Java:自定义JsonSerializer中到JSON的Gson - hashmap

import com.google.gson.*;
import java.lang.reflect.Type;
import java.util.HashMap;

public class HashMapSerializer implements JsonSerializer<HashMap<?, ?>> {

    @Override
    public JsonElement serialize(HashMap<?, ?> src, Type typeOfSrc, JsonSerializationContext context) {
        JsonObject jsonObject = new JsonObject();
        for (Object key : src.keySet()) {
            Object value = src.get(key);
            JsonElement jsonValue = context.serialize(value);
            jsonObject.add(key.toString(), jsonValue);
        }
        return jsonObject;
    }
}

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.registerTypeAdapter(HashMap.class, new HashMapSerializer());
        Gson gson = gsonBuilder.create();

        HashMap<String, String> hashMap = new HashMap<>();
        hashMap.put("key1", "value1");
        hashMap.put("key2", "value2");

        String json = gson.toJson(hashMap);
        System.out.println(json);
    }
}