pack-priv final Class Enhancer

Each enhancer has the same number of constructors as the class it enhances, but each constructor takes an additional handler array before the rest of the expected arguments.

Enhanced methods are overridden to call the handler with the same index as the method. The handler delegates to the interceptor stack. Once the last interceptor returns the handler will call back into the trampoline with the method index, which invokes the superclass method.

The trampoline also provides access to constructor invokers that take a context object (the handler array) with an argument array and invokes the appropriate enhanced constructor. These invokers are used in the proxy factory to create enhanced instances.

Enhanced classes have the following pseudo-Java structure:

public class HostClass$$EnhancerByGuice
  extends HostClass
{
  // InterceptorStackCallbacks, one per enhanced method
  private final InvocationHandler[] GUICE$HANDLERS;

  public HostClass$$EnhancerByGuice(InvocationHandler[] handlers, ...) {
     // JVM lets us store this before calling the superclass constructor
    GUICE$HANDLERS = handlers;
    super(...);
  }

  public static Object GUICE$TRAMPOLINE(int index, Object context, Object[] args) {
    switch (index) {
      case 0: {
        return new HostClass$$EnhancerByGuice((InvocationHandler[]) context, ...);
      }
      case 1: {
        return context.super.instanceMethod(...); // call original unenhanced method
      }
    }
    return null;
  }

  // enhanced method
  public final Object instanceMethod(...) {
    // pack arguments and trigger the associated InterceptorStackCallback
    return GUICE$HANDLERS[0].invoke(this, null, args);
  }

  // ...
}