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Explore the source code of the class 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
 * [email protected] GsonBuilder#registerTypeAdapter(Type, Object)} method before Gson will be able to use
 * them.
 * 

Let us look at an example where defining an InstanceCreator might be useful. The * [email protected] Id} class defined below does not have a default no-args constructor.

* *
 * 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;
 *   }
 * }
 * 
* *

If Gson encounters an object of type [email protected] 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:

* *
 * private Id() {
 *   this(Object.class, 0L);
 * }
 * 
* *

However, let us assume that the developer does not have access to the source-code of the * [email protected] Id} class, or does not want to define a no-args constructor for it. The developer * can solve this problem by defining an [email protected] InstanceCreator} for [email protected] Id}:

* *
 * class IdInstanceCreator implements InstanceCreator<Id> {
 *   public Id createInstance(Type type) {
 *     return new Id(Object.class, 0L);
 *   }
 * }
 * 
* *

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 * new object is returned, not a common object since its fields will be overwritten. * The developer will need to register [email protected] IdInstanceCreator} with Gson as follows:

* *
 * Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdInstanceCreator()).create();
 * 
* * @param the type of object that will be created by this implementation. * * @author Inderjeet Singh * @author Joel Leitch */ public interface InstanceCreator { /** * 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 [email protected] new} to create a new instance. * * @param type the parameterized T represented as a [email protected] Type}. * @return a default object instance of type T. */ public T createInstance(Type type); }