gson-comments/gson/src/main/java/com/google/gson/annotations/JsonAdapter.java

/*
 * 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.
 *
 * <p>Here is an example of how this annotation is used:
 *
 * <pre>
 * &#64;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&lt;User&gt; {
 *   &#64;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();
 *   }
 *
 *   &#64;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]);
 *   }
 * }
 * </pre>
 *
 * Since {@code User} class specified {@code UserJsonAdapter.class} in {@code @JsonAdapter}
 * annotation, it will automatically be invoked to serialize/deserialize {@code User} instances.
 *
 * <p>Here is an example of how to apply this annotation to a field.
 *
 * <pre>
 * private static final class Gadget {
 *   &#64;JsonAdapter(UserJsonAdapter.class)
 *   final User user;
 *
 *   Gadget(User user) {
 *     this.user = user;
 *   }
 * }
 * </pre>
 *
 * 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.
 *
 * <p>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.
 *
 * <p>The type adapter is created in the same way Gson creates instances of custom classes during
 * deserialization, that means:
 *
 * <ol>
 *   <li>If a custom {@link InstanceCreator} has been registered for the adapter class, it will be
 *       used to create the instance
 *   <li>Otherwise, if the adapter class has a no-args constructor (regardless of which visibility),
 *       it will be invoked to create the instance
 *   <li>Otherwise, JDK {@code Unsafe} will be used to create the instance; see {@link
 *       GsonBuilder#disableJdkUnsafe()} for the unexpected side-effects this might have
 * </ol>
 *
 * <p>{@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}.
   *
   * <p>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;
}