How to build JSON array of different objects using Jackson?

Question

I'm building an android app that saves info on incoming/outgoing calls in a CallLog class, incoming/outgoing SMSes in an SmsLog class, and bytes sent/received via a DataLog class. I made them all implement a JsonLog interface so I can create a single ArrayList of JsonLogs, hoping that it will be easily convertible to a JSON array of different objects via Jackson.

However, I kept getting this error whenever I'm deserializing the JSON file:

com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance
of com.project.pojos.JsonLog, problem: abstract types either need to be mapped
to concrete types, have custom deserializer, or be instantiated with additional
type information

How can I solve this? All JsonLog classes have a string attribute type, which is either in_call or out_call for CallLogs, in_sms or out_sms for SmsLogs, and data for DataLog.

KARASZI István · Accepted Answer · 2018-12-13 12:05:38Z

3

You'll need to have a type parameter in your JSON data or a custom deserializer to do the job.

The easier solution is the former, because you won't need to code that much, and it can be easily extended and from the JSON data you could simply tell which entry is what type.

Type information

If you have a type parameter:

[
  {
    "type": "SMS",
    "id": 1,
    "data": { }
  },
  {
    "type": "CALL",
    "id": 2,
    "somethingOtherData": {}
  }
]

Then in your abstract JsonLog class you can set the type mapping information:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
@JsonSubTypes({
    @Type(name = "SMS", value = SmsLog.class),
    @Type(name = "CALL", value = CallLog.class)
})
public abstract class JsonLog {
}

Deserializer

If you cannot have this type information in your JSON you need to write your own deserializer. In that deserializer you'll need to decide from the actual entry whether this entry is an SmsLog or a CallLog.

edited Dec 13, 2018 at 12:05

answered Sep 11, 2012 at 9:59

KARASZI István

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6 Comments

MLQ Over a year ago

Because my SmsLog and CallLog classes both serve to hold two different types (one for incoming and outgoing), I guess it'll also be easier to just create separate classes for logs of incoming SMS and calls, and outgoing SMS and calls, right?

KARASZI István Over a year ago

If you want to have your entries in one JSON array, then these two solutions could work. If you can have separate arrays for the SMS and the Call logs, then it's much easier.

MLQ Over a year ago

Oh, well the reason why I'm putting them all in one JSON array is I'll have to upload them altogether as one from the mobile app. If I separated them in their own JSON arrays, I'll have to make multiple connections and calls to the server.

KARASZI István Over a year ago

No, you can do that like this: { "sms": [ ], "call": [ ] }

MLQ Over a year ago

^Ah, I get it. I think that was my initial design but I wanted to avoid the nesting JSON arrays and objects, so it's like this now: [{"type": "in_call", ...}, {"type": "out_sms", ...}, ...]

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