java - 使用 Retrofit GsonConverterFactory 反序列化 false 或对象的 json

Question

我正在使用返回 json 的服务器。其中一个元素要么是一个对象，要么是 false - 如果它不存在。我知道这是服务器响应的非常糟糕的实现，并且有很多这样的情况，但这就是我必须处理的。我该如何处理这种情况？如果有一个对象，我成功反序列化它，如果没有 - 我收到错误 - EXPECTED OBJECT FOUND BOOLEAN。更糟糕的是，我不知道将来在这个项目中我会在哪里遇到这样的情况。这是示例 json:

    {
  "course": {
    "id": "47902",
    "course": "3844",
    "group": "1825",
    "teacher": "59502",
    "table": "1447",
    "client": "1",
    "course_id": "3844",
    "description": ""
  },
  "teacher": {
    "id": "59502",
    "post": "0",
    "experience": "",
    "dep_experience": "",
    "rank": "0",
    "online": "1458891283",
    "departments": [
      null
    ]
  },
  "depart": {
    "id": "100",
    "postcode": "",
    "public": "1",
    "alias": "",
    "faculty": "97",
    "client": "1"
  },
  "progress": false,
  "files": [
    {
      "teacher": {
        "id": "59502",
        "code": "53bd7c21ad05b03e",
        "photo": "1"
      },
      "files": [
        {
          "id": "0ffe41e5003ee5c0",
          "owner": "59502",
          "address": "0ffe41e5003ee5c0",
          "type": "f",
          "size": "0",
          "time": "2015-07-10 14:39:15",
          "data": ""
        }
      ]
    }
  ]
}

如您所见，这里的progress 是错误的。其他时候它是普通对象，如 depart。反序列化由Retrofit 2完成。

非常感谢。

Answer 1

我假设您有一个类似于以下映射的顶级映射，并且已为 Gson 配置了您的 Retrofit 实例:

final class Response {

    @SerializedName("progress")
    @JsonAdapter(FalseAsNullTypeAdapterFactory.class)
    final Progress progress = null;

}

final class Progress {

    final String foo = null;

}

请注意，progress 属性使用 @JsonAdapter 注释进行注释:我们假设这是 progress 属性可以使用的唯一位置是一个 boolean 值(如果您有很多这样的地方，您可以使用此注释来注释每个字段，或者通过 GsonBuilder 来注释每个字段；如果是 .registerTypeAdapterFactory() 工厂必须检查已知类型，以免“拦截”所有类型。

现在，这是一个类型适配器工厂来处理您的问题:

final class FalseAsNullTypeAdapterFactory
        implements TypeAdapterFactory {

    // Let Gson instantiate it itself
    private FalseAsNullTypeAdapterFactory() {
    }

    @Override
    public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
        // Get a downstream parser (for simplicity: get the default parser for the given type)
        final TypeAdapter<T> delegateTypeAdapter = gson.getDelegateAdapter(this, typeToken);
        return new TypeAdapter<T>() {
            @Override
            public void write(final JsonWriter out, final T value) {
                throw new UnsupportedOperationException();
            }

            @Override
            public T read(final JsonReader in)
                    throws IOException {
                // Peek whether the next JSON token is a boolean
                if ( in.peek() == BOOLEAN ) {
                    // And take the this JSON token as a boolean value
                    // Is it true?
                    if ( in.nextBoolean() ) {
                        // Then it's not something we can handle -- probably a boolean field annotated with @JsonAdapter(FalseAsNullTypeAdapterFactory.class)?
                        throw new MalformedJsonException("Unexpected boolean marker: true");
                    }
                    // We're assuming it's null
                    return null;
                }
                // If it's not a boolean value, then we just delegate parsing to the original type adapter
                return delegateTypeAdapter.read(in);
            }
        };
    }

}

现在测试一下:

try ( final Reader reader = getPackageResourceReader(Q43231983.class, "success.json") ) {
    final Response response = gson.fromJson(reader, Response.class);
    System.out.println(response.progress.foo);
}
try ( final Reader reader = getPackageResourceReader(Q43231983.class, "failure.json") ) {
    final Response response = gson.fromJson(reader, Response.class);
    System.out.println(response.progress);
}

给定的资源在哪里:

• success.json 为 {"progress":{"foo": "bar"}};
• failure.json 为 {"progress":false}。

输出如下:

bar
null