Jackson java.util.Date value in Map<String, Object> (de-)serialization

Question

Consider this property

@JsonProperty
private Map<String, Object> myMap;

When a contained java.util.Date value is serialized as long, it will not be deserialized to Date again because the type information is not present in Map<String, Object>. How can I bypass the problem? I read answers about this question which would be a work around but there would be no way to distinguish strings containing dates from dates serialized as strings in the map. Can I tell Jackson to include type information for each map value such that Jackson can deserialize them correctly?

Answer 1

Implement a custom Deserializer and add the Annotation @JsonDeserialize(using = DateDeserializer.class) to your field.

Take a look at this example:

Your Json-Bean:

public class Foo {

    private String            name;

    @JsonProperty
    @JsonDeserialize(using = DateDeserializer.class)
    private Map<String, Object> dates;

    [...] // getter, setter, equals, hashcode
}

Deserializer:

public class DateDeserializer extends JsonDeserializer<Map<String, Object>> {

    private TypeReference<HashMap<String, Object>> typeRef = new TypeReference<HashMap<String, Object>>() {};

    @Override
    public Map<String, Object> deserialize(JsonParser p, DeserializationContext ctxt, Map<String, Object> target) throws IOException, JsonProcessingException {

        Map<String, Long> map = new ObjectMapper().readValue(p, typeRef);

        for(Entry<String, Long> e : map.entrySet()){

            Long value = e.getValue();
            String key = e.getKey();

            if(value instanceof Long){ // or if("date".equals(key)) ...
                target.put(key, new Date(value));
            } else {
                target.put(key, value); // leave as is
            }

        }

        return target;
    }

    @Override
    public Map<String, Object> deserialize(JsonParser paramJsonParser, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        return this.deserialize(paramJsonParser, ctxt, new HashMap<>());
    }

}

Simple test:

public static void main(String[] args) throws Exception {

    Foo foo1 = new Foo();
    foo1.setName("foo");
    foo1.setData(new HashMap<String, Object>(){{
        put("date",   new Date());
        put("bool",   true);
        put("string", "yeah");
    }});
    ObjectMapper mapper = new ObjectMapper();
    String jsonStr = mapper.writeValueAsString(foo1);
    System.out.println(jsonStr);
    Foo foo2 = mapper.readValue(jsonStr, Foo.class);

    System.out.println(foo2.equals(foo1));

}