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/*
* 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 GsonBuilder#registerTypeAdapter(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 Type}.
* @return a default object instance of type T.
*/
public T createInstance(Type type);
}
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