/* * Copyright (C) 2014 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.annotations; import com.google.gson.Gson; import com.google.gson.GsonBuilder; import com.google.gson.InstanceCreator; import com.google.gson.JsonDeserializer; import com.google.gson.JsonSerializer; import com.google.gson.TypeAdapter; import com.google.gson.TypeAdapterFactory; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; /** * An annotation that indicates the Gson {@link TypeAdapter} to use with a class or field. * *
Here is an example of how this annotation is used: * *
* @JsonAdapter(UserJsonAdapter.class) * public class User { * public final String firstName, lastName; * * private User(String firstName, String lastName) { * this.firstName = firstName; * this.lastName = lastName; * } * } * * public class UserJsonAdapter extends TypeAdapter<User> { * @Override public void write(JsonWriter out, User user) throws IOException { * // implement write: combine firstName and lastName into name * out.beginObject(); * out.name("name"); * out.value(user.firstName + " " + user.lastName); * out.endObject(); * } * * @Override public User read(JsonReader in) throws IOException { * // implement read: split name into firstName and lastName * in.beginObject(); * in.nextName(); * String[] nameParts = in.nextString().split(" "); * in.endObject(); * return new User(nameParts[0], nameParts[1]); * } * } ** * Since {@code User} class specified {@code UserJsonAdapter.class} in {@code @JsonAdapter} * annotation, it will automatically be invoked to serialize/deserialize {@code User} instances. * *
Here is an example of how to apply this annotation to a field. * *
* private static final class Gadget { * @JsonAdapter(UserJsonAdapter.class) * final User user; * * Gadget(User user) { * this.user = user; * } * } ** * It's possible to specify different type adapters on a field, that field's type, and in the {@link * GsonBuilder}. Field annotations take precedence over {@code GsonBuilder}-registered type * adapters, which in turn take precedence over annotated types. * *
The class referenced by this annotation must be either a {@link TypeAdapter} or a {@link * TypeAdapterFactory}, or must implement one or both of {@link JsonDeserializer} or {@link * JsonSerializer}. Using {@link TypeAdapterFactory} makes it possible to delegate to the enclosing * {@link Gson} instance. By default the specified adapter will not be called for {@code null} * values; set {@link #nullSafe()} to {@code false} to let the adapter handle {@code null} values * itself. * *
The type adapter is created in the same way Gson creates instances of custom classes during * deserialization, that means: * *
{@code Gson} instances might cache the adapter they create for a {@code @JsonAdapter} * annotation. It is not guaranteed that a new adapter is created every time the annotated class or * field is serialized or deserialized. * * @since 2.3 * @author Inderjeet Singh * @author Joel Leitch * @author Jesse Wilson */ // Note that the above example is taken from AdaptAnnotationTest. @Retention(RetentionPolicy.RUNTIME) @Target({ElementType.TYPE, ElementType.FIELD}) public @interface JsonAdapter { /** * Either a {@link TypeAdapter} or {@link TypeAdapterFactory}, or one or both of {@link * JsonDeserializer} or {@link JsonSerializer}. */ Class> value(); /** * Whether the adapter referenced by {@link #value()} should be made {@linkplain * TypeAdapter#nullSafe() null-safe}. * *
If {@code true} (the default), it will be made null-safe and Gson will handle {@code null} * Java objects on serialization and JSON {@code null} on deserialization without calling the * adapter. If {@code false}, the adapter will have to handle the {@code null} values. */ boolean nullSafe() default true; }