Deserialization of map with integer keys using Jackson

Question

I have to serialize a simple integer-to-string map as JSON and then read it back. The serialization is pretty simple, however since JSON keys must be strings the resulting JSON looks like:

{
  "123" : "hello",
  "456" : "bye",
}

When I read it using code like:

new ObjectMapper().readValue(json, Map.class)

I get Map<String, String> instead of Map<Integer, String> that I need.

I tried to add key deserializer as following:

    Map<Integer, String> map1 = new HashMap<>();
    map1.put(1, "foo");
    map1.put(2, "bar");


    ObjectMapper mapper = new ObjectMapper();
    SimpleModule module = new SimpleModule();
    module.addKeyDeserializer(Integer.class, new KeyDeserializer() {
        @Override
        public Object deserializeKey(String key, DeserializationContext ctxt) throws IOException, JsonProcessingException {
            System.out.println("deserialize " + key);
            return Integer.parseInt(key);
        }
    });

    mapper.registerModule(module);
    String json = mapper.writeValueAsString(map1);


    Map map2 = mapper.readValue(json, Map.class);
    System.out.println(map2);
    System.out.println(map2.keySet().iterator().next().getClass());

Unfortunately my key deserialzier is never called and map2 is in fact Map<String, String>, so my example prints:

{1=foo, 2=bar}
class java.lang.String

What am I doing wrong and how to fix the problem?

Answer 1

Use

Map<Integer, String> map2 = 
        mapper.readValue(json, new TypeReference<Map<Integer, String>>(){});

or

    Map<Integer, String> map2 = 
        mapper.readValue(json, TypeFactory.defaultInstance()
                         .constructMapType(HashMap.class, Integer.class, String.class));

Your program will output below text:

deserialize 1
deserialize 2
{1=foo, 2=bar}
class java.lang.Integer