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.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:</p>
 * <pre>
 * @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();
 *     // implement the write method
 *   }
 *   @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 User class specified UserJsonAdapter.class in @JsonAdapter annotation, it
 * will automatically be invoked to serialize/deserialize User instances. <br>
 *
 * <p> Here is an example of how to apply this annotation to a field.
 * <pre>
 * private static final class Gadget {
 *   @JsonAdapter(UserJsonAdapter2.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 com.google.gson.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}. Using the factory interface
 * makes it possible to delegate to the enclosing {@code Gson} instance.
 *
 * @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}. */
  Class<?> value();

}