Move gson to its own folder

[unical.git] / gson / com / google / gson / InstanceCreator.java

diff --git a/gson/com/google/gson/InstanceCreator.java b/gson/com/google/gson/InstanceCreator.java
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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&lt;T&gt; {
+ *   private final Class&lt;T&gt; clazz;
+ *   private final long value;
+ *   public Id(Class&lt;T&gt; 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&lt;Id&gt; {
+ *   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);
+}