InstanceCreator.java

/*
 * Copyright (C) 2008 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;

import java.lang.reflect.Type;

/**
 * This interface is implemented to create instances of a class that does not define a no-args
 * constructor. If you can modify the class, you should instead add a private, or public
 * no-args constructor. However, that is not possible for library classes, such as JDK classes, or
 * a third-party library that you do not have source-code of. In such cases, you should define an
 * instance creator for the class. Implementations of this interface should be registered with
 * {@link com.google.gson.GsonBuilder#registerTypeAdapter(java.lang.reflect.Type, Object)} method before Gson will be able to use
 * them.
 * <p>Let us look at an example where defining an InstanceCreator might be useful. The
 * {@code Id} class defined below does not have a default no-args constructor.</p>
 *
 * <pre>
 * public class Id<T> {
 *   private final Class<T> clazz;
 *   private final long value;
 *   public Id(Class<T> clazz, long value) {
 *     this.clazz = clazz;
 *     this.value = value;
 *   }
 * }
 * </pre>
 *
 * <p>If Gson encounters an object of type {@code Id} during deserialization, it will throw an
 * exception. The easiest way to solve this problem will be to add a (public or private) no-args
 * constructor as follows:</p>
 *
 * <pre>
 * private Id() {
 *   this(Object.class, 0L);
 * }
 * </pre>
 *
 * <p>However, let us assume that the developer does not have access to the source-code of the
 * {@code Id} class, or does not want to define a no-args constructor for it. The developer
 * can solve this problem by defining an {@code InstanceCreator} for {@code Id}:</p>
 *
 * <pre>
 * class IdInstanceCreator implements InstanceCreator<Id> {
 *   public Id createInstance(Type type) {
 *     return new Id(Object.class, 0L);
 *   }
 * }
 * </pre>
 *
 * <p>Note that it does not matter what the fields of the created instance contain since Gson will
 * overwrite them with the deserialized values specified in Json. You should also ensure that a
 * <i>new</i> object is returned, not a common object since its fields will be overwritten.
 * The developer will need to register {@code IdInstanceCreator} with Gson as follows:</p>
 *
 * <pre>
 * Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdInstanceCreator()).create();
 * </pre>
 *
 * @param <T> the type of object that will be created by this implementation.
 *
 * @author Inderjeet Singh
 * @author Joel Leitch
 */
public interface InstanceCreator<T> {

  /**
   * Gson invokes this call-back method during deserialization to create an instance of the
   * specified type. The fields of the returned instance are overwritten with the data present
   * in the Json. Since the prior contents of the object are destroyed and overwritten, do not
   * return an instance that is useful elsewhere. In particular, do not return a common instance,
   * always use {@code new} to create a new instance.
   *
   * @param type the parameterized T represented as a {@link java.lang.reflect.Type}.
   * @return a default object instance of type T.
   */
  public T createInstance(Type type);
}