/*
* 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 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 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 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 Id class, or does not want to define a no-args constructor for it. The developer can solve this problem by defining an InstanceCreator for 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 IdInstanceCreator with Gson as follows:
Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdInstanceCreator()).create();
Author: Inderjeet Singh, Joel Leitch Type parameters: - <T> – the type of object that will be created by this implementation.
/**
* 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 new to create a new instance. Params: - type – the parameterized T represented as a
Type.
Returns: a default object instance of type 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);
}