94 lines
3.6 KiB
Java
94 lines
3.6 KiB
Java
/*
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* Copyright (C) 2008 Google Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package com.google.gson;
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import java.lang.reflect.Type;
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/**
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* This interface is implemented to create instances of a class that does not define a no-args
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* constructor. If you can modify the class, you should instead add a private, or public no-args
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* constructor. However, that is not possible for library classes, such as JDK classes, or a
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* third-party library that you do not have source-code of. In such cases, you should define an
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* instance creator for the class. Implementations of this interface should be registered with
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* {@link GsonBuilder#registerTypeAdapter(Type, Object)} method before Gson will be able to use
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* them.
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*
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* <p>Let us look at an example where defining an InstanceCreator might be useful. The {@code Id}
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* class defined below does not have a default no-args constructor.
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*
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* <pre>
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* public class Id<T> {
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* private final Class<T> clazz;
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* private final long value;
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* public Id(Class<T> clazz, long value) {
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* this.clazz = clazz;
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* this.value = value;
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* }
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* }
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* </pre>
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*
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* <p>If Gson encounters an object of type {@code Id} during deserialization, it will throw an
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* exception. The easiest way to solve this problem will be to add a (public or private) no-args
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* constructor as follows:
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*
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* <pre>
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* private Id() {
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* this(Object.class, 0L);
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* }
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* </pre>
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*
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* <p>However, let us assume that the developer does not have access to the source-code of the
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* {@code Id} class, or does not want to define a no-args constructor for it. The developer can
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* solve this problem by defining an {@code InstanceCreator} for {@code Id}:
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*
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* <pre>
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* class IdInstanceCreator implements InstanceCreator<Id> {
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* public Id createInstance(Type type) {
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* return new Id(Object.class, 0L);
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* }
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* }
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* </pre>
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*
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* <p>Note that it does not matter what the fields of the created instance contain since Gson will
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* overwrite them with the deserialized values specified in JSON. You should also ensure that a
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* <i>new</i> object is returned, not a common object since its fields will be overwritten. The
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* developer will need to register {@code IdInstanceCreator} with Gson as follows:
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*
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* <pre>
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* Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdInstanceCreator()).create();
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* </pre>
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*
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* @param <T> the type of object that will be created by this implementation.
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* @see GsonBuilder#registerTypeAdapter(Type, Object)
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* @author Inderjeet Singh
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* @author Joel Leitch
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*/
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public interface InstanceCreator<T> {
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/**
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* Gson invokes this call-back method during deserialization to create an instance of the
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* specified type. The fields of the returned instance are overwritten with the data present in
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* the JSON. Since the prior contents of the object are destroyed and overwritten, do not return
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* an instance that is useful elsewhere. In particular, do not return a common instance, always
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* use {@code new} to create a new instance.
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*
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* @param type the parameterized T represented as a {@link Type}.
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* @return a default object instance of type T.
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*/
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public T createInstance(Type type);
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}
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