RuntimeTypeAdapterFactory.java

/*
 * Copyright (C) 2011 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.ylinor.library.utils;

import java.io.IOException;
import java.util.LinkedHashMap;
import java.util.Map;

import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;
import com.google.gson.JsonPrimitive;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.internal.Streams;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;


/**
 * Adapts values whose runtime type may differ from their declaration type. This
 * is necessary when a field's type is not the same type that GSON should create
 * when deserializing that field. For example, consider these types:
 * 
 * <pre>
 * {
 *     &#64;code
 *     abstract class Shape
 *     {
 *         int x;
 *         int y;
 *     }
 *     class Circle extends Shape
 *     {
 *         int radius;
 *     }
 *     class Rectangle extends Shape
 *     {
 *         int width;
 *         int height;
 *     }
 *     class Diamond extends Shape
 *     {
 *         int width;
 *         int height;
 *     }
 *     class Drawing
 *     {
 *         Shape bottomShape;
 *         Shape topShape;
 *     }
 * }
 * </pre>
 * <p>
 * Without additional type information, the serialized JSON is ambiguous. Is the
 * bottom shape in this drawing a rectangle or a diamond?
 * 
 * <pre>
 *    {@code
 *   {
 *     "bottomShape": {
 *       "width": 10,
 *       "height": 5,
 *       "x": 0,
 *       "y": 0
 *     },
 *     "topShape": {
 *       "radius": 2,
 *       "x": 4,
 *       "y": 1
 *     }
 *   }}
 * </pre>
 * 
 * This class addresses this problem by adding type information to the
 * serialized JSON and honoring that type information when the JSON is
 * deserialized:
 * 
 * <pre>
 *    {@code
 *   {
 *     "bottomShape": {
 *       "type": "Diamond",
 *       "width": 10,
 *       "height": 5,
 *       "x": 0,
 *       "y": 0
 *     },
 *     "topShape": {
 *       "type": "Circle",
 *       "radius": 2,
 *       "x": 4,
 *       "y": 1
 *     }
 *   }}
 * </pre>
 * 
 * Both the type field name ({@code "type"}) and the type labels ({@code
 * "Rectangle"}) are configurable.
 *
 * <h3>Registering Types</h3> Create a {@code RuntimeTypeAdapterFactory} by
 * passing the base type and type field name to the {@link #of} factory method.
 * If you don't supply an explicit type field name, {@code "type"} will be used.
 * 
 * <pre>
 * {
 *     &#64;code
 *     RuntimeTypeAdapterFactory<Shape> shapeAdapterFactory = RuntimeTypeAdapterFactory.of(Shape.class, "type");
 * }
 * </pre>
 * 
 * Next register all of your subtypes. Every subtype must be explicitly
 * registered. This protects your application from injection attacks. If you
 * don't supply an explicit type label, the type's simple name will be used.
 * 
 * <pre>
 *    {@code
 *   shapeAdapter.registerSubtype(Rectangle.class, "Rectangle");
 *   shapeAdapter.registerSubtype(Circle.class, "Circle");
 *   shapeAdapter.registerSubtype(Diamond.class, "Diamond");
 * }
 * </pre>
 * 
 * Finally, register the type adapter factory in your application's GSON
 * builder:
 * 
 * <pre>
 * {
 *     &#64;code
 *     Gson gson = new GsonBuilder().registerTypeAdapterFactory(shapeAdapterFactory).create();
 * }
 * </pre>
 * 
 * Like {@code GsonBuilder}, this API supports chaining:
 * 
 * <pre>
 * {
 *     &#64;code
 *     RuntimeTypeAdapterFactory<Shape> shapeAdapterFactory = RuntimeTypeAdapterFactory.of(Shape.class).registerSubtype(Rectangle.class).registerSubtype(Circle.class).registerSubtype(Diamond.class);
 * }
 * </pre>
 */
public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory
{
    private final Class<?> baseType;
    private final String typeFieldName;
    public final Map<String, Class<?>> labelToSubtype = new LinkedHashMap<String, Class<?>>();
    private final Map<Class<?>, String> subtypeToLabel = new LinkedHashMap<Class<?>, String>();

    private RuntimeTypeAdapterFactory(Class<?> baseType, String typeFieldName)
    {
        if (typeFieldName == null || baseType == null)
        {
            throw new NullPointerException();
        }
        this.baseType = baseType;
        this.typeFieldName = typeFieldName;
    }

    /**
     * Creates a new runtime type adapter using for {@code baseType} using
     * {@code
     * typeFieldName} as the type field name. Type field names are case
     * sensitive.
     */
    public static <T> RuntimeTypeAdapterFactory<T> of(Class<T> baseType, String typeFieldName)
    {
        return new RuntimeTypeAdapterFactory<T>(baseType, typeFieldName);
    }

    /**
     * Creates a new runtime type adapter for {@code baseType} using
     * {@code "type"} as the type field name.
     */
    public static <T> RuntimeTypeAdapterFactory<T> of(Class<T> baseType)
    {
        return new RuntimeTypeAdapterFactory<T>(baseType, "type");
    }

    /**
     * Registers {@code type} identified by {@code label}. Labels are case
     * sensitive.
     *
     * @throws IllegalArgumentException if either {@code type} or {@code label}
     *         have already been registered on this type adapter.
     */
    public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type, String label)
    {
        if (type == null || label == null)
        {
            throw new NullPointerException();
        }
        if (subtypeToLabel.containsKey(type) || labelToSubtype.containsKey(label))
        {
            throw new IllegalArgumentException("types and labels must be unique");
        }
        labelToSubtype.put(label, type);
        subtypeToLabel.put(type, label);
        return this;
    }

    /**
     * Registers {@code type} identified by its {@link Class#getSimpleName
     * simple name}. Labels are case sensitive.
     *
     * @throws IllegalArgumentException if either {@code type} or its simple
     *         name have already been registered on this type adapter.
     */
    public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type)
    {
        return registerSubtype(type, type.getSimpleName());
    }

    public <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type)
    {
        if (type == null || !baseType.isAssignableFrom(type.getRawType()))
        {
            return null;
        }

        final Map<String, TypeAdapter<?>> labelToDelegate = new LinkedHashMap<String, TypeAdapter<?>>();
        final Map<Class<?>, TypeAdapter<?>> subtypeToDelegate = new LinkedHashMap<Class<?>, TypeAdapter<?>>();
        for (Map.Entry<String, Class<?>> entry : labelToSubtype.entrySet())
        {
            TypeAdapter<?> delegate = gson.getDelegateAdapter(this, TypeToken.get(entry.getValue()));
            labelToDelegate.put(entry.getKey(), delegate);
            subtypeToDelegate.put(entry.getValue(), delegate);
        }

        return new TypeAdapter<R>() {
            @Override
            public R read(JsonReader in) throws IOException
            {
                JsonElement jsonElement = Streams.parse(in);
                JsonElement labelJsonElement = jsonElement.getAsJsonObject().remove(typeFieldName);
                if (labelJsonElement == null)
                {
                    throw new JsonParseException("cannot deserialize " + baseType + " because it does not define a field named " + typeFieldName);
                }
                String label = labelJsonElement.getAsString();
                @SuppressWarnings("unchecked") // registration requires that subtype extends T
                TypeAdapter<R> delegate = (TypeAdapter<R>) labelToDelegate.get(label);
                if (delegate == null)
                {
                    throw new JsonParseException("cannot deserialize " + baseType + " subtype named " + label + "; did you forget to register a subtype?");
                }
                return delegate.fromJsonTree(jsonElement);
            }

            @Override
            public void write(JsonWriter out, R value) throws IOException
            {
                Class<?> srcType = value.getClass();
                String label = subtypeToLabel.get(srcType);
                @SuppressWarnings("unchecked") // registration requires that subtype extends T
                TypeAdapter<R> delegate = (TypeAdapter<R>) subtypeToDelegate.get(srcType);
                if (delegate == null)
                {
                    throw new JsonParseException("cannot serialize " + srcType.getName() + "; did you forget to register a subtype?");
                }
                JsonObject jsonObject = delegate.toJsonTree(value).getAsJsonObject();
                if (jsonObject.has(typeFieldName))
                {
                    throw new JsonParseException("cannot serialize " + srcType.getName() + " because it already defines a field named " + typeFieldName);
                }
                JsonObject clone = new JsonObject();
                clone.add(typeFieldName, new JsonPrimitive(label));
                for (Map.Entry<String, JsonElement> e : jsonObject.entrySet())
                {
                    clone.add(e.getKey(), e.getValue());
                }
                Streams.write(clone, out);
            }
        }.nullSafe();
    }
}