Instances.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.apache.twill.internal.utils;

import com.google.common.base.Defaults;
import com.google.common.base.Preconditions;
import com.google.common.base.Throwables;
import com.google.common.reflect.TypeToken;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;

/**
 * Utility class to help instantiate object instance from class.
 */
public final class Instances {

  private static final Object UNSAFE;
  private static final Method UNSAFE_NEW_INSTANCE;

  static {
    Object unsafe;
    Method newInstance;
    try {
      Class<?> clz = Class.forName("sun.misc.Unsafe");
      Field f = clz.getDeclaredField("theUnsafe");
      f.setAccessible(true);
      unsafe = f.get(null);

      newInstance = clz.getMethod("allocateInstance", Class.class);
    } catch (Exception e) {
      unsafe = null;
      newInstance = null;
    }
    UNSAFE = unsafe;
    UNSAFE_NEW_INSTANCE = newInstance;
  }

  /**
   * Creates a new instance of the given class. It will use the default constructor if it is presents.
   * Otherwise it will try to use {@link sun.misc.Unsafe#allocateInstance(Class)} to create the instance.
   * @param clz Class of object to be instantiated.
   * @param <T> Type of the class
   * @return An instance of type {@code <T>}
   */
  @SuppressWarnings("unchecked")
  public static <T> T newInstance(Class<T> clz) {
    try {
      try {
        Constructor<T> cons = clz.getDeclaredConstructor();
        if (!cons.isAccessible()) {
          cons.setAccessible(true);
        }
        return cons.newInstance();
      } catch (Exception e) {
        // Try to use Unsafe
        Preconditions.checkState(UNSAFE != null, "Fail to instantiate with Unsafe.");
        return unsafeCreate(clz);
      }
    } catch (Exception e) {
      throw Throwables.propagate(e);
    }
  }


  /**
   * Creates an instance of the given using Unsafe. It also initialize all fields into default values.
   */
  private static <T> T unsafeCreate(Class<T> clz) throws InvocationTargetException, IllegalAccessException {
    T instance = (T) UNSAFE_NEW_INSTANCE.invoke(UNSAFE, clz);

    for (TypeToken<?> type : TypeToken.of(clz).getTypes().classes()) {
      if (Object.class.equals(type.getRawType())) {
        break;
      }
      for (Field field : type.getRawType().getDeclaredFields()) {
        if (Modifier.isStatic(field.getModifiers())) {
          continue;
        }
        if (!field.isAccessible()) {
          field.setAccessible(true);
        }
        field.set(instance, Defaults.defaultValue(field.getType()));
      }
    }

    return instance;
  }


  private Instances() {
    // Protect instantiation of this class
  }
}