Custom deserialization of List using Jackson

Question

I am trying to write a custom deserializer in order to trim down a big set of data I receive from somewhere else. I return a List of custom objects from the deserializer.

My question is, how do I do that, if this is my custom deserializer :

public class MyCustomDeserializer extends JsonDeserializer<List<CustomClass>> { ... }

I certainly can't do this :

final SimpleModule module = new SimpleModule();
module.addDeserializer(List<CustomClass>.class, new MyCustomDeserializer());

Will something like this work ?

final List<CustomClass> response = Arrays.asList(objectMapper.readValue(stringBean, CustomClass[].class));

If this indeed works, I find it a bit confusing and "dangerous" ? Isn't the deserialization done inside the asList method invocation ? So it basically maps a List to an array[] ?

I learned about TypeReference so I can probably use that like so :

objectMapper.readValue(stringBean, new TypeReference<List<CustomClass>>(){});

but I heard it is slower.

I also don't want to create a container for the list, and return that in the deserialization because that means it will be wrapped in another json object, and I simply want my endpoint to produce something like :

[{object1}, {object2}]

// instead of

{"Output" : [{object1}, {object2}]}

EDIT:

It seems that I have misinterpreted how jackson is using my deserializer in both cases :

final List<CustomClass> response = Arrays.asList(objectMapper.readValue(stringBean, CustomClass[].class));
// or
objectMapper.readValue(stringBean, new TypeReference<List<CustomClass>>(){});

It looks like the deserializer is called twice, once for each object in the array. I thought that the entire array would be considered as a whole. To clear the confusion, here is what I mean:

The json I receive and try to deserialize looks like so :

[
  {
    "Data" : {
      "id" : "someId",
      "otherThing" : "someOtherThing"
    },
    "Message" : "OK"
  },
  {
    "Data" : null,
    "Message" : "Object not found for id blabla"
  }
]

and so I though this is what I would have inside my deserializer, but as I said before it seems that i actually get each "entry" from that array and call it multiple times.

Answer 1

First of all, If you registered your custom deserializer using annotation on the bean CustomClass then the deserializer should handle one instance of CustomClass and not a collection and thus should be defined:

public class MyCustomDeserializer extends JsonDeserializer<CustomClass> {
        @Override
        public CustomClass deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException
        {
            ...
        }
}

and now you can use Jackson's type factory to pass the mapper the required type information

JavaType customClassCollection = objectMapper.getTypeFactory().constructCollectionType(List.class, CustomClass.class);
List<CustomClass> beanList = (List<CustomClass>)objectMapper.readValue(stringBean, customClassCollection);