Demo for defining class loaders in Java

CLoaderExample.java

import static java.lang.System.out;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

/**
 * Demo code for http://www.romanenco.com/defining-class-loader-java
 * Compile and run from command line:
 * javac CLoaderExample.java
 * java -cp . CLoaderExample
 */
public class CLoaderExample {

	public static void main(String[] args) throws Exception {
		out.println("With delegation");
		/*
		 * Note
		 * dlc will receive only one call for loadClass
		 * and class loader of Custom is system one
		 */
		final DelegatingCL dcl = new DelegatingCL();
		Class<?> c = dcl.loadClass("Custom");
		out.println("Class loader of C: " + c.getClassLoader());
		Class<?> cParent = c.getSuperclass();
		out.println("Class loader of super: " + cParent.getClassLoader());
		
		out.println("With interception");
		/*
		 * Note
		 * icl receives loadclass call twice, for Custom and it's parent (java.lang.Object)
		 * Class loader of Custom is icl
		 * Class loader of Object - null - this means bootstrap.
		 */
		final InterceptingCL icl = new InterceptingCL();
		Class<?> d = icl.loadClass("Custom");
		out.println("Class loader of C: " + d.getClassLoader());
		Class<?> dParent = d.getSuperclass();
		out.println("Class loader of super: " + dParent.getClassLoader());
	} 
	
}

/**
 * Delegating class loader does not make any resolutions, just print calls.
 */
class DelegatingCL extends ClassLoader {

	@Override
	protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
		out.println("CLoader.loadClass(s, b): " + name + ", " + resolve);
		return super.loadClass(name, resolve);
	}

	@Override
	protected Class<?> findClass(String name) throws ClassNotFoundException {
		out.println("CLoader.findClass(s): " + name);
		return super.findClass(name);
	}
		
}

/**
 * This class loader delegates to parent all calls, other than for class Custom.
 */
class InterceptingCL extends ClassLoader {

	@Override
	protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
		out.println("CLoader.loadClass(s, b): " + name + ", " + resolve);
		if (name.equals("Custom")) {
			return readAndDefine(name);
		}
		return super.loadClass(name, resolve);
	}

	@Override
	protected Class<?> findClass(String name) throws ClassNotFoundException {
		out.println("CLoader.findClass(s): " + name);
		return super.findClass(name);
	}
	
	private Class<?> readAndDefine(String name) {
		Path path = Paths.get(name + ".class");
		byte[] data;
		try {
			data = Files.readAllBytes(path);
		} catch (IOException e) {
			throw new RuntimeException("Can't read file", e);
		}
		return defineClass(name, data, 0, data.length);
	}
	
}

class Custom {
	public static final int I = 99;
}