Create example JSON from nested empty objects in Java

Question

I'm trying to create a utility to print example JSON structures from any given POJO. I've been trying Jackson and Gson to print all of the fields from a given object. I've created the following three objects as examples.

public class Model {
    private String val1;
    private Child child;    
//getters and setters
}

public class Child {
    private String val2;
    private ArrayList subChildren;
//getters and setters
}

public class SubChild {
    private String val3;
//getters and setters
}

I would like to have an example serializer that prints these objects with all of their field names even when the children are null. Below is my target output:

{
  "val1" : "",
  "child" : {
    "val2" : "",
    "subChildren" : [ {
      "val3" : ""
    } ]
  }
}

These are the methods I've tried to print these pojos along with their outputs which don't quite match my needs

Jackson:

ObjectMapper map = new ObjectMapper().setSerializationInclusion(Include.ALWAYS);
Model testModel = new Model()
map.writerWithDefaultPrettyPrinter().writeValueAsString(testModel);

Output:

{
  "val1" : null,
  "child" : null
}

Gson:

Gson gson = builder.serializeNulls().setPrettyPrinting().create();
Model testModel = new Model();
gson.toJson(testModel);

Output:

{
  "val1" : null,
  "child" : null
}

Is there a simple way to achieve my goal without populating all of the child fields? I would like to be able to use this utility on a generic class where I wouldn't know which methods to call to populate the objects with empty values.

SirCipher · Accepted Answer

I cannot see how this would be possible. What if you had a Long as a field? Jackson would not know what to do there. What Jackson and Gson have done is correct and they have printed null.

What you could do is write a utility to set the fields manually. However, you would have to handle different types accordingly. Something like this achieves what you have asked for, but only for List:

public static void main(String args[]) throws IOException, IllegalAccessException {
        ObjectMapper map = new ObjectMapper().setSerializationInclusion(JsonInclude.Include.ALWAYS);
        Model testModel = new Model();

        instantiateFields(testModel);

        String result = map.writerWithDefaultPrettyPrinter().writeValueAsString(testModel);
        System.out.println(result);
    }

    private static void instantiateFields(Object o) throws IllegalAccessException {
        Field[] fields = o.getClass().getDeclaredFields();

        for (Field field : fields) {
            field.setAccessible(true);

            if (field.get(o) == null) {
                Type type = field.getType();

                try {
                    Class clazz = (Class) type;
                    Object instance = clazz.newInstance();

                    if (List.class.isAssignableFrom(clazz)) {
                        instantiateList(clazz, field, instance);
                    }

                    field.set(o, instance);
                    instantiateFields(instance);

                } catch (ClassCastException | InstantiationException e) {
                    // Handle this or leave field null
                }
            }
        }
    }

    private static void instantiateList(Class clazz, Field field, Object instance) throws IllegalAccessException, InstantiationException {
        ParameterizedType listType = (ParameterizedType) field.getGenericType();
        Class listClass = (Class) listType.getActualTypeArguments()[0];

        Object listTypeInstance = listClass.newInstance();

        instantiateFields(listTypeInstance);

        List

Create example JSON from nested empty objects in Java

Answers (2)

Related Questions