136 lines
5.0 KiB
Java
136 lines
5.0 KiB
Java
/*
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* Copyright (C) 2014 Google Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package com.google.gson.annotations;
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import com.google.gson.Gson;
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import com.google.gson.GsonBuilder;
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import com.google.gson.InstanceCreator;
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import com.google.gson.JsonDeserializer;
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import com.google.gson.JsonSerializer;
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import com.google.gson.TypeAdapter;
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import com.google.gson.TypeAdapterFactory;
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import java.lang.annotation.ElementType;
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import java.lang.annotation.Retention;
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import java.lang.annotation.RetentionPolicy;
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import java.lang.annotation.Target;
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/**
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* An annotation that indicates the Gson {@link TypeAdapter} to use with a class or field.
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*
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* <p>Here is an example of how this annotation is used:
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*
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* <pre>
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* @JsonAdapter(UserJsonAdapter.class)
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* public class User {
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* public final String firstName, lastName;
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*
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* private User(String firstName, String lastName) {
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* this.firstName = firstName;
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* this.lastName = lastName;
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* }
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* }
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*
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* public class UserJsonAdapter extends TypeAdapter<User> {
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* @Override public void write(JsonWriter out, User user) throws IOException {
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* // implement write: combine firstName and lastName into name
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* out.beginObject();
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* out.name("name");
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* out.value(user.firstName + " " + user.lastName);
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* out.endObject();
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* }
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*
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* @Override public User read(JsonReader in) throws IOException {
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* // implement read: split name into firstName and lastName
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* in.beginObject();
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* in.nextName();
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* String[] nameParts = in.nextString().split(" ");
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* in.endObject();
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* return new User(nameParts[0], nameParts[1]);
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* }
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* }
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* </pre>
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*
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* Since {@code User} class specified {@code UserJsonAdapter.class} in {@code @JsonAdapter}
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* annotation, it will automatically be invoked to serialize/deserialize {@code User} instances.
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*
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* <p>Here is an example of how to apply this annotation to a field.
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*
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* <pre>
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* private static final class Gadget {
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* @JsonAdapter(UserJsonAdapter.class)
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* final User user;
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*
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* Gadget(User user) {
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* this.user = user;
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* }
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* }
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* </pre>
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*
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* It's possible to specify different type adapters on a field, that field's type, and in the {@link
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* GsonBuilder}. Field annotations take precedence over {@code GsonBuilder}-registered type
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* adapters, which in turn take precedence over annotated types.
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*
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* <p>The class referenced by this annotation must be either a {@link TypeAdapter} or a {@link
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* TypeAdapterFactory}, or must implement one or both of {@link JsonDeserializer} or {@link
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* JsonSerializer}. Using {@link TypeAdapterFactory} makes it possible to delegate to the enclosing
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* {@link Gson} instance. By default the specified adapter will not be called for {@code null}
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* values; set {@link #nullSafe()} to {@code false} to let the adapter handle {@code null} values
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* itself.
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*
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* <p>The type adapter is created in the same way Gson creates instances of custom classes during
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* deserialization, that means:
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*
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* <ol>
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* <li>If a custom {@link InstanceCreator} has been registered for the adapter class, it will be
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* used to create the instance
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* <li>Otherwise, if the adapter class has a no-args constructor (regardless of which visibility),
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* it will be invoked to create the instance
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* <li>Otherwise, JDK {@code Unsafe} will be used to create the instance; see {@link
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* GsonBuilder#disableJdkUnsafe()} for the unexpected side-effects this might have
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* </ol>
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*
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* <p>{@code Gson} instances might cache the adapter they create for a {@code @JsonAdapter}
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* annotation. It is not guaranteed that a new adapter is created every time the annotated class or
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* field is serialized or deserialized.
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*
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* @since 2.3
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* @author Inderjeet Singh
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* @author Joel Leitch
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* @author Jesse Wilson
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*/
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// Note that the above example is taken from AdaptAnnotationTest.
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@Retention(RetentionPolicy.RUNTIME)
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@Target({ElementType.TYPE, ElementType.FIELD})
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public @interface JsonAdapter {
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/**
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* Either a {@link TypeAdapter} or {@link TypeAdapterFactory}, or one or both of {@link
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* JsonDeserializer} or {@link JsonSerializer}.
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*/
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Class<?> value();
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/**
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* Whether the adapter referenced by {@link #value()} should be made {@linkplain
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* TypeAdapter#nullSafe() null-safe}.
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*
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* <p>If {@code true} (the default), it will be made null-safe and Gson will handle {@code null}
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* Java objects on serialization and JSON {@code null} on deserialization without calling the
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* adapter. If {@code false}, the adapter will have to handle the {@code null} values.
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*/
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boolean nullSafe() default true;
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}
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