java - gson - 多态变量的自定义反序列化器

Question

我正在使用 gson 将 java 对象与 json 对象相互转换。我遇到了多态性问题。

我有这些看起来像这样的请求:

{
  "method": "getUser",
  "methodParameters": {
    "a": "b",
    "c": "d",
    "e": "f",
    "data": {
      "u": "v",
      "x": "y"
    }
  },
  "version": "1.3"
}

每个请求方法都有不同类型的数据对象。自然地，每个数据对象都扩展了一个名为 RequestData 的基类。

我试图创建一个自定义反序列化器，但由于它是请求对象而不是保存该方法的数据对象，我无法找到一种方法来知道要反序列化到哪个对象。

是否有可能在反序列化数据对象时以某种方式获取方法值，或者是否有其他更好的方法来解决这个问题？谢谢。

Answer 1

我遇到了类似的问题:正如 Tomek 所指出的，您需要一个自定义反序列化器，以及一个特定的 JSON 字段，您将在运行时使用它来决定要实例化哪个子类。

考虑以下类作为基类:

// Base class for a server sent event
public class ServerEvent 
{
    public static final String TYPEFIELDNAME = "eventType";

    // Event type is used during deserialization to choose the right subclass
    private final String eventType;

    public ServerEvent(String eventType) 
    {
        this.eventType = eventType;
    }

    public String toString()
    {
        return "eventType: " + eventType;
    }
}

ServerEvent 有两个子类，每个子类都有不同的属性:

public class EventA extends ServerEvent 
{   
    private static final String EVENTTYPE = "eventa";
    private int data;

    public EventA() 
    {
        super(EVENTTYPE);
    }

    // Helper function to register this class with ServerEventDeserializer
    public static void register(ServerEventDeserializer deserializer) 
    {
        deserializer.registerEvent(EVENTTYPE, EventA.class);
    }

    public String toString()
    {
        return super.toString() + ", data = " + data;
    }
}

public class EventB extends ServerEvent 
{   
    private static final String EVENTTYPE = "eventb";
    private String name;

    public EventB() 
    {
        super(EVENTTYPE);
    }

    // Helper function to register this class with ServerEventDeserializer
    public static void register(ServerEventDeserializer deserializer) 
    {
        deserializer.registerEvent(EVENTTYPE, EventB.class);
    }

    public String toString()
    {
        return super.toString() + ", name = " + name;
    }
}

两种可能的输入如下:

{ "eventType" : "eventa", "data" : 15 }
{ "eventType" : "eventb", "name" : "test" }

这是多态反序列化器:

// This class handles the polymorphic deserialization of ServerEvent class
public class ServerEventDeserializer implements JsonDeserializer<ServerEvent>
{
    // Gson engine
    private Gson gson;

    // Map of subclasses
    private Map<String, Class<? extends ServerEvent>> eventRegistry;

    public ServerEventDeserializer()
    {
        gson = new Gson();
        eventRegistry = new HashMap<String, Class<? extends ServerEvent>>();
    }

    // Registers a ServerEvent subclass
    public void registerEvent(String event, Class<? extends ServerEvent> eventInstanceClass)
    {
        eventRegistry.put(event, eventInstanceClass);
    }

    @Override
    public ServerEvent deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException
    {
        try
        {
            // Get the JSON object
            JsonObject commandObject = json.getAsJsonObject();

            // Read the field named "ServerEvent.TYPEFIELDNAME"
            JsonElement commandTypeElement = commandObject.get(ServerEvent.TYPEFIELDNAME);

            // Query the eventRegistry map to instance the right subclass
            Class<? extends ServerEvent> commandInstanceClass = eventRegistry.get(commandTypeElement.getAsString());
            ServerEvent command = gson.fromJson(json, commandInstanceClass);

            return command;
        }
        catch (Exception e)
        {
            throw new RuntimeException(e);
        }
    }
}

我写了一个可以下载的最小工作示例 here .