How can GSON be configured to recognize ISO date fields as parse them as Date objects?

This code:

Map<String, Object> map = new HashMap<>();
map.put("now", new Date());

Gson gson = new GsonBuilder()
        .setDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'")
        .create();
String json = gson.toJson(map);
System.out.println(json);

produces something like {"now":"2023-03-11T12:34:56.789Z"}

Contrarily, undoing the action like this:

Map<String, Object> map = gson.fromJson(json, new TypeToken<Map<String, Object>>(){}.getType());

fails to produce the input data - instead giving output like {"now":"2023-03-11T12:34:56.789Z"}.

The date string stays a string instead of being converted back to a Date object.

How may the string be parsed back into a Map so that GSON recognises the date string and parses it back to a Date object.

The regex pattern "\\d{1,4}-\\d{2}-\\d{2}T\\d{2}\\:\\d{2}\\:\\d{2}\\.\\d{3}Z" roughly correlates to the date pattern.

Basically, I need to tell GSON by configuration: If the string matches regex part (DATE_PATTERN) deserialize it as a date.

Note: I am aware that deserializing to a class with date field would work properly since the field type gives GSON a hint. I need GSON to determine the target type simply based on the format of the string.

Solution

This is not possible; Gson does not allow overwriting the default type adapter factory for Object.

There are probably the following alternatives:

Manually process the Map after it was deserilized with Gson and convert the values
Have a model class with Object as field type (if directly specifying Date is not possible for some reason), and then place a @JsonAdapter annotation on the field specifying your custom adapter factory
Specify one of the general purpose interfaces implemented by Date (Serializable, Cloneable or Comparable) as value type of the map, e.g. Map<String, Serializable>, and then register a type adapter factory for that interface. This is possibly not a very clean solution and of course only works if the other deserialized values of the map implement that interface as well.
Register a custom type adapter factory for Map which performs conversion of the values