Make the Jackson fail on duplicate property in a JSON

Question

I use Jackson to deserialize a JSON to an immutable custom Java object. Here is the class:

final class DataPoint {
  private final int count;
  private final int lower;
  private final int median;
  private final int upper;

  @JsonCreator
  DataPoint(
      @JsonProperty("count") int count,
      @JsonProperty("lower") int lower,
      @JsonProperty("median") int median,
      @JsonProperty("upper") int upper) {
    if (count <= 0) {
      throw new IllegalArgumentException("...");
    }
    this.count = count;
    this.lower = lower;
    this.median = median;
    this.upper = upper;
  }

  // getters...
}

Here is the JSON I deserialize:

{
  "count": 3,
  "lower": 2,
  "median": 3,
  "upper": 4
}

It works fine. Now I break the JSON, i.e. douplicate the lower property:

{
  "count": 4,
  "lower": 2,
  "lower": 3,
  "median": 4,
  "upper": 5
}

Now I get count == 4, and lower == 3. Instead, I would like the Jackson to fail deserilizing, since there is a duplicate property in the JSON (lower).

Here is the deserializing code:

String json = "..."; // the second JSON
ObjectMapper mapper = new ObjectMapper().enable(
    DeserializationFeature.FAIL_ON_MISSING_CREATOR_PROPERTIES,
    DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES,
    DeserializationFeature.FAIL_ON_READING_DUP_TREE_KEY)
    .disable(DeserializationFeature.ACCEPT_FLOAT_AS_INT);
DataPoint data = mapper.readValue(json, DataPoint.class);

Folks, can I make the Jackson to fail when desierializing a JSON with duplicate property keys?

Thank you a lot, guys!

Answer 1

You can enable STRICT_DUPLICATE_DETECTION to make the parser throw an Exception if there's a duplicate property, e.g.:

String s = "{\"count\": 4,\"lower\": 2,\"lower\": 3,\"median\": 4,\"upper\": 5}";
ObjectMapper mapper = new ObjectMapper();
mapper.enable(JsonParser.Feature.STRICT_DUPLICATE_DETECTION);
JsonNode readTree = mapper.readTree(s);
System.out.println(readTree.toString());

This will throw the following exception:

Exception in thread "main" com.fasterxml.jackson.core.JsonParseException: Duplicate field 'lower'

Here's the documentation.