How to deserialize nested JSON array into a Map using Java's Jackson library?

Question

I have a structure like so:

{
    "name": "user",
    "values":[["0.00207760","18.48000000"],["0.00207740","40.00000000"],["0.00207710","2.26000000"]]
}

I'd like to deserialize into a class like so using the popular Jackson library:

public class Values {

    public String name;

    public Map<BigDecimal, BigDecimal> values = new HashMap<>();

}

Where each entry in the values property becomes a key/value entry in the class' map.

However, if I attempt a simple deserialization with Jackson I get this error:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.util.TreeMap<java.lang.Object,java.lang.Object>` out of START_ARRAY token
 at [Source: (String)"{"name": "user","values":[["0.00207760","18.48000000"],["0.00207740","40.00000000"],["0.00207710","2.26000000"]]...
    at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:59)
    at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1442)
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1216)
    ...

How can this be accomplished using Jackson?

Thanks!

Eduardo

Michał Ziober · Accepted Answer · 2019-11-29 23:06:49Z

You expect Map which on JSON side is represented by JSON Object but there is a JSON Array which can be mapped by default to List, Set or array. I suggest to use List<List<BigDecimal>> in POJO and create a method which converts data to Map:

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.json.JsonMapper;

import java.io.File;
import java.math.BigDecimal;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class JsonPathApp {

    public static void main(String[] args) throws Exception {
        File jsonFile = new File("./resource/test.json").getAbsoluteFile();

        ObjectMapper mapper = JsonMapper.builder().build();
        Values values = mapper.readValue(jsonFile, Values.class);
        System.out.println(values.getValuesAsMap());
    }
}

class Values {

    private String name;
    private List<List<BigDecimal>> values;

    public Map<BigDecimal, BigDecimal> getValuesAsMap() {
        return values.stream().collect(Collectors.toMap(
                k -> k.get(0),
                v -> v.get(1),
                (u, v) -> {
                    throw new IllegalStateException(String.format("Duplicate key %s", u));
                },
                LinkedHashMap::new));
    }

    // getters, setters, toString
}

Above code prints:

{0.00207760=18.48000000, 0.00207740=40.00000000, 0.00207710=2.26000000}

In other case, you need to implement a custom deserialiser for a Map and register it for this field using @JsonDeserialize annotation.

William Jones · Accepted Answer · 2022-04-04 17:00:51Z

List<List> works with the same data type in the inner array. I however got a map that was converted into a nested jsonArray with the value being a jsonObject.

https://riptutorial.com/java/example/18575/deserialize-json-collection-to-collection-of-objects-using-jackson gave me the answer.

JSON:

{
 "history": [
  [
    2,
    {
      "trx_id": "4014ae64ae9c45861ed4f0af30224b6741dcbb46",
      ...
      "operation_id": 0
    }
  ],
  [
    3,
    {
      "trx_id": "a4023a3641ffc112324427523945f641d88c5eae",
      ...
      "operation_id": 0
    }
  ]
 ]
}

3 class answer:

public class HistoryListResponse {
   private List<HistoryMapEntryResponse> history;
}

2ed class:

@JsonFormat(shape = JsonFormat.Shape.ARRAY)
@JsonPropertyOrder({ "key", "trx"})
public class HistoryMapEntryResponse {
  private int key;
  private HistoryTrxResponse trx;
}

And then back to normal json:

public class HistoryTrxResponse {
   private String trx_id;
   ...
   private int operation_id;
}

Still will need a method to convert the list back into a Map, but it is pretty close, and the inner data gets parsed/deserialized.

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