Serialization and Deserialization of generic Map for Java and external use

Question

I have a Map<String, Object> which I am using as a mapping for a JSON document, however want to create and maintain Java type information at the same time as retaining the structure of the document.

I'm attempting to use Jackson to create the document and it seems to work fine but I'm seeing something strange when attempting to deserialize it. A very simple serialization example:

final ObjectMapper mapper = new ObjectMapper().enableDefaultTyping(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE, JsonTypeInfo.As.EXTERNAL_PROPERTY);
final Map<String, Object> map = Maps.newHashMap();
map.put("test", new Date());
final String ser = mapper.writeValueAsString(map);
final Map<String, Object> deser = mapper.readValue(ser, new TypeReference<HashMap<String, Object>>(){});
System.err.println(deser.get("test").getClass());

Gives the serialized form {"test":1410721662084,"@class":"java.util.Date"} which seems fine but when deserializing returns the type of "test" to be Long .

If I change the type serialization to use WRAPPER_ARRAY rather than EXTERNAL_PROPERTY then the type of "test" is correctly returned as Date , but doing this alters the structure of the JSON document so is not something I'm allowed to do. How do I retain the structure of the document as well as allow deserialization back to the correct types?

This is against Jackson 2.4.2.

Answer 1

Deserialization with maps is always tricky as maps don't preserve type information, which makes Jackson resort to @class and that not something you usually want. Instead, you can create a simple class:

public class TestClass {
   private Date test;
   //getters and setters omitted 
}

This class has concrete structure and JSON will serialize it as

{ "test" : 1410721662084}

which is much cleaner and type-safe representation of your object. Then you just need to pass TestClass.class to readValue() method and your test attribute will be magically converted to proper type ( Date )