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 class NumberTypeAdapter 
      implements JsonSerializer<Number>, JsonDeserializer<Number>,
InstanceCreator<Number> {

    public JsonElement serialize(Number src, Type typeOfSrc, JsonSerializationContext
context) {
      return new JsonPrimitive(src);
    }

    public Number deserialize(JsonElement json, Type typeOfT,
JsonDeserializationContext context)
        throws JsonParseException {
      JsonPrimitive jsonPrimitive = json.getAsJsonPrimitive();
      if (jsonPrimitive.isNumber()) {
        return jsonPrimitive.getAsNumber();
      } else {
        throw new IllegalStateException("Expected a number field, but was " + json);
      }
    }

    public Number createInstance(Type type) {
      return 1L;
    }
  }

  public static void main(String[] args) {
    Map<String, Number> map = new HashMap<String, Number>();    
    map.put("int", 123);
    map.put("long", 1234567890123456789L);
    map.put("double", 1234.5678D);
    map.put("float", 1.2345F);
    Type mapType = new TypeToken<Map<String, Number>>() {}.getType();

    Gson gson = new GsonBuilder().registerTypeAdapter(Number.class, new
NumberTypeAdapter()).create();
    String json = gson.toJson(map, mapType);
    System.out.println(json);

    Map<String, Number> deserializedMap = gson.fromJson(json, mapType);
    System.out.println(deserializedMap);
  }

Cool and that works, but it seems like so much overhead (a whole Type Adapter class?). I have used other JSON libraries like JSONLib and they let you build a map in the following way:

JSONObject json = new JSONObject();
for(Entry<String,Integer> entry : map.entrySet()){
     json.put(entry.getKey(), entry.getValue());
}

Or if I have a custom class something like the following:

JSONObject json = new JSONObject();
for(Entry<String,MyClass> entry : map.entrySet()){
 JSONObject myClassJson =  JSONObject.fromObject(entry.getValue());
     json.put(entry.getKey(), myClassJson);
}

The process is more manual, but requires way less code and doesn't have the overhead of haivng to create a custom Type Adapter for Number or in most cases for my own custom class.

Is this the only way to serialize a map with Gson, or has anyone found a way that beats out what Gson recommends in the link above.

Bluestocking

Which version of Gson are you using? What should the output of the example be (never mind if the type adapter is provided by the user)? – Duron

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

Calesta

Lol, I don't pay attention to my own code when answering questions sometimes. Yes, Maps/Lists/etc are serializable by default but you need to use the Type = new TypeToken<T>.getType() statement above. You do still have to create TypeAdapters for types that aren't primitive (or the primitive wrapper classes). Thanks for providing the correct answer, arturo. – Devindevina

@zygimantus TypeToken the class is public while its default constructor is protected, in order to instantiate the object you can extend it as an anonymous inner class: new TypeToken<...>() {} – Gertie

val typeOfHashMap = object:TypeToken<HashMap<String,String>>() {}.type //in Kotlin – Mckeehan

what is the underlying Map implementation this uses? (HashMap etc.?) – Abad

The implementation is com.google.gson.internal.LinkedTreeMap. – Starwort

Is there any way to get Gson to create a default HashMap instead of their builtin LinkedTreeMap? – Amuck

Answer 2

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.

Sheena

Why isn't complex map key serialization default behavior? – Irrecusable

@Irrecusable I upvoted your comment, but i guess that as the name implies, since it only affects the keys, this might have been added as an after-thought because it is only needed when you go from POJO to json and not vice-versa. – Incognizant

Answer 3

In Gson 2.7.2 it's as easy as

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

Heavyladen

This only works for primitve data types. For a complex type like in the example you have to use the answer of matt burns. – Infer

Answer 4

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.

Devindevina

Answer 5

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

Byproduct

That is the method for deserializing not serializing as my question states. – Bluestocking