Gson多态序列化

Question

使用Gson 2.2.2，我尝试序列化POJO(行为)的数组列表。

我有一个适配器，它几乎是我在网上看到的东西的副本：

public class BehaviorAdapter implements JsonSerializer<Behavior> {

    private static final String CLASSNAME = "CLASSNAME";
    private static final String INSTANCE = "INSTANCE";

    @Override
    public JsonElement serialize(Behavior src, Type typeOfSrc,
            JsonSerializationContext context) {

        JsonObject retValue = new JsonObject();
        String className = src.getClass().getCanonicalName();
        retValue.addProperty(CLASSNAME, className);
        JsonElement elem = context.serialize(src);
        retValue.add(INSTANCE, elem);
        return retValue;
    }
}

我像这样注册它：

GsonBuilder builder = new GsonBuilder();        
builder.registerTypeHierarchyAdapter(Behavior.class, new BehaviorAdapter());
gson = builder.create();

然后当我尝试序列化我的ArrayList时：

String json2 = gson.toJson(behaviors);

我得到一个堆栈溢出。

看起来是这样的：

JsonElement elem = context.serialize(src);

它开始一个递归循环，一次又一次地通过我的序列化程序。那么我如何注册它才不会发生这种情况呢？我需要序列化列表并维护多态。

Answer 1

看起来你找到了无限循环the JsonSerializer docs warn about

然而，你永远不应该在src对象本身调用它，因为那会导致无限循环(Gson会再次调用你的回调方法)。

我能想到的最简单的方法是创建一个没有安装处理程序的新Gson实例，然后运行您的实例。

作为结束运行，您可以只序列化List<Behavior>：

public class BehaviorListAdapter implements JsonSerializer<List<Behavior>> {

    private static final String CLASSNAME = "CLASSNAME";
    private static final String INSTANCE = "INSTANCE";

    @Override
    public JsonElement serialize(List<Behavior> src, Type typeOfSrc,
            JsonSerializationContext context) {
        JsonArray array = new JsonArray();
        for (Behavior behavior : src) {
            JsonObject behaviorJson = new JsonObject();
            String className = behavior.getClass().getCanonicalName();
            behaviorJson.addProperty(CLASSNAME, className);
            JsonElement elem = context.serialize(behavior);
            behaviorJson.add(INSTANCE, elem);
            array.add(behaviorJson);
        }
        return array;
    }
}

GsonBuilder builder = new GsonBuilder();
// use a TypeToken to make a Type instance for a parameterized type
builder.registerTypeAdapter(
    (new TypeToken<List<Behavior>>() {}).getType(),
    new BehaviorListAdapter());
gson = builder.create();

Answer 2

看看RuntimeTypeAdapterFactory吧。该类的test有一个示例：

RuntimeTypeAdapterFactory<BillingInstrument> rta = RuntimeTypeAdapterFactory.of(
    BillingInstrument.class)
    .registerSubtype(CreditCard.class);
Gson gson = new GsonBuilder()
    .registerTypeAdapterFactory(rta)
    .create();

CreditCard original = new CreditCard("Jesse", 234);
assertEquals("{\"type\":\"CreditCard\",\"cvv\":234,\"ownerName\":\"Jesse\"}",
    gson.toJson(original, BillingInstrument.class));
BillingInstrument deserialized = gson.fromJson(
    "{type:'CreditCard',cvv:234,ownerName:'Jesse'}", BillingInstrument.class);
assertEquals("Jesse", deserialized.ownerName);
assertTrue(deserialized instanceof CreditCard);

这个类不在核心Gson中；您需要将它复制到您的项目中才能使用它。

Answer 3

我知道你想在这里做什么，我也有同样的问题。

我写完了一个简单的抽象类。

public abstract class TypedJsonizable extends Jsonizable {}

并将TypeHierarchyAdapter注册到我的Gson实例

    protected static Gson gson = new GsonBuilder()
    .registerTypeHierarchyAdapter
    (TypedJsonizable.class,new TypedJsonizableSerializer());

这个TypeAdapter的关键不是调用context.serialize和context.deserialize，因为这会导致一个无限循环，正如Jeff Bowman在他的回答中所说的，这个TypeAdapter使用反射来避免这种情况。

import com.google.gson.*;
import org.apache.log4j.Logger;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Type;

public class TypedJsonizableSerializer implements JsonSerializer<TypedJsonizable>, JsonDeserializer<TypedJsonizable> {
static final String CLASSNAME_FIELD = "_className";
Logger logger = Logger.getLogger(TypedJsonizable.class);

@Override
public JsonElement serialize(TypedJsonizable src, Type typeOfSrc, JsonSerializationContext context) {
    JsonObject contentObj = new JsonObject();
    contentObj.addProperty(CLASSNAME_FIELD,src.getClass().getCanonicalName());

    for (Field field : src.getClass().getDeclaredFields()) {
        field.setAccessible(true);
        try {
            if (field.get(src)!=null)
                contentObj.add(field.getName(),context.serialize(field.get(src)));
        } catch (IllegalAccessException e) {
            logger.error(e.getMessage(),e);
        }
    }
    return contentObj;
}

@Override
public TypedJsonizable deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
    JsonObject jsonObject = json.getAsJsonObject();
    String className = jsonObject.get(CLASSNAME_FIELD).getAsString();
    if (className == null || className.isEmpty())
        throw new JsonParseException("Cannot find _className field. Probably this instance has not been serialized using Jsonizable jsonizer");
    try {
        Class<?> clazz = Class.forName(className);
        Class<?> realClazz = (Class<?>) typeOfT;
        if (!realClazz.equals(clazz))
            throw new JsonParseException(String.format("Cannot serialize object of class %s to %s", clazz.getCanonicalName(),realClazz.getCanonicalName()));
        Object o  = clazz.getConstructor().newInstance();
        for (Field field : o.getClass().getDeclaredFields()) {
            field.setAccessible(true);
            if (jsonObject.has(field.getName())) {
                field.set(o,context.deserialize(jsonObject.get(field.getName()) , field.getGenericType()));
            }
        }
        return (TypedJsonizable) o;
    } catch (ClassNotFoundException e) {
        throw new JsonParseException(String.format("Cannot find class with name %s . Maybe the class has been refactored or sender and receiver are not using the same jars",className));
    } catch (IllegalAccessException e){
        throw new JsonParseException(String.format("Cannot deserialize, got illegalAccessException %s ",e.getMessage()));
    } catch (NoSuchMethodException | InstantiationException | InvocationTargetException e) {
        throw new JsonParseException(String.format("Cannot deserialize object of class %s, unable to create a new instance invoking empty constructor",className));
    }
}

}