REPO: Replace current metadata solution with a bytecode generated subclass in a classloader solution

[jasonewall]

Here is a really basic example using Javassist as the bytecode generator.

import ca.thejayvm.playground.java8.Person;
import javassist.*;

import java.lang.reflect.Modifier;

/**
 * Created by jason on 14-11-16.
 */
public class JavassistMain {
    public static void main(String[] args) throws NotFoundException, CannotCompileException, IllegalAccessException, InstantiationException {
        Class<? extends Person> personMetaClass = createClass();
        Person peep = personMetaClass.newInstance();
        peep.setFirstName("Jason");
        peep.setLastName("Wall");
        System.out.println(peep.getFirstName());
        InvocationTracker tracker = (InvocationTracker)peep;
        tracker.getInvocations().forEach(System.out::println);
    }

    @SuppressWarnings("unchecked")
    public static Class<? extends Person> createClass() throws NotFoundException, CannotCompileException {
        return getCtClass().toClass();
    }

    private static CtClass getCtClass() throws NotFoundException, CannotCompileException {
        ClassPool pool = ClassPool.getDefault();
        CtClass list = pool.get(java.util.List.class.getName());
        CtClass invocationTracker = pool.get(InvocationTracker.class.getName());
        CtClass subClass = pool.makeClass(Person.class.getName() + "Meta");
        final CtClass superClass = pool.get(Person.class.getName());
        subClass.setSuperclass(superClass);
        subClass.setInterfaces(new CtClass[]{ invocationTracker });
        subClass.setModifiers(Modifier.PUBLIC);

        CtField invocations = CtField.make("public java.util.List invocations = new java.util.ArrayList();", subClass);
        subClass.addField(invocations);

        CtMethod getInvocations = CtNewMethod.make(list, "getInvocations", new CtClass[]{}, new CtClass[]{}, "{ return invocations; }", subClass);
        subClass.addMethod(getInvocations);

        CtMethod setFirstName = CtNewMethod.make("public void setFirstName(String s) { invocations.add(\"firstName\"); super.setFirstName(s); }", subClass);
        subClass.addMethod(setFirstName);
        return subClass;
    }
}

The Metadata classloader also needs to be a child of the hex ClassLoader layer. It also needs to be cached at that level so it probably cannot be a singleton. An individual application will probably be responsible for declaring and storing its own metadata cache.