MapAsArrayTypeAdapter.java

/*
 * Copyright (C) 2010 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.smartandroid.sa.json;

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

/**
 * Adapts maps containing complex keys as arrays of map entries.
 * 
 * <h3>Maps as JSON objects</h3> The standard GSON map type adapter converts
 * Java {@link Map Maps} to JSON Objects. This requires that map keys can be
 * serialized as strings; this is insufficient for some key types. For example,
 * consider a map whose keys are points on a grid. The default JSON form encodes
 * reasonably:
 * 
 * <pre>
 * {
 * 	@code
 * 	Map<Point, String> original = new LinkedHashMap<Point, String>();
 * 	original.put(new Point(5, 6), "a");
 * 	original.put(new Point(8, 8), "b");
 * 	System.out.println(gson.toJson(original, type));
 * }
 * </pre>
 * 
 * The above code prints this JSON object:
 * 
 * <pre>
 * {@code
 *   {
 *     "(5,6)": "a",
 *     "(8,8)": "b"
 *   }
 * }
 * </pre>
 * 
 * But GSON is unable to deserialize this value because the JSON string name is
 * just the {@link Object#toString() toString()} of the map key. Attempting to
 * convert the above JSON to an object fails with a parse exception:
 * 
 * <pre>
 * com.google.gson.JsonParseException: Expecting object found: "(5,6)"
 *   at com.google.gson.JsonObjectDeserializationVisitor.visitFieldUsingCustomHandler
 *   at com.google.gson.ObjectNavigator.navigateClassFields
 *   ...
 * </pre>
 * 
 * <h3>Maps as JSON arrays</h3> An alternative approach taken by this type
 * adapter is to encode maps as arrays of map entries. Each map entry is a two
 * element array containing a key and a value. This approach is more flexible
 * because any type can be used as the map's key; not just strings. But it's
 * also less portable because the receiver of such JSON must be aware of the map
 * entry convention.
 * 
 * <p>
 * Register this adapter when you are creating your GSON instance.
 * 
 * <pre>
 * {
 * 	@code
 * 	Gson gson = new GsonBuilder().registerTypeAdapter(Map.class,
 * 			new MapAsArrayTypeAdapter()).create();
 * }
 * </pre>
 * 
 * This will change the structure of the JSON emitted by the code above. Now we
 * get an array. In this case the arrays elements are map entries:
 * 
 * <pre>
 * {@code
 *   [
 *     [
 *       {
 *         "x": 5,
 *         "y": 6
 *       },
 *       "a",
 *     ],
 *     [
 *       {
 *         "x": 8,
 *         "y": 8
 *       },
 *       "b"
 *     ]
 *   ]
 * }
 * </pre>
 * 
 * This format will serialize and deserialize just fine as long as this adapter
 * is registered.
 * 
 * <p>
 * This adapter returns regular JSON objects for maps whose keys are not
 * complex. A key is complex if its JSON-serialized form is an array or an
 * object.
 */
final class MapAsArrayTypeAdapter extends BaseMapTypeAdapter implements
		JsonSerializer<Map<?, ?>>, JsonDeserializer<Map<?, ?>> {

	public Map<?, ?> deserialize(JsonElement json, Type typeOfT,
			JsonDeserializationContext context) throws JsonParseException {
		Map<Object, Object> result = constructMapType(typeOfT, context);
		Type[] keyAndValueType = typeToTypeArguments(typeOfT);
		if (json.isJsonArray()) {
			JsonArray array = json.getAsJsonArray();
			for (int i = 0; i < array.size(); i++) {
				JsonArray entryArray = array.get(i).getAsJsonArray();
				Object k = context.deserialize(entryArray.get(0),
						keyAndValueType[0]);
				Object v = context.deserialize(entryArray.get(1),
						keyAndValueType[1]);
				result.put(k, v);
			}
			checkSize(array, array.size(), result, result.size());
		} else {
			JsonObject object = json.getAsJsonObject();
			for (Map.Entry<String, JsonElement> entry : object.entrySet()) {
				Object k = context.deserialize(
						new JsonPrimitive(entry.getKey()), keyAndValueType[0]);
				Object v = context.deserialize(entry.getValue(),
						keyAndValueType[1]);
				result.put(k, v);
			}
			checkSize(object, object.entrySet().size(), result, result.size());
		}
		return result;
	}

	public JsonElement serialize(Map<?, ?> src, Type typeOfSrc,
			JsonSerializationContext context) {
		Type[] keyAndValueType = typeToTypeArguments(typeOfSrc);
		boolean serializeAsArray = false;
		List<JsonElement> keysAndValues = new ArrayList<JsonElement>();
		for (Map.Entry<?, ?> entry : src.entrySet()) {
			JsonElement key = serialize(context, entry.getKey(),
					keyAndValueType[0]);
			serializeAsArray |= key.isJsonObject() || key.isJsonArray();
			keysAndValues.add(key);
			keysAndValues.add(serialize(context, entry.getValue(),
					keyAndValueType[1]));
		}

		if (serializeAsArray) {
			JsonArray result = new JsonArray();
			for (int i = 0; i < keysAndValues.size(); i += 2) {
				JsonArray entryArray = new JsonArray();
				entryArray.add(keysAndValues.get(i));
				entryArray.add(keysAndValues.get(i + 1));
				result.add(entryArray);
			}
			return result;
		} else {
			JsonObject result = new JsonObject();
			for (int i = 0; i < keysAndValues.size(); i += 2) {
				result.add(keysAndValues.get(i).getAsString(),
						keysAndValues.get(i + 1));
			}
			checkSize(src, src.size(), result, result.entrySet().size());
			return result;
		}
	}

	private Type[] typeToTypeArguments(Type typeOfT) {
		if (typeOfT instanceof ParameterizedType) {
			Type[] typeArguments = ((ParameterizedType) typeOfT)
					.getActualTypeArguments();
			if (typeArguments.length != 2) {
				throw new IllegalArgumentException(
						"MapAsArrayTypeAdapter cannot handle " + typeOfT);
			}
			return typeArguments;
		}
		return new Type[] { Object.class, Object.class };
	}

	private void checkSize(Object input, int inputSize, Object output,
			int outputSize) {
		if (inputSize != outputSize) {
			throw new JsonSyntaxException("Input size " + inputSize
					+ " != output size " + outputSize + " for input " + input
					+ " and output " + output);
		}
	}
}