jackson serialization for Java object with Map?

Question

I have a Java class like this and want to convert to JSON using Jackson. Thanks for your help.

1. Java Class

   public class myClass {
      String Id;
      Map<String, Object> optionalData = new LinkedHashMap<String, Object>();
   }
   

2. How to serialization it to JSON using Jackson ObjectMapper ?

For example, suppose the optionalData is a Map saving two entries <"type", "book"> and <"year", "2014"> I want the output to be as follow. Please note that the key/value of optionalData could be changed on the fly (so, I cannot create a "static" Java object for this without using Map)

  [ 
    { 
      id: "book-id1",
      type: "book",
      year: "2014"
    },
    { 
      id: "book-id2",
      type: "book",
      year: "2013"
     }
  ]

Answer 1

You can use the @JsonAnyGetter annotation on a getter method that returns the map of optional values. Please refer to this blog plost that explains that in details.

Here is an example:

public class JacksonAnyGetter {

    public static class myClass {
        final String Id;
        private final Map<String, Object> optionalData = new LinkedHashMap<>();

        public myClass(String id, String key1, Object value1, String key2, Object value2) {
            Id = id;
            optionalData.put(key1, value1);
            optionalData.put(key2, value2);
        }

        public String getid() {
            return Id;
        }

        @JsonAnyGetter
        public Map<String, Object> getOptionalData() {
            return optionalData;
        }
    }

    public static void main(String[] args) throws JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        List<myClass> objects = Arrays.asList(
                new myClass("book-id1", "type", "book", "year", 2013),
                new myClass("book-id2", "type", "book", "year", 2014)
        );
        System.out.println(mapper.writerWithDefaultPrettyPrinter().writeValueAsString(objects));
    }

}

Output:

[ {
  "id" : "book-id1",
  "type" : "book",
  "year" : 2013
}, {
  "id" : "book-id2",
  "type" : "book",
  "year" : 2014
} ]

Answer 2

You need to write your own Jackson JsonSerializer to create custom JSON string from Java object as per the need.

Here are the nice posts along with example

• JSON Serializer & DeserializerHow do I use a custom Serializer with Jackson?

• How Do I Write a Jackson JSON Serializer & Deserializer?

---

The same thing you can achieve using GSON JsonSerializer

Here are some examples

• Serialize java object with GSON

• GSON Serialiser Example - javacreed

---

Here is the code using GSON Serialiser

List<MyClass> list = new ArrayList<MyClass>();

MyClass myClass1 = new MyClass();
myClass1.setId("book-id1");
myClass1.getOptionalData().put("type", "book");
myClass1.getOptionalData().put("year", "2014");
list.add(myClass1);

MyClass myClass2 = new MyClass();
myClass2.setId("book-id2");
myClass2.getOptionalData().put("type", "book");
myClass2.getOptionalData().put("year", "2013");
list.add(myClass2);

class MyClassSerialiser implements JsonSerializer<MyClass> {

    @Override
    public JsonElement serialize(final MyClass obj, final Type typeOfSrc,
            final JsonSerializationContext context) {
        final JsonObject jsonObject = new JsonObject();

        jsonObject.addProperty("id", obj.getId());
        Map<String, String> optioanlData = obj.getOptionalData();
        if (optioanlData.size() > 0) {
            for (Map.Entry<String, String> entry : optioanlData.entrySet()) {
                jsonObject.addProperty(entry.getKey(), entry.getValue());
            }
        }

        return jsonObject;
    }
}

String jsonString = new GsonBuilder().setPrettyPrinting().
        .registerTypeAdapter(MyClass.class, new MyClassSerialiser()).create()
        .toJson(list);

System.out.println(jsonString);

output:

[
    {
        "id": "book-id1",
        "type": "book",
        "year": "2014"
    },
    {
        "id": "book-id2",
        "type": "book",
        "year": "2013"
    }
]