stevebot stevebot - 1 year ago 72
Java Question

Gson: Is there an easier way to serialize a map

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
String json = gson.toJson(map, mapType);

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

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.

Answer Source

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);


Type typeOfHashMap = new TypeToken<Map<String, String>>() { }.getType();
Map<String, String> newMap = gson.fromJson(json, typeOfHashMap); // This type must match TypeToken


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