/*
 * Copyright (C) 2011 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.gson.internal;

import java.io.ObjectInputStream;
import java.io.ObjectStreamClass;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;

Do sneaky things to allocate objects without invoking their constructors.
Author:
Joel Leitch, Jesse Wilson/**
 * Do sneaky things to allocate objects without invoking their constructors.
 *
 * @author Joel Leitch
 * @author Jesse Wilson
 */
public abstract class UnsafeAllocator {
  public abstract <T> T newInstance(Class<T> c) throws Exception;

  public static UnsafeAllocator create() {
    // try JVM
    // public class Unsafe {
    //   public Object allocateInstance(Class<?> type);
    // }
    try {
      Class<?> unsafeClass = Class.forName("sun.misc.Unsafe");
      Field f = unsafeClass.getDeclaredField("theUnsafe");
      f.setAccessible(true);
      final Object unsafe = f.get(null);
      final Method allocateInstance = unsafeClass.getMethod("allocateInstance", Class.class);
      return new UnsafeAllocator() {
        @Override
        @SuppressWarnings("unchecked")
        public <T> T newInstance(Class<T> c) throws Exception {
          assertInstantiable(c);
          return (T) allocateInstance.invoke(unsafe, c);
        }
      };
    } catch (Exception ignored) {
    }

    // try dalvikvm, post-gingerbread
    // public class ObjectStreamClass {
    //   private static native int getConstructorId(Class<?> c);
    //   private static native Object newInstance(Class<?> instantiationClass, int methodId);
    // }
    try {
      Method getConstructorId = ObjectStreamClass.class
          .getDeclaredMethod("getConstructorId", Class.class);
      getConstructorId.setAccessible(true);
      final int constructorId = (Integer) getConstructorId.invoke(null, Object.class);
      final Method newInstance = ObjectStreamClass.class
          .getDeclaredMethod("newInstance", Class.class, int.class);
      newInstance.setAccessible(true);
      return new UnsafeAllocator() {
        @Override
        @SuppressWarnings("unchecked")
        public <T> T newInstance(Class<T> c) throws Exception {
          assertInstantiable(c);
          return (T) newInstance.invoke(null, c, constructorId);
        }
      };
    } catch (Exception ignored) {
    }

    // try dalvikvm, pre-gingerbread
    // public class ObjectInputStream {
    //   private static native Object newInstance(
    //     Class<?> instantiationClass, Class<?> constructorClass);
    // }
    try {
      final Method newInstance = ObjectInputStream.class
          .getDeclaredMethod("newInstance", Class.class, Class.class);
      newInstance.setAccessible(true);
      return new UnsafeAllocator() {
        @Override
        @SuppressWarnings("unchecked")
        public <T> T newInstance(Class<T> c) throws Exception {
          assertInstantiable(c);
          return (T) newInstance.invoke(null, c, Object.class);
        }
      };
    } catch (Exception ignored) {
    }

    // give up
    return new UnsafeAllocator() {
      @Override
      public <T> T newInstance(Class<T> c) {
        throw new UnsupportedOperationException("Cannot allocate " + c);
      }
    };
  }

  
Check if the class can be instantiated by unsafe allocator. If the instance has interface or abstract modifiers throw an UnsupportedOperationException 
Params:
c – instance of the class to be checked/**
   * Check if the class can be instantiated by unsafe allocator. If the instance has interface or abstract modifiers
   * throw an {@link java.lang.UnsupportedOperationException}
   * @param c instance of the class to be checked
   */
  static void assertInstantiable(Class<?> c) {
    int modifiers = c.getModifiers();
    if (Modifier.isInterface(modifiers)) {
      throw new UnsupportedOperationException("Interface can't be instantiated! Interface name: " + c.getName());
    }
    if (Modifier.isAbstract(modifiers)) {
      throw new UnsupportedOperationException("Abstract class can't be instantiated! Class name: " + c.getName());
    }
  }
}