Disaligning a card with integer keys using Jackson

I need to serialize a simple integer map as JSON and then read it back. Serialization is pretty straightforward, however, since the JSON keys must be strings, the resulting JSON looks like this:

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

      

        
        
        
      

    

When I read it using the following code:

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

      

        
        
        
      

    

I get Map<String, String>

instead of Map<Integer, String>

what I need.

I tried to add a key deserializer like this:

    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 deserializer never gets called, and map2

in fact Map<String, String>

, so my example prints:

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

      

        
        
        
      

    

What am I doing wrong and how do I fix this problem?