ClassNotFoundException for nested classes on Paper 1.21.x

[ErrorsErrors]

Description
When running on Paper 1.21.x (Java 21), ConfigLib fails to load nested ($) classes during polymorphic deserialization.
Top-level classes load correctly, but nested ones throw a ClassNotFoundException.

Stack trace

Caused by: java.lang.ClassNotFoundException: com.test.testplugin.Event$FightConfig
    at io.papermc.paper.plugin.entrypoint.classloader.BytecodeModifyingURLClassLoader.findClass(...)
    ...
    at de.exlll.configlib.PolymorphicSerializer.tryFindClass(PolymorphicSerializer.java:121)

Environment

• Server: Paper 1.21.4 (build 231)
• Java: 21
• ConfigLib version: 4.x
• Plugin setup: ConfigLib loaded via libraries:

[Exlll]

Hi @ErrorsErrors, thanks for the report. Could you please paste your whole configuration class so that I can reproduce the issue?

Also, please note that inner classes are generally not supported as described in section Declaring configuration types.

[ErrorsErrors]

import de.exlll.configlib.Configuration;
import de.exlll.configlib.Polymorphic;

public class Event {
    @Configuration
    public static class MainConfig {
        public int a = 10;
        public LocalConfiguration local = new FightConfig();
    }

    @Configuration
    @Polymorphic
    public static class LocalConfiguration { }
    
    public static class FightConfig extends LocalConfiguration {
        public int durationSeconds = 300;
    }
}

If you save and then load the MainConfig configuration, it writes an error. As I understand it, the scope of visibility on Paper is limited and for some reason the library cannot see the nested classes, rather it is even a bug of the Paper itself.
As a solution, when resolving a class you can specify the plugin’s class loader, but to do this you need access to any class from the plugin.

[Exlll]

I can add a method to the ConfigurationProperties class through which you can set a specific ClassLoader instance of your choice. This ClassLoader instance would then be used for the Class.forName calls.

Would that resolve you issue?

[ErrorsErrors]

Yeah, that would fix the problem, but it would be more of a workaround. Ideally, we should take a closer look at how scope works in Paper. I tried to figure it out, but I couldn’t really understand what exactly causes this bug. I debugged the code in BytecodeModifyingURLClassLoader, and in that class it fails to find the class — both for the nested class and for other classes as well. However, for the nested class it throws an error, while for the other classes it doesn’t.

[Exlll]

Can you shade ConfigLib into your plugin (instead of loading it via libraries:)? Then you shoud not have this issue.

[Exlll]

How would like me to proceed here, should I add a method to ConfigurationProperties as mentioned above? Also, have you tried shading ConfigLib?