Gson: Is there an easier way to serialize a map

Question

This link from the Gson project seems to indicate that I would have to do something like the following for serializing a typed Map to JSON:

    public static         


        

Answer 1

Only the TypeToken part is neccesary (when there are Generics involved).

Map<String, String> myMap = new HashMap<String, String>();
myMap.put("one", "hello");
myMap.put("two", "world");

Gson gson = new GsonBuilder().create();
String json = gson.toJson(myMap);

System.out.println(json);

Type typeOfHashMap = new TypeToken<Map<String, String>>() { }.getType();
Map<String, String> newMap = gson.fromJson(json, typeOfHashMap); // This type must match TypeToken
System.out.println(newMap.get("one"));
System.out.println(newMap.get("two"));

Output:

{"two":"world","one":"hello"}
hello
world

Answer 2

In Gson 2.7.2 it's as easy as

Gson gson = new Gson();
String serialized = gson.toJson(map);

Answer 3

Default

The default Gson implementation of Map serialization uses toString() on the key:

Gson gson = new GsonBuilder()
        .setPrettyPrinting().create();
Map<Point, String> original = new HashMap<>();
original.put(new Point(1, 2), "a");
original.put(new Point(3, 4), "b");
System.out.println(gson.toJson(original));

Will give:

{
  "java.awt.Point[x\u003d1,y\u003d2]": "a",
  "java.awt.Point[x\u003d3,y\u003d4]": "b"
}

---

Using enableComplexMapKeySerialization

If you want the Map Key to be serialized according to default Gson rules you can use enableComplexMapKeySerialization. This will return an array of arrays of key-value pairs:

Gson gson = new GsonBuilder().enableComplexMapKeySerialization()
        .setPrettyPrinting().create();
Map<Point, String> original = new HashMap<>();
original.put(new Point(1, 2), "a");
original.put(new Point(3, 4), "b");
System.out.println(gson.toJson(original));

Will return:

[
  [
    {
      "x": 1,
      "y": 2
    },
    "a"
  ],
  [
    {
      "x": 3,
      "y": 4
    },
    "b"
  ]
]

More details can be found here.

Answer 4

Map<String, Object> config = gson.fromJson(reader, Map.class);

Answer 5

I'm pretty sure GSON serializes/deserializes Maps and multiple-nested Maps (i.e. Map<String, Map<String, Object>>) just fine by default. The example provided I believe is nothing more than just a starting point if you need to do something more complex.

Check out the MapTypeAdapterFactory class in the GSON source: http://code.google.com/p/google-gson/source/browse/trunk/gson/src/main/java/com/google/gson/internal/bind/MapTypeAdapterFactory.java

So long as the types of the keys and values can be serialized into JSON strings (and you can create your own serializers/deserializers for these custom objects) you shouldn't have any issues.