Serialization of Keys with Circular References using Jackson - serialization

Question

I am trying to serialize a HashMap from Objects to Strings, but the specific Object has a reference to the current class leading to an infinite recursion, which doesn't seem to be solved with the usual JsonIdentifyInfo annotation. Here's an example:
public class CircularKey {
public void start() throws IOException {
ObjectMapper mapper = new ObjectMapper();
Cat cat = new Cat();
// Encode
String json = mapper.writeValueAsString(cat);
System.out.println(json);
// Decode
Cat cat2 = mapper.readValue(json, Cat.class);
System.out.println(mapper.writeValueAsString(cat2));
}
}
#JsonIdentityInfo(generator = ObjectIdGenerators.IntSequenceGenerator.class, property = "#id")
#JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY, property = "#class")
class Mouse {
int id;
#JsonProperty
Cat cat;
}
#JsonIdentityInfo(generator = ObjectIdGenerators.IntSequenceGenerator.class, property = "#id")
#JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY, property = "#class")
class Cat {
int id;
#JsonSerialize(keyUsing = MouseMapKeySerializer.class)
#JsonDeserialize(keyUsing = MouseMapKeyDeserializer.class)
#JsonProperty
HashMap<Mouse, String> status = new HashMap<Mouse, String>();
public Cat() {
Mouse m = new Mouse();
m.cat = this;
status.put(m, "mike");
}
}
Here's the serializer/deserializer for the key:
class MouseMapKeySerializer extends JsonSerializer<Mouse> {
static ObjectMapper mapper = new ObjectMapper();
#Override
public void serialize(Mouse value, JsonGenerator generator,
SerializerProvider provider) throws IOException,
JsonProcessingException {
String json = mapper.writeValueAsString(value);
generator.writeFieldName(json);
}
}
class MouseMapKeyDeserializer extends KeyDeserializer {
static ObjectMapper mapper = new ObjectMapper();
#Override
public Mouse deserializeKey(String c, DeserializationContext ctx)
throws IOException, JsonProcessingException {
return mapper.readValue(c, Mouse.class);
}
}
If I switch the map to HashMap (String,Object) it works but I cannot change the original mapping. Any ideas?

Answer 1

It looks like you found your answer at http://jackson-users.ning.com/forum/topics/serializing-hashmap-with-object-key-and-recursion. This doesn't seem to be possible because:
Complex keys are tricky, and it is not a use case I ever considered. Then again, there is nothing specifically preventing use of standard components; main concern was just the limitations than JSON has (must be String-value, JsonParser/JsonGenerator expose keys as different tokens).
There is no explicit support for either polymorphic types or object ids for Object keys. Standard serializers/deserializers are mostly for relatively simple types that can be easily and reliably converted to/from Strings; numbers, Dates, UUIDs.
So: unlike with value handlers, where modular design (with separation of TypeSerializer/JsonSerializer) makes sense, I think what you need to do is to have custom (de)serializers that handle all aspects. You should be able to use code from existing value (de)serializers, type (de)serializers, but not classes themselves.
Your use case does sound interesting, but for better or worse, it is pushing the envelope quite a bit. :-)