cmpt213-a3/src/cmpt213/assignment3/packagedeliveries/gson/extras/RuntimeTypeAdapterFactory.java

package cmpt213.assignment3.packagedeliveries.gson.extras;

import com.google.gson.*;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;

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

/**
 * 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>   {@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 cmpt213.assignment2.packagedeliveriestracker.gson.extras.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>   {@code
 *   cmpt213.assignment2.packagedeliveriestracker.gson.extras.RuntimeTypeAdapterFactory<Shape> shapeAdapterFactory
 *       = cmpt213.assignment2.packagedeliveriestracker.gson.extras.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
 *   shapeAdapterFactory.registerSubtype(Rectangle.class, "Rectangle");
 *   shapeAdapterFactory.registerSubtype(Circle.class, "Circle");
 *   shapeAdapterFactory.registerSubtype(Diamond.class, "Diamond");
 * }</pre>
 * Finally, register the type adapter factory in your application's GSON builder:
 * <pre>   {@code
 *   Gson gson = new GsonBuilder()
 *       .registerTypeAdapterFactory(shapeAdapterFactory)
 *       .create();
 * }</pre>
 * Like {@code GsonBuilder}, this API supports chaining: <pre>   {@code
 *   cmpt213.assignment2.packagedeliveriestracker.gson.extras.RuntimeTypeAdapterFactory<Shape> shapeAdapterFactory = cmpt213.assignment2.packagedeliveriestracker.gson.extras.RuntimeTypeAdapterFactory.of(Shape.class)
 *       .registerSubtype(Rectangle.class)
 *       .registerSubtype(Circle.class)
 *       .registerSubtype(Diamond.class);
 * }</pre>
 *
 * <h3>Serialization and deserialization</h3>
 * In order to serialize and deserialize a polymorphic object,
 * you must specify the base type explicitly.
 * <pre>   {@code
 *   Diamond diamond = new Diamond();
 *   String json = gson.toJson(diamond, Shape.class);
 * }</pre>
 * And then:
 * <pre>   {@code
 *   Shape shape = gson.fromJson(json, Shape.class);
 * }</pre>
 */
public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
    private final Class<?> baseType;
    private final String typeFieldName;
    private final Map<String, Class<?>> labelToSubtype = new LinkedHashMap<>();
    private final Map<Class<?>, String> subtypeToLabel = new LinkedHashMap<>();
    private final boolean maintainType;

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

    /**
     * Creates a new runtime type adapter using for {@code baseType} using {@code
     * typeFieldName} as the type field name. Type field names are case sensitive.
     * {@code maintainType} flag decide if the type will be stored in pojo or not.
     */
    public static <T> RuntimeTypeAdapterFactory<T> of(Class<T> baseType, String typeFieldName, boolean maintainType) {
        return new RuntimeTypeAdapterFactory<>(baseType, typeFieldName, maintainType);
    }

    /**
     * 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<>(baseType, typeFieldName, false);
    }

    /**
     * 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<>(baseType, "type", false);
    }

    /**
     * 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());
    }

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

        final TypeAdapter<JsonElement> jsonElementAdapter = gson.getAdapter(JsonElement.class);
        final Map<String, TypeAdapter<?>> labelToDelegate = new LinkedHashMap<>();
        final Map<Class<?>, TypeAdapter<?>> subtypeToDelegate = new LinkedHashMap<>();
        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 = jsonElementAdapter.read(in);
                JsonElement labelJsonElement;
                if (maintainType) {
                    labelJsonElement = jsonElement.getAsJsonObject().get(typeFieldName);
                } else {
                    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 (maintainType) {
                    jsonElementAdapter.write(out, jsonObject);
                    return;
                }

                JsonObject clone = new JsonObject();

                if (jsonObject.has(typeFieldName)) {
                    throw new JsonParseException("cannot serialize " + srcType.getName()
                            + " because it already defines a field named " + typeFieldName);
                }
                clone.add(typeFieldName, new JsonPrimitive(label));

                for (Map.Entry<String, JsonElement> e : jsonObject.entrySet()) {
                    clone.add(e.getKey(), e.getValue());
                }
                jsonElementAdapter.write(out, clone);
            }
        }.nullSafe();
    }
}