Deserialize a JSON array partially into a HashMap with the help of jackson

Question

I have the following JSON:

[
  {
    "a": 1,         
    "b": 2,                  
    "c": "one",
    "d": 3                     
  },
  {
    "a": 4,         
    "b": 5,                  
    "c": "two",
    "d": 6                     
  },
  {
    "a": 7,         
    "b": 8,                  
    "c": "three",
    "d": 9                     
  },
]

I want to cast this with the help of jackson into a HashMap<String,Integer> like this:

{
  "one": 2,
  "two": 5,
  "three": 8
}

I have tried a custom deserializer but I can't get it registered correctly and I don't really know if this is the correct way to do it. The next problem is that i don't have any nesting object as an actual basis for my JSON, means that I get it straight as a string like mentioned above, so I can't use any annotation like this:

@JsonDeserialize(using = CustomDeserializer.class).

So at the end I need to register the Deserializer in the ObjectMapper itself, but this won't work either with the help of a configuration like this:

ObjectMapper objectMapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule();
simpleModule.addDeserializer(new TypeReference<HashMap<String,Integer>>(){}, new CustomDeserializer());

Am I missing something here? I hope someone can help me with this problem.

Answer 1

Maybe you have missed with adding module.addDeserializer

HashMapValueDeserializer.class

public class HashMapValueDeserializer extends JsonDeserializer<HashMap<String, String>> {
    @Override
    public HashMap<String, String> deserialize(JsonParser parser, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {
        HashMap<String, String> ret = new HashMap<String, String>();

        ObjectCodec codec = parser.getCodec();
        TreeNode node = codec.readTree(parser);

        for (JsonNode n : (ArrayNode)node){
            JsonNode c = n.get("c");
            JsonNode b = n.get("b");
            ret.put(c.asText(), b.asText());
        }
        return ret;
    }
}

Test.class

public class Test {
  public static void main(String[] args) throws FileNotFoundException, MalformedURLException, JsonProcessingException {
    String jsonArray = "[{ \"a\" : \"1\", \"b\" : \"2\", \"c\" : \"one\", \"d\" : \"3\" }, { \"a\" : \"4\", \"b\" : \"5\", \"c\" : \"two\", \"d\" : \"6\" }, { \"a\" : \"7\", \"b\" : \"8\", \"c\" : \"three\", \"d\" : \"9\" }]";

    ObjectMapper mapper = new ObjectMapper();

    SimpleModule module =
        new SimpleModule("HashMapValueDeserializer", new Version(1, 0, 0, null, null, null));
    module.addDeserializer(HashMap.class, new HashMapValueDeserializer());
    mapper.registerModule(module);

    HashMap<String, Integer> result = mapper.readValue(jsonArray, HashMap.class);
    System.out.println(result);
  }

}

Output :

{one=2, two=5, three=8}