Serialize an object with no data in Jackson

Question

Is it possible to serialize an object with no fields in Jackson using only annotations? When I attempt to serialize such an object with no annotations I get:

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: No serializer found for class [redacted].SubjectObjectFeatureExtractor and no properties discovered to create BeanSerializer

I have examined the list of Jackson annotations without seeing a way to annotate the class as having no serializable data. I tried putting @JsonCreator on the empty constructor (not expecting it to work, since it's a deserialization annotation), and I got the same error. There are no accessors or fields to put @JsonProperty on. Any ideas?

Update: The reason for this is that I have a list of objects which represent transformations which can be applied to a certain type of data. Some of these transformations are defined by parameters which needs to be serialized, but some of these are parameter-less (the data-less objects in question). I'd like to be able to serialize and deserialize a sequence of these transformations. Also, I'm using DefaultTyping.NON_FINAL so that the class name will be serialized.

Update: An example class would be

class ExtractSomeFeature implements FeatureExtractor<SomeOtherType> {
    public void extractFeature(SomeOtherType obj, WeightedFeatureList output) {
          // do stuff
    }
 }

I don't particularly care how the JSON for this looks like, as long as I can deserialize List<FeatureExtractor>s properly. My impression is that using default typing, the expected JSON would be something like:

['com.mycompany.foo.ExtractSomeFeature', {}]

Other sub-classes of FeatureExtractor would have real parameters, so they would presumably look something like:

[`com.mycompany.foo.SomeParameterizedFeature', {some actual JSON stuff in here}]

I think I could use @JsonValue on some toJSONString() method to return {}, but if possible I'd like to hide such hackery from end-users who will be creating FeatureExtractor sub-classes.

So you just want an empty {}? What purpose does that have? — Sotirios Delimanolis
@SotiriosDelimanolis: I've updated the question in response to your questions. — Ryan Gabbard
Please give an example of the type of object you'd want to serialize and what you'd expect the generated JSON to be. — Sotirios Delimanolis
There is a relevant issue in Jackon's JIRA: jira.codehaus.org/browse/…, which seems to indicate the issue was fixed, so that POJOs with other Jackson annotations (e.g. @JsonCreator on the empty constructor, I assume) should not throw an exception. However, this does not seem to be the case. — Ryan Gabbard

wmlynarski wmlynarski · Accepted Answer · 2016-12-05T11:11:08

You have to configure your object mapper to support this case.

ObjectMapper objectMapper = ...
  objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);

The documentation of this feature can be found here : Fail on empty beans

Feature that determines what happens when no accessors are found for a type (and there are no annotations to indicate it is meant to be serialized). If enabled (default), an exception is thrown to indicate these as non-serializable types; if disabled, they are serialized as empty Objects, i.e. without any properties.

Serialize an object with no data in Jackson

4 Answers