这可以通过自定义 TypeAdapterFactory 它将委托给默认适配器，然后调用 IndexedPojo.setKeySet(...) 方法：

class IndexedPojoTypeAdapterFactory implements TypeAdapterFactory {
  public static final IndexedPojoTypeAdapterFactory INSTANCE = new IndexedPojoTypeAdapterFactory();

  private IndexedPojoTypeAdapterFactory() { }

  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
    // Only handle IndexedPojo and subclasses
    if (!IndexedPojo.class.isAssignableFrom(type.getRawType())) {
      return null;
    }

    // Get the default adapter as delegate
    // Cast is safe due to `type` check at method start
    @SuppressWarnings("unchecked")
    TypeAdapter<IndexedPojo> delegate = (TypeAdapter<IndexedPojo>) gson.getDelegateAdapter(this, type);
    // Cast is safe because `T` is IndexedPojo or subclass (due to `type` check at method start)
    @SuppressWarnings("unchecked")
    TypeAdapter<T> adapter = (TypeAdapter<T>) new TypeAdapter<IndexedPojo>() {
      @Override
      public void write(JsonWriter out, IndexedPojo value) throws IOException {
        delegate.write(out, value);
      }

      @Override
      public IndexedPojo read(JsonReader in) throws IOException {
        // Read JsonObject from JsonReader to be able to pass it to `IndexedPojo.setKeySet(...)`
        // afterwards
        // Note: JsonParser automatically parses in lenient mode, which cannot be disabled
        // Note: Might have to add handling for JSON null values
        JsonObject jsonObject = JsonParser.parseReader(in).getAsJsonObject();

        IndexedPojo value = delegate.fromJsonTree(jsonObject);
        value.setKeySet(jsonObject);
        return value;
      }
    };

    return adapter;
  }
}

然后在你的工厂里使用这个工厂 deserializeJsonTo(...) 方法：

public static <T extends IndexedPojo> T deserializeJsonTo(JsonObject parameters, Class<T> tClass) {
  // Note: Could also store Gson instance in `static final` field
  Gson gson = new GsonBuilder()
    .registerTypeAdapterFactory(IndexedPojoTypeAdapterFactory.INSTANCE)
    .create();
  return gson.fromJson(parameters, tClass);
}