如何用GSON序列化HashMap<Object,Object>？

Question

我正在尝试序列化 HashMap，它具有自定义对象（相同类型）作为键和值。然而 gson 正在输出这样的东西：

{ key.toString():{value}}

基本上，不是序列化在地图中用作键的对象，而是使用它的 toString 值。值对象得到很好的序列化。结果 Json 显然无法反序列化。我如何说服 GSON 完全序列化关键对象？

编辑：存储在 HashMap 中的对象包含有关玩家的信息（我正在为我们的棋盘游戏组构建自定义媒人）。它们看起来像这样：

public class Player implements Serializable{
private static final long serialVersionUID = 42L;
private String playerName,faction,teamName;
private HashMap<String,Integer> resources;

散列图应该包含有关即将到来的比赛的信息，基本上是像这样的键值对战：

HashMap<Player,Player> matchMap=new HashMap<>();
matchMap.put(player1,opponentForPlayer1);

Answer 1

没有具体的例子很难得出准确的答案。

您可以使用 SerializedName 注释更改密钥的命名方式：

private class SomeObject {
  @SerializedName("custom_naming") private final String someField;
  private final String someOtherField;

  public SomeObject(String a, String b) {
    this.someField = a;
    this.someOtherField = b;
  }
}

SomeObject someObject = new SomeObject("first", "second");
Gson gson = new GsonBuilder().setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE).create();
String jsonRepresentation = gson.toJson(someObject);
System.out.println(jsonRepresentation);

输出为：

{"custom_naming": "first", "SomeOtherField": "second"}

（无耻地从文档中窃取的示例 :)

要自定义序列化，请查看文档：https://github.com/google/gson/blob/master/UserGuide.md#custom-serialization-and-deserialization

// Step 1: writing a custom serializer


/** Here is an example of how to write a custom serializer for JodaTime DateTime class. */
private class DateTimeSerializer implements JsonSerializer<DateTime> {
  public JsonElement serialize(DateTime src, Type typeOfSrc, JsonSerializationContext context) {
    return new JsonPrimitive(src.toString());
  }
}

// Step 2: registering it
GsonBuilder gson = new GsonBuilder();
gson.registerTypeAdapter(MyType.class, new DateTimeSerializer());

registerTypeAdapter call checks if the type adapter implements more than one of these interfaces and register it for all of them.

Answer 2

JSON 键只能是 Strings。尝试以下使用自定义 JsonSerializer 和 JsonDeserializer.

的解决方案

自定义 JsonSerializer 将在序列化时将键（Player 对象）转换为 JSON String：

import java.lang.reflect.Type;
import java.util.Map;
import java.util.Map.Entry;

import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;

public class CustomMapSerializer implements JsonSerializer<Map<Player, Player>> {

    @Override
    public JsonElement serialize(Map<Player, Player> src, Type typeOfSrc, JsonSerializationContext context) {
        JsonObject json = new JsonObject();

        Gson gson = new Gson();

        for (Entry<Player, Player> entry : src.entrySet()) {
            json.add(gson.toJson(entry.getKey()), gson.toJsonTree(entry.getValue()));
        }

        return json;
    }

}

自定义 JsonDeserializer 以在反序列化期间将键 (JSON String) 转换回 Player 对象:

import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;

import com.google.gson.Gson;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;

public class CustomMapDeserializer implements JsonDeserializer<Map<Player, Player>> {

    @Override
    public Map<Player, Player> deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
            throws JsonParseException {
        Map<Player, Player> players = new HashMap<Player, Player>();

        Gson gson = new Gson();

        JsonObject object = json.getAsJsonObject();

        for (Entry<String, JsonElement> entry : object.entrySet()) {
            players.put(gson.fromJson(entry.getKey(), Player.class), gson.fromJson(entry.getValue(), Player.class));
        }

        return players;
    }
}

参考下面的例子 serialization 和 deserialization:

import java.util.HashMap;
import java.util.Map;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class Solutiony {

    public static void main(String[] args) {
        Map<String, Integer> data = new HashMap<>();
        data.put("value", 100);

        Player p = new Player();
        p.setFaction("0.9");
        p.setPlayerName("x");
        p.setTeamName("A");
        p.setResources(data);

        Player p2 = new Player();
        p2.setFaction("1.0");
        p2.setPlayerName("y");
        p2.setTeamName("B");
        p2.setResources(data);

        Map<Player, Player> map = new HashMap<Player, Player>();
        map.put(p, p2);

        Gson gson = new GsonBuilder().registerTypeAdapter(map.getClass(), new CustomMapSerializer())
                .registerTypeAdapter(map.getClass(), new CustomMapDeserializer())
                .setPrettyPrinting().create();

        //Serialization
        String val = gson.toJson(map);

        //De-serialization
        Map<Player, Player> map2 = gson.fromJson(val, map.getClass());
    }
}

序列化后的 JSON 看起来像：

{
  "{\"playerName\":\"x\",\"faction\":\"0.9\",\"teamName\":\"A\",\"resources\":{\"value\":100}}": {
    "playerName": "y",
    "faction": "1.0",
    "teamName": "B",
    "resources": {
      "value": 100
    }
  }
}

Answer 3

根据 fluffys 的评论，最简单的解决方案是在像这样创建 gson 对象时使用 enableComplexMapKeySerialization()：

Gson gson=new GsonBuilder().enableComplexMapKeySerialization().create();

如果那不可用或出于某种原因无法工作也可以。

如何用GSON序列化HashMap<Object,Object>？

How to serialize HashMap<Object,Object> with GSON?

java

hashmap

gson