Gson反序列化Long精度丢失问题

1.问题复现
json long类型反序列化时，出现精度丢失问题，测试代码：

    @Test
    public void t1() {
        String j = "{\"id\":123456789012345678, \"type\":\"auto\"}";
        Map m = new Gson().fromJson(j, Map.class);
        System.out.println(m);
    }

打印:{id=1.2345678901234568E17, type=auto}

2.分析原因
debug可以发现对于Map类型，其Gson对应的adapter为MapTypeAdapterFactory，在读取json值时对应的adapter为ObjectTypeAdapter。
ObjectTypeAdapter解析json值主要在read方法中：

@Override public Object read(JsonReader in) throws IOException {
    JsonToken token = in.peek();
    switch (token) {
    case BEGIN_ARRAY:
      List<Object> list = new ArrayList<Object>();
      in.beginArray();
      while (in.hasNext()) {
        list.add(read(in));
      }
      in.endArray();
      return list;

    case BEGIN_OBJECT:
      Map<String, Object> map = new LinkedTreeMap<String, Object>();
      in.beginObject();
      while (in.hasNext()) {
        map.put(in.nextName(), read(in));
      }
      in.endObject();
      return map;

    case STRING:
      return in.nextString();

    case NUMBER:
      return in.nextDouble();

    case BOOLEAN:
      return in.nextBoolean();

    case NULL:
      in.nextNull();
      return null;

    default:
      throw new IllegalStateException();
    }
  }

可以看到数字类型NUMBER读取时转换为了double类型。
double类型整数最多16位（十进制），所以超过16就会存在精度丢失。

3.解决方法
a.声明反序列化具体类型。
声明反序列化具体类型，Gson会匹配对应的adapter，Long类型对应的adapter为：

public static final TypeAdapter<Number> LONG = new TypeAdapter<Number>() {
    @Override
    public Number read(JsonReader in) throws IOException {
      if (in.peek() == JsonToken.NULL) {
        in.nextNull();
        return null;
      }
      try {
        return in.nextLong();
      } catch (NumberFormatException e) {
        throw new JsonSyntaxException(e);
      }
    }
    @Override
    public void write(JsonWriter out, Number value) throws IOException {
      out.value(value);
    }
  };

可以看到读取的值转换为long类型，不存在精度问题。

b.通过自定义TypeAdapter改变其NUMBER类型转换规则，特殊处理Long类型，下面的CustomObjectTypeAdapter将大于Integer.MAX_VALUE都按string类型返回：

public static final class CustomObjectTypeAdapter extends TypeAdapter<Object> {

        CustomObjectTypeAdapter() {
        }

        @Override
        public Object read(JsonReader in) throws IOException {
            JsonToken token = in.peek();
            switch (token) {
                case BEGIN_ARRAY:
                    List<Object> list = new ArrayList<>();
                    in.beginArray();
                    while (in.hasNext()) {
                        list.add(read(in));
                    }
                    in.endArray();
                    return list;

                case BEGIN_OBJECT:
                    Map<String, Object> map = new LinkedTreeMap<>();
                    in.beginObject();
                    while (in.hasNext()) {
                        map.put(in.nextName(), read(in));
                    }
                    in.endObject();
                    return map;

                case STRING:
                    return in.nextString();

                case NUMBER:
                    String s = in.nextString();
                    double d = Double.parseDouble(s);
                    if (d > Integer.MAX_VALUE) {
                        return s;
                    } else {
                        int integer = (int) d;
                        //noinspection RedundantIfStatement
                        if (integer == d) {
                            return integer;
                        }
                        return d;
                    }

                case BOOLEAN:
                    return in.nextBoolean();

                case NULL:
                    in.nextNull();
                    return null;

                default:
                    throw new IllegalStateException();
            }
        }

        @Override
        public void write(JsonWriter out, Object value) {
        }
    }

将其添加到Gson 反序列化adapter中：

        GsonBuilder builder = new GsonBuilder();
        builder.setLongSerializationPolicy(LongSerializationPolicy.STRING);
        builder.setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES);
        builder.registerTypeAdapter(Map.class, new CustomObjectTypeAdapter());
        GSON = builder.create();

这样map类型反序列化时long类型将返回string类型，eg：

        String j = "{\"id\":123456789012345678, \"type\":\"auto\", \"maps\":{123456789012345678: 1234567890123456789}}";
        Map m = GSON.fromJson(j, Map.class);
        System.out.println(GSON.toJson(m));

输出：{"id":"123456789012345678","type":"auto","maps":{"123456789012345678":"1234567890123456789"}}

对于非map类型，并且没有明确定义的类型（如Object）。反序列化时可以转一遍map再转为目标类型：

        if (!(GSON.getAdapter(obj) instanceof CustomObjectTypeAdapter)) {
            Map map = GSON.fromJson(json, Map.class);
            json = GSON.toJson(map);
        }
        return GSON.fromJson(json, obj);