Fix IllegalAccessException for JDK module classes in ClassRealm

[Copilot]

Fixes an issue where JNDI and other JDK services fail with IllegalAccessException when running in Maven plugins with Java 9+.

Problem

When using Maven plugins with Java 9+, code that uses JNDI (such as Netty's DNS resolver) fails with:

javax.naming.NoInitialContextException: Cannot instantiate class: com.sun.jndi.dns.DnsContextFactory 
[Root exception is java.lang.IllegalAccessException: class javax.naming.spi.NamingManager (in module java.naming) 
cannot access class com.sun.jndi.dns.DnsContextFactory (in module jdk.naming.dns) because module jdk.naming.dns 
does not export com.sun.jndi.dns to module java.naming]

This occurs when:

• The thread context classloader is set to a ClassRealm instance (as Maven does for plugins)
• JNDI tries to load factory classes from JDK modules
• The Java 9+ module system's access checks fail

The same code works fine when run as a regular Java application.

Root Cause

Java 9+ introduced module-aware class loading through ClassLoader.findClass(String moduleName, String name). When moduleName is not null, it indicates the JVM is trying to find a class in a specific named module.

The existing implementation simply returned null when moduleName != null, which meant:

• ClassRealm couldn't find classes from named JDK modules
• The module context was lost, breaking module access rules
• JNDI and other services couldn't properly instantiate JDK internal classes

Solution

Modified ClassRealm.findClass(String moduleName, String name) to properly delegate to the parent classloader (or system classloader as fallback) when a module name is specified. This ensures JDK module classes are loaded in the correct module context, allowing JNDI and other services to work properly.

Testing

• Added testLoadJdkModuleClassThroughJNDI() test case that reproduces the exact scenario from the issue (Netty using JNDI in Maven plugins)
• All existing tests continue to pass (93 tests, 1 skipped)
• Verified the fix works with explicit JNDI factory class names as used by Netty

Impact

• Minimal changes: Only 16 lines added to findClass method
• No breaking changes: All existing functionality preserved
• Enables JNDI and other JDK services to work correctly in Maven plugins on Java 9+

Fixes #XX

Original prompt

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This section details on the original issue you should resolve

<issue_title>IllegalAccessException for com.sun.jndi.dns.DnsContextFactory</issue_title>
<issue_description>I'm not quite sure where to place this bug - here, with Maven, with Netty, or with the affected Maven plugin. So I'm going to start here...

I am using the apicurio-registry-maven-plugin in my Maven build with Java 21.

Under the hood, this uses Netty. Under Windows, Netty uses JNDI to get the system DNS servers, essentially doing this:

        Hashtable<String, String> env = new Hashtable<String, String>();
        env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
        env.put("java.naming.provider.url", "dns://");

        DirContext ctx = new InitialDirContext(env);
        String dnsUrls = (String) ctx.getEnvironment().get("java.naming.provider.url");

When this code runs in a Maven plugin, this fails with

javax.naming.NoInitialContextException: Cannot instantiate class: com.sun.jndi.dns.DnsContextFactory [Root exception is java.lang.IllegalAccessException: class javax.naming.spi.NamingManager (in module java.naming) cannot access class com.sun.jndi.dns.DnsContextFactory (in module jdk.naming.dns) because module jdk.naming.dns does not export com.sun.jndi.dns to module java.naming]

When I run the same code directly as a Java application, with the same JVM options, classpath etc., it works fine. I suspect that this has something to do with the plexus-classworlds classloader system, but I'm at a loss what, why and how.

System:
Windows 11
OpenJDK 21.0.5 (Eclipse Temurin-21.0.5+11)
Maven 3.9.9
apicurio-registry-maven-plugin 3.0.4
Netty 4.1.111
</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #119

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