms.common.dependencies.classloader;
 
-import com.google.common.base.Supplier;
-import com.google.common.base.Suppliers;
-
 import me.lucko.luckperms.common.plugin.bootstrap.LuckPermsBootstrap;
 
 import java.lang.reflect.InvocationTargetException;
@@ -38,10 +35,28 @@ import java.net.URLClassLoader;
 import java.nio.file.Path;
 
 public class ReflectionClassLoader implements PluginClassLoader {
-    private final URLClassLoader classLoader;
 
-    @SuppressWarnings("Guava") // we can't use java.util.Function because old Guava versions are used at runtime
-    private final Supplier<Method> addUrlMethod;
+    private static final Method ADD_URL_METHOD;
+
+    static {
+        // If on Java 9+, open the URLClassLoader module to this module
+        // so we can access its API via reflection without producing a warning.
+        try {
+            openUrlClassLoaderModule();
+        } catch (Throwable e) {
+            // ignore exception - will throw on Java 8 since the Module classes don't exist
+        }
+
+        // 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);
+        }
+    }
+
+    private final URLClassLoader classLoader;
 
     public ReflectionClassLoader(LuckPermsBootstrap bootstrap) throws IllegalStateException {
         ClassLoader classLoader = bootstrap.getClass().getClassLoader();
@@ -50,42 +65,35 @@ public class ReflectionClassLoader implements PluginClassLoader {
         } else {
             throw new IllegalStateException("ClassLoader is not instance of URLClassLoader");
         }
-
-        this.addUrlMethod = Suppliers.memoize(() -> {
-            if (isJava9OrNewer()) {
-                bootstrap.getPluginLogger().info("It is safe to ignore any warning printed following this message " +
-                        "starting with 'WARNING: An illegal reflective access operation has occurred, Illegal reflective " +
-                        "access by " + getClass().getName() + "'. This is intended, and will not have any impact on the " +
-                        "operation of LuckPerms.");
-            }
-
-            try {
-                Method addUrlMethod = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
-                addUrlMethod.setAccessible(true);
-                return addUrlMethod;
-            } catch (NoSuchMethodException e) {
-                throw new RuntimeException(e);
-            }
-        });
     }
 
     @Override
     public void addJarToClasspath(Path file) {
         try {
-            this.addUrlMethod.get().invoke(this.classLoader, file.toUri().toURL());
+            ADD_URL_METHOD.invoke(this.classLoader, file.toUri().toURL());
         } catch (IllegalAccessException | InvocationTargetException | MalformedURLException e) {
             throw new RuntimeException(e);
         }
     }
 
     @SuppressWarnings("JavaReflectionMemberAccess")
-    private static boolean isJava9OrNewer() {
-        try {
-            // method was added in the Java 9 release
-            Runtime.class.getMethod("version");
-            return true;
-        } catch (NoSuchMethodException e) {
-            return false;
-        }
+    private static void openUrlClassLoaderModule() throws Exception {
+        // This is effectively calling:
+        //
+        // URLClassLoader.class.getModule().addOpens(
+        //     URLClassLoader.class.getPackageName(),
+        //     ReflectionClassLoader.class.getModule()
+        // );
+        //
+        // We use reflection since we build against Java 8.
+
+        Class<?> moduleClass = Class.forName("java.lang.Module");
+        Method getModuleMethod = Class.class.getMethod("getModule");
+        Method addOpensMethod = moduleClass.getMethod("addOpens", String.class, moduleClass);
+
+        Object urlClassLoaderModule = getModuleMethod.invoke(URLClassLoader.class);
+        Object thisModule = getModuleMethod.invoke(ReflectionClassLoader.class);
+
+        addOpensMethod.invoke(urlClassLoaderModule, URLClassLoader.class.getPackage().getName(), thisModule);
     }
 }