@Test
public void testSerialize() throws JsonProcessingException{
    ObjectMapper om = new ObjectMapper();
    om.configure(SerializationFeature.WRITE_NULL_MAP_VALUES,false);
    om.configure(SerializationFeature.INDENT_OUTPUT,true);
    om.setSerializationInclusion(Include.NON_NULL);

    SortedMap<String,String> sortedMap = new TreeMap<String,String>();
    Map<String,String> map = new HashMap<String,String>();
    map.put("aaa","AAA");

    map.put("bbb","BBB");
    map.put("ccc","CCC");
    map.put("ddd","DDD");

    sortedMap.putAll(map);

    System.out.println(om.writeValueAsString(map));

    System.out.println(om.writeValueAsString(sortedMap));


}

{
  "aaa" : "AAA",
  "ddd" : "DDD",
  "ccc" : "CCC",
  "bbb" : "BBB"
}

{
  "aaa" : "AAA",
  "bbb" : "BBB",
  "ccc" : "CCC",
  "ddd" : "DDD"
}