https://github.com/google/gson/gson: Gson JSON library 
/*
 * 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.google.gson.internal.bind;

import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSyntaxException;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.internal.$Gson$Types;
import com.google.gson.internal.ConstructorConstructor;
import com.google.gson.internal.JsonReaderInternalAccess;
import com.google.gson.internal.ObjectConstructor;
import com.google.gson.internal.Streams;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;
import java.io.IOException;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;


Adapts maps to either JSON objects or JSON arrays.

Maps as JSON objects
 For primitive keys or when complex map key serialization is not enabled, this converts Java 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: 
 
  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));


The above code prints this JSON object:
 
  {
    "(5,6)": "a",
    "(8,8)": "b"
  }

 But GSON is unable to deserialize this value because the JSON string name is just the toString() of the map key. Attempting to convert the above JSON to an object fails with a parse exception: 
com.google.gson.JsonParseException: Expecting object found: "(5,6)"
  at com.google.gson.JsonObjectDeserializationVisitor.visitFieldUsingCustomHandler
  at com.google.gson.ObjectNavigator.navigateClassFields
  ...


Maps as JSON arrays

An alternative approach taken by this type adapter when it is required and
complex map key serialization is enabled 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.

Register this adapter when you are creating your GSON instance.

 
  Gson gson = new GsonBuilder()
    .registerTypeAdapter(Map.class, new MapAsArrayTypeAdapter())
    .create();


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:

 
  [
    [
      {
        "x": 5,
        "y": 6
      },
      "a",
    ],
    [
      {
        "x": 8,
        "y": 8
      },
      "b"
    ]
  ]


This format will serialize and deserialize just fine as long as this adapter
is registered.

/**
 * Adapts maps to either JSON objects or JSON arrays.
 *
 * <h3>Maps as JSON objects</h3>
 * For primitive keys or when complex map key serialization is not enabled, this
 * 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 when it is required and
 * complex map key serialization is enabled 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.
 */
public final class MapTypeAdapterFactory implements TypeAdapterFactory {
  private final ConstructorConstructor constructorConstructor;
  final boolean complexMapKeySerialization;

  public MapTypeAdapterFactory(ConstructorConstructor constructorConstructor,
      boolean complexMapKeySerialization) {
    this.constructorConstructor = constructorConstructor;
    this.complexMapKeySerialization = complexMapKeySerialization;
  }

  @Override public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
    Type type = typeToken.getType();

    Class<? super T> rawType = typeToken.getRawType();
    if (!Map.class.isAssignableFrom(rawType)) {
      return null;
    }

    Class<?> rawTypeOfSrc = $Gson$Types.getRawType(type);
    Type[] keyAndValueTypes = $Gson$Types.getMapKeyAndValueTypes(type, rawTypeOfSrc);
    TypeAdapter<?> keyAdapter = getKeyAdapter(gson, keyAndValueTypes[0]);
    TypeAdapter<?> valueAdapter = gson.getAdapter(TypeToken.get(keyAndValueTypes[1]));
    ObjectConstructor<T> constructor = constructorConstructor.get(typeToken);

    @SuppressWarnings({"unchecked", "rawtypes"})
    // we don't define a type parameter for the key or value types
    TypeAdapter<T> result = new Adapter(gson, keyAndValueTypes[0], keyAdapter,
        keyAndValueTypes[1], valueAdapter, constructor);
    return result;
  }

  
Returns a type adapter that writes the value as a string.

/**
   * Returns a type adapter that writes the value as a string.
   */
  private TypeAdapter<?> getKeyAdapter(Gson context, Type keyType) {
    return (keyType == boolean.class || keyType == Boolean.class)
        ? TypeAdapters.BOOLEAN_AS_STRING
        : context.getAdapter(TypeToken.get(keyType));
  }

  private final class Adapter<K, V> extends TypeAdapter<Map<K, V>> {
    private final TypeAdapter<K> keyTypeAdapter;
    private final TypeAdapter<V> valueTypeAdapter;
    private final ObjectConstructor<? extends Map<K, V>> constructor;

    public Adapter(Gson context, Type keyType, TypeAdapter<K> keyTypeAdapter,
        Type valueType, TypeAdapter<V> valueTypeAdapter,
        ObjectConstructor<? extends Map<K, V>> constructor) {
      this.keyTypeAdapter =
        new TypeAdapterRuntimeTypeWrapper<K>(context, keyTypeAdapter, keyType);
      this.valueTypeAdapter =
        new TypeAdapterRuntimeTypeWrapper<V>(context, valueTypeAdapter, valueType);
      this.constructor = constructor;
    }

    @Override public Map<K, V> read(JsonReader in) throws IOException {
      JsonToken peek = in.peek();
      if (peek == JsonToken.NULL) {
        in.nextNull();
        return null;
      }

      Map<K, V> map = constructor.construct();

      if (peek == JsonToken.BEGIN_ARRAY) {
        in.beginArray();
        while (in.hasNext()) {
          in.beginArray(); // entry array
          K key = keyTypeAdapter.read(in);
          V value = valueTypeAdapter.read(in);
          V replaced = map.put(key, value);
          if (replaced != null) {
            throw new JsonSyntaxException("duplicate key: " + key);
          }
          in.endArray();
        }
        in.endArray();
      } else {
        in.beginObject();
        while (in.hasNext()) {
          JsonReaderInternalAccess.INSTANCE.promoteNameToValue(in);
          K key = keyTypeAdapter.read(in);
          V value = valueTypeAdapter.read(in);
          V replaced = map.put(key, value);
          if (replaced != null) {
            throw new JsonSyntaxException("duplicate key: " + key);
          }
        }
        in.endObject();
      }
      return map;
    }

    @Override public void write(JsonWriter out, Map<K, V> map) throws IOException {
      if (map == null) {
        out.nullValue();
        return;
      }

      if (!complexMapKeySerialization) {
        out.beginObject();
        for (Map.Entry<K, V> entry : map.entrySet()) {
          out.name(String.valueOf(entry.getKey()));
          valueTypeAdapter.write(out, entry.getValue());
        }
        out.endObject();
        return;
      }

      boolean hasComplexKeys = false;
      List<JsonElement> keys = new ArrayList<JsonElement>(map.size());

      List<V> values = new ArrayList<V>(map.size());
      for (Map.Entry<K, V> entry : map.entrySet()) {
        JsonElement keyElement = keyTypeAdapter.toJsonTree(entry.getKey());
        keys.add(keyElement);
        values.add(entry.getValue());
        hasComplexKeys |= keyElement.isJsonArray() || keyElement.isJsonObject();
      }

      if (hasComplexKeys) {
        out.beginArray();
        for (int i = 0, size = keys.size(); i < size; i++) {
          out.beginArray(); // entry array
          Streams.write(keys.get(i), out);
          valueTypeAdapter.write(out, values.get(i));
          out.endArray();
        }
        out.endArray();
      } else {
        out.beginObject();
        for (int i = 0, size = keys.size(); i < size; i++) {
          JsonElement keyElement = keys.get(i);
          out.name(keyToString(keyElement));
          valueTypeAdapter.write(out, values.get(i));
        }
        out.endObject();
      }
    }

    private String keyToString(JsonElement keyElement) {
      if (keyElement.isJsonPrimitive()) {
        JsonPrimitive primitive = keyElement.getAsJsonPrimitive();
        if (primitive.isNumber()) {
          return String.valueOf(primitive.getAsNumber());
        } else if (primitive.isBoolean()) {
          return Boolean.toString(primitive.getAsBoolean());
        } else if (primitive.isString()) {
          return primitive.getAsString();
        } else {
          throw new AssertionError();
        }
      } else if (keyElement.isJsonNull()) {
        return "null";
      } else {
        throw new AssertionError();
      }
    }
  }
}