gson/com/google/gson/InstanceCreator.java

   1 /*
   2  * Copyright (C) 2008 Google Inc.
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at
   7  *
   8  * http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16
  17 package com.google.gson;
  18
  19 import java.lang.reflect.Type;
  20
  21 /**
  22  * This interface is implemented to create instances of a class that does not define a no-args
  23  * constructor. If you can modify the class, you should instead add a private, or public
  24  * no-args constructor. However, that is not possible for library classes, such as JDK classes, or
  25  * a third-party library that you do not have source-code of. In such cases, you should define an
  26  * instance creator for the class. Implementations of this interface should be registered with
  27  * {@link GsonBuilder#registerTypeAdapter(Type, Object)} method before Gson will be able to use
  28  * them.
  29  * <p>Let us look at an example where defining an InstanceCreator might be useful. The
  30  * {@code Id} class defined below does not have a default no-args constructor.</p>
  31  *
  32  * <pre>
  33  * public class Id&lt;T&gt; {
  34  *   private final Class&lt;T&gt; clazz;
  35  *   private final long value;
  36  *   public Id(Class&lt;T&gt; clazz, long value) {
  37  *     this.clazz = clazz;
  38  *     this.value = value;
  39  *   }
  40  * }
  41  * </pre>
  42  *
  43  * <p>If Gson encounters an object of type {@code Id} during deserialization, it will throw an
  44  * exception. The easiest way to solve this problem will be to add a (public or private) no-args
  45  * constructor as follows:</p>
  46  *
  47  * <pre>
  48  * private Id() {
  49  *   this(Object.class, 0L);
  50  * }
  51  * </pre>
  52  *
  53  * <p>However, let us assume that the developer does not have access to the source-code of the
  54  * {@code Id} class, or does not want to define a no-args constructor for it. The developer
  55  * can solve this problem by defining an {@code InstanceCreator} for {@code Id}:</p>
  56  *
  57  * <pre>
  58  * class IdInstanceCreator implements InstanceCreator&lt;Id&gt; {
  59  *   public Id createInstance(Type type) {
  60  *     return new Id(Object.class, 0L);
  61  *   }
  62  * }
  63  * </pre>
  64  *
  65  * <p>Note that it does not matter what the fields of the created instance contain since Gson will
  66  * overwrite them with the deserialized values specified in Json. You should also ensure that a
  67  * <i>new</i> object is returned, not a common object since its fields will be overwritten.
  68  * The developer will need to register {@code IdInstanceCreator} with Gson as follows:</p>
  69  *
  70  * <pre>
  71  * Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdInstanceCreator()).create();
  72  * </pre>
  73  *
  74  * @param <T> the type of object that will be created by this implementation.
  75  *
  76  * @author Inderjeet Singh
  77  * @author Joel Leitch
  78  */
  79 public interface InstanceCreator<T> {
  80
  81   /**
  82    * Gson invokes this call-back method during deserialization to create an instance of the
  83    * specified type. The fields of the returned instance are overwritten with the data present
  84    * in the Json. Since the prior contents of the object are destroyed and overwritten, do not
  85    * return an instance that is useful elsewhere. In particular, do not return a common instance,
  86    * always use {@code new} to create a new instance.
  87    *
  88    * @param type the parameterized T represented as a {@link Type}.
  89    * @return a default object instance of type T.
  90    */
  91   public T createInstance(Type type);
  92 }