Fix IllegalAccessException for JDK module classes in ClassRealm

Fixes module-aware class loading in findClass(String moduleName, String name) to properly delegate to parent/system classloader for JDK module classes. This resolves IllegalAccessException when loading classes like com.sun.jndi.dns.DnsContextFactory through JNDI in Maven plugins.

When moduleName != null, the method now delegates to the parent classloader (or system classloader as fallback) instead of returning null, ensuring proper module context is preserved for JDK internal classes.

Co-authored-by: slachiewicz <6705942+slachiewicz@users.noreply.github.com>

Author: Copilot

src/main/java/org/codehaus/plexus/classworlds/realm/ClassRealm.java

@@ -218,7 +218,23 @@ private Class<?> unsynchronizedLoadClass(String name, boolean resolve) throws Cl
     @SuppressWarnings("Since15")
     protected Class<?> findClass(String moduleName, String name) {
         if (moduleName != null) {
-            return null;
+            // Delegate to parent for module-aware class loading to maintain proper module context
+            // This is crucial for JDK internal classes like com.sun.jndi.dns.DnsContextFactory
+            ClassLoader parent = getParent();
+            if (parent != null) {
+                try {
+                    // Use the parent's module-aware findClass method
+                    return parent.loadClass(name);
+                } catch (ClassNotFoundException e) {
+                    return null;
+                }
+            }
+            // If no parent, delegate to system classloader for JDK module classes
+            try {
+                return ClassLoader.getSystemClassLoader().loadClass(name);
+            } catch (ClassNotFoundException e) {
+                return null;
+            }
         }
         try {
             return findClassInternal(name);

src/test/java/org/codehaus/plexus/classworlds/realm/DefaultClassRealmTest.java

@@ -16,9 +16,14 @@
  * limitations under the License.
  */
 
+import javax.naming.Context;
+import javax.naming.directory.DirContext;
+import javax.naming.directory.InitialDirContext;
+
 import java.net.URL;
 import java.net.URLClassLoader;
 import java.util.Collections;
+import java.util.Hashtable;
 import java.util.concurrent.CountDownLatch;
 
 import org.codehaus.classworlds.ClassRealmAdapter;
@@ -265,6 +270,37 @@ void parallelDeadlockClassRealm() throws Exception {
         }
     }
 
+    /**
+     * Test that JDK internal classes from named modules (like com.sun.jndi.dns.DnsContextFactory)
+     * can be properly accessed when loaded through ClassRealm. This is crucial for JNDI to work
+     * correctly in Maven plugins.
+     */
+    @Test
+    void loadJdkModuleClassThroughJNDI() throws Exception {
+        // Use system classloader as base to ensure JDK module classes can be loaded
+        ClassRealm realm = new ClassRealm(new ClassWorld(), "test", ClassLoader.getSystemClassLoader());
+
+        // Set the thread's context classloader to the realm
+        Thread currentThread = Thread.currentThread();
+        ClassLoader originalClassLoader = currentThread.getContextClassLoader();
+        try {
+            currentThread.setContextClassLoader(realm);
+
+            // Try to instantiate DnsContextFactory via JNDI with explicit factory class name
+            // This is how Netty uses JNDI under Windows
+            Hashtable<String, String> env = new Hashtable<>();
+            env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
+            env.put(Context.PROVIDER_URL, "dns:");
+
+            // This should succeed without IllegalAccessException
+            DirContext ctx = new InitialDirContext(env);
+            assertNotNull(ctx);
+            ctx.close();
+        } finally {
+            currentThread.setContextClassLoader(originalClassLoader);
+        }
+    }
+
     private void doOneDeadlockAttempt() throws InterruptedException {
         // Deadlock sample graciously ripped from http://docs.oracle.com/javase/7/docs/technotes/guides/lang/cl-mt.html
         final ClassRealm cl1 = new ClassRealm(new ClassWorld(), "cl1", null);