Gson反序列化具有多种对象类型的JSON数组 [英] Gson deserialize JSON array with multiple object types

问题描述

我有一些奇怪的JSON，例如：

  [
 {
type：0 ，
value：我的字符串
}，
 {
type：1，
value：42 
 }，
 {
type：2，
value：{
} 
} 
] 
  

基于某些字段，数组中的对象是某种类型。
使用Gson，我的想法是有一个TypeAdapterFactory，它将这些特定类型的委托适配器发送给TypeAdapter，但是我很想理解一种阅读该type字段的好方法，以知道要创建哪种类型。
在TypeAdapter中，

  Object read（JsonReader in）抛出IOException {
 String type = in。 nextString（）; 
 switch（type）{
 //委托创建某些类型。 
 
 
 
 
 $ b $ p 
 $ b $ p $会假定type字段在我的第一位JSON。有没有一种体面的方式来消除这种假设？
解决方案
以下是我编写的用于处理NewsFeedArticle和NewsFeedAd项目数组的代码在Json。这两个项目都实现了一个标记接口NewsFeedItem，以便我轻松检查TypeAdater是否应该用于特定的字段。
  public class NewsFeedItemTypeAdapterFactory实现TypeAdapterFactory {
 $ b $ @覆盖
 @SuppressWarnings（未选中）
 public< T> TypeAdapter< T>创建（Gson gson，TypeToken< T>类型）{
 if（！NewsFeedItem.class.isAssignableFrom（type.getRawType（）））{
 return null; 
} 
 
 TypeAdapter< JsonElement> jsonElementAdapter = gson.getAdapter（JsonElement.class）; 
 TypeAdapter< NewsFeedArticle> newsFeedArticleAdapter = gson.getDelegateAdapter（this，TypeToken.get（NewsFeedArticle.class））; 
 TypeAdapter< NewsFeedAd> newsFeedAdAdapter = gson.getDelegateAdapter（this，TypeToken.get（NewsFeedAd.class））; 
 
 return（TypeAdapter< T>）new NewsFeedItemTypeAdapter（jsonElementAdapter，newsFeedArticleAdapter，newsFeedAdAdapter）.nullSafe（）; 
} 
 
 private static class NewsFeedItemTypeAdapter extends TypeAdapter< NewsFeedItem> {
 
 private final TypeAdapter< JsonElement> jsonElementAdapter; 
 private final TypeAdapter< NewsFeedArticle> newsFeedArticleAdapter; 
 private final TypeAdapter< NewsFeedAd> newsFeedAdAdapter; 
 
 NewsFeedItemTypeAdapter（TypeAdapter< JsonElement> jsonElementAdapter，
 TypeAdapter< NewsFeedArticle> newsFeedArticleAdapter，
 TypeAdapter< NewsFeedAd> newsFeedAdAdapter）{
 this.jsonElementAdapter = jsonElementAdapter; 
 this.newsFeedArticleAdapter = newsFeedArticleAdapter; 
 this.newsFeedAdAdapter = newsFeedAdAdapter; 
 
 $ b @Override 
 public void write（JsonWriter out，NewsFeedItem value）抛出IOException {
 if（value.getClass（）。isAssignableFrom（NewsFeedArticle.class） ）{
 newsFeedArticleAdapter.write（out，（NewsFeedArticle）value）; 
 
} else if（value.getClass（）。isAssignableFrom（NewsFeedAd.class））{
 newsFeedAdAdapter.write（out，（NewsFeedAd）value）; 
 
 
} 
 
 @Override 
 public NewsFeedItem read（JsonReader in）throws IOException {
 JsonObject objectJson = jsonElementAdapter .read（在）.getAsJsonObject（）; 
 
 if（objectJson.has（Title））{
 return newsFeedArticleAdapter.fromJsonTree（objectJson）; 
 
} else if（objectJson.has（CampaignName））{
 return newsFeedAdAdapter.fromJsonTree（objectJson）; 
} 
 
返回null; 
} 
} 
} 
  
然后您可以注册Gson使用以下代码。
 
 
 返回新的GsonBuilder（）
 .registerTypeAdapterFactory（new NewsFeedItemTypeAdapterFactory（））
 .create（）; 
  
 
I have some odd JSON like:
[
  {
    "type":"0",
    "value":"my string"
  },
  {
    "type":"1",
    "value":42
  },
  {
    "type":"2",
    "value": {
    }
  }
]
Based on some field, the object in the array is a certain type.
Using Gson, my thought is to have a TypeAdapterFactory that sends delegate adapters for those certain types to a TypeAdapter, but I'm hung up on understanding a good way of reading that "type" field to know which type to create.
In the TypeAdapter,
Object read(JsonReader in) throws IOException {
  String type = in.nextString();
  switch (type) {
    // delegate to creating certain types.
  }
}
would assume the "type" field comes first in my JSON. Is there a decent way to remove that assumption?
解决方案
Here is some code I wrote to handle an array of NewsFeedArticle and NewsFeedAd items in Json. Both items implement a marker interface NewsFeedItem to allow me to easily check if the TypeAdater should be used for a particular field.
public class NewsFeedItemTypeAdapterFactory implements TypeAdapterFactory {

    @Override
    @SuppressWarnings("unchecked")
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        if (!NewsFeedItem.class.isAssignableFrom(type.getRawType())) {
            return null;
        }

        TypeAdapter<JsonElement> jsonElementAdapter = gson.getAdapter(JsonElement.class);
        TypeAdapter<NewsFeedArticle> newsFeedArticleAdapter = gson.getDelegateAdapter(this, TypeToken.get(NewsFeedArticle.class));
        TypeAdapter<NewsFeedAd> newsFeedAdAdapter = gson.getDelegateAdapter(this, TypeToken.get(NewsFeedAd.class));

        return (TypeAdapter<T>) new NewsFeedItemTypeAdapter(jsonElementAdapter, newsFeedArticleAdapter, newsFeedAdAdapter).nullSafe();
    }

    private static class NewsFeedItemTypeAdapter extends TypeAdapter<NewsFeedItem> {

        private final TypeAdapter<JsonElement> jsonElementAdapter;
        private final TypeAdapter<NewsFeedArticle> newsFeedArticleAdapter;
        private final TypeAdapter<NewsFeedAd> newsFeedAdAdapter;

        NewsFeedItemTypeAdapter(TypeAdapter<JsonElement> jsonElementAdapter,
                TypeAdapter<NewsFeedArticle> newsFeedArticleAdapter,
                TypeAdapter<NewsFeedAd> newsFeedAdAdapter) {
            this.jsonElementAdapter = jsonElementAdapter;
            this.newsFeedArticleAdapter = newsFeedArticleAdapter;
            this.newsFeedAdAdapter = newsFeedAdAdapter;
        }

        @Override
        public void write(JsonWriter out, NewsFeedItem value) throws IOException {
            if (value.getClass().isAssignableFrom(NewsFeedArticle.class)) {
                newsFeedArticleAdapter.write(out, (NewsFeedArticle) value);

            } else if (value.getClass().isAssignableFrom(NewsFeedAd.class)) {
                newsFeedAdAdapter.write(out, (NewsFeedAd) value);

            }

        }

        @Override
        public NewsFeedItem read(JsonReader in) throws IOException {
            JsonObject objectJson = jsonElementAdapter.read(in).getAsJsonObject();

            if (objectJson.has("Title")) {
                return newsFeedArticleAdapter.fromJsonTree(objectJson);

            } else if (objectJson.has("CampaignName")) {
                return newsFeedAdAdapter.fromJsonTree(objectJson);
            }

            return null;
        }
    }
}
You can then register this with Gson using the following code.
return new GsonBuilder()
        .registerTypeAdapterFactory(new NewsFeedItemTypeAdapterFactory())
        .create();


                        
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