How would you implement a custom ClassLoader to load classes from a non-standard source, such as loading plugin JARs dynamically at runtime, and what challenges arise with class unloading?

A custom ClassLoader extends ClassLoader (or URLClassLoader for JAR/URL-based loading) and overrides findClass() to define how to locate and read the raw bytecode from your custom source (a database, a plugin directory scanned at runtime, a network location), then calls defineClass() to convert those bytes into an actual Class object -- a major challenge is that a loaded class (and all its instances) can only be garbage collected once both the Class object itself AND the ClassLoader that loaded it become entirely unreferenced, meaning improperly discarding a plugin's ClassLoader reference (while still holding references to instances it created) causes a classloader leak, a notoriously tricky category of memory leak.

public class PluginClassLoader extends URLClassLoader {
    public PluginClassLoader(URL[] urls, ClassLoader parent) {
        super(urls, parent);
    }
}

// Dynamically load a plugin JAR at runtime
URL pluginUrl = new File("plugins/my-plugin.jar").toURI().toURL();
PluginClassLoader loader = new PluginClassLoader(new URL[]{pluginUrl}, Main.class.getClassLoader());
Class<?> pluginClass = loader.loadClass("com.example.MyPlugin");
Object plugin = pluginClass.getDeclaredConstructor().newInstance();

// To allow unloading: discard ALL references to loader, pluginClass, and any instances it created