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.smartandroid.sa.json;

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);
}