Question

I’m serializing some java.util.Dates within a Map. The dates are serialized into Longs (Jackson writes the Long value of the Date instance to the JSON string), however, they’re not being de-serialized back to Date instances, but as Long instances.

I’d like Jackson to de-serialize the dates back to Date objects (rather than formatted Strings or Longs), how can I achieve this?

Map<String, Comparable<?>> change = new HashMap<String, Comparable<?>>();
    change.put("DESCRIPTION", "LIBOR");
    change.put("RATE", "1.8");
    change.put("DATE", Util.newDate(2009, 7, 1)); // Returns a java.util.Date

Produces

{"DESCRIPTION":"LIBOR"},{"RATE":"1.8"},{"DATE":1246402800000}, ... }

Which is okay. However, the date String is deserialized (inflated) back into an instance of java.lang.Long, when I want it to be an instance of java.util.Date – which is what it started as.
i.e. Map change now contains three entries; Description as a String, Rate as a Float and Date as a Long.

Answer 1

You want to use org.codehaus.jackson.map.JsonDeserializer and write the custom deserialization code. Something like:

public class DateDeserializer extends JsonDeserializer<Long> {
    @Override
    public Long deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
       ... custom logic
    }
}

I guess you have to figure out when a Long property has to be deserialized to Date. Maybe using annotations on your pojos?