package com.atlassian.plugins.rest.module.util;
   
   import java.lang.reflect.Constructor;
   import java.lang.reflect.InvocationHandler;
   import java.lang.reflect.Method;
   import java.lang.reflect.Modifier;
   import java.lang.reflect.Proxy;
   import java.util.concurrent.ExecutionException;
   
  import net.sf.cglib.proxy.Callback;
  import net.sf.cglib.proxy.Enhancer;
  import net.sf.cglib.proxy.Factory;
  
  import com.atlassian.plugins.rest.module.ChainingClassLoader;
  import com.google.common.cache.CacheBuilder;
  import com.google.common.cache.CacheLoader;
  import com.google.common.cache.LoadingCache;
  
  /**
   * Proxy creation utilities
   */
  public class ProxyUtils {
      private static LoadingCache<Class<?>, ConstructorAndArgs> generatorCache = CacheBuilder.newBuilder().build(new CacheLoader<Class<?>, ConstructorAndArgs>() {
          @Override
          public ConstructorAndArgs load(Class<?> from) throws Exception {
              return new ConstructorAndArgs(from);
          }
      });
  
      public static <T> T create(Class<T> clazz, Callback callback) {
          try {
              return (T) generatorCache.get(clazz).create(callback);
          } catch (ExecutionException e) {
              throw new RuntimeException("Failed to construct class: ", e);
          }
      }
  }
  
  /*
  This class encapsulates a proxy and it's construction args. These should be cached per class.
   */
  
  class ConstructorAndArgs {
      private Class<?> clazz;
      private Object prototype;
      private Object[] args;
      private Constructor<?> constructor;
  
      ConstructorAndArgs(Class<?> clazz) {
          this.clazz = clazz;
          initialise();
      }
  
      private void initialise() {
          Enhancer enhancer = new Enhancer();
          enhancer.setSuperclass(clazz);
          enhancer.setCallback(new UnsupportedOperationInvocationHandler());
          enhancer.setClassLoader(new ChainingClassLoader(ProxyUtils.class.getClassLoader(), clazz.getClassLoader()));
  
          Constructor<?>[] constructors = clazz.getConstructors();
  
          for (Constructor constructor : constructors) {
              if ((constructor.getModifiers() & Modifier.PUBLIC) != 0) {
                  this.constructor = constructor;
  
                  int size = constructor.getParameterTypes().length;
                  args = new Object[size];
  
                  for (int i = 0; i < args.length; i++)
                      args[i] = createEmptyValue(constructor.getParameterTypes()[i]);
  
                  prototype = clazz.cast(enhancer.create(constructor.getParameterTypes(), args));
                  return;
              }
          }
  
          throw new IllegalArgumentException("Class has no accessible constructor");
      }
  
  
      private static Object createEmptyValue(Class aClass) {
          //todo: add more types
          if (aClass.isInterface())
              return stubInterface(aClass);
          else if (aClass == Long.TYPE)
              return 0L;
          else
              return null;
      }
  
      private static Object stubInterface(Class _interface) {
          return Proxy.newProxyInstance(_interface.getClassLoader(), new Class[]{_interface}, UnsupportedOperationInvocationHandler.INSTANCE);
      }
  
      public Object create(Callback... callback) {
          return clazz.cast(((Factory) prototype).newInstance(constructor.getParameterTypes(), args, callback));
      }
  }
  
 class UnsupportedOperationInvocationHandler implements InvocationHandler, net.sf.cglib.proxy.InvocationHandler {
     public static UnsupportedOperationInvocationHandler INSTANCE = new UnsupportedOperationInvocationHandler();
 
     public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
         throw new UnsupportedOperationException();
     }
 }