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Diffstat (limited to 'gson/src/main/java/com/google/gson/InstanceCreator.java')
-rw-r--r-- | gson/src/main/java/com/google/gson/InstanceCreator.java | 92 |
1 files changed, 92 insertions, 0 deletions
diff --git a/gson/src/main/java/com/google/gson/InstanceCreator.java b/gson/src/main/java/com/google/gson/InstanceCreator.java new file mode 100644 index 00000000..d5096a07 --- /dev/null +++ b/gson/src/main/java/com/google/gson/InstanceCreator.java @@ -0,0 +1,92 @@ +/* + * 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<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); +} |