Jackson Object mapper to convert java map to json maintaining order of keys

Question

I am using jackson.map.ObjectMapper API to convert map to json string. I am using writeValueAsString method to achieve this.

I pass a map sorted on the basis of values to writeValueAsString method. The JSON string which I get is resorted on the basis of keys.

Is there a way to convert maps to JSON string using jackson without disturbing the order of items in the map.

I tried setting Feature.SORT_PROPERTIES_ALPHABETICALLY to false, but as per documentation it is applicable only for POJO types.

Any idea to implement the said behavior.

Answer 1

with Jackson 2.3.1 (don't know for previous versions) you can serialize a SortedMap, for example a TreeMap, the order will be respected.

Here is an exempale in junit 4:

    @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));


}

and here is the result:`

with a Map

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

with a SortedMap

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

`

The 1st serialization with a Map will not be ordered, The second one with a TreeMap will be ordered alphabeticaly using keys. you can pass a Comparator to the treeMap for a different order.

Edit: It also work on Jackson with a LinkedHashMap() even if this is not a SortedMap. This is an implementation of Map that keep the order which keys were inserted into the map. This could be what your are looking for.