How to make readerForUpdating.readValue() not to override non null values?

I have following two classes :

public class Simple {

    String a;
    String b;
    SubSimple subSimple;
}

public class SubSimple {

    String a;
    String b;
}

What changes can I do to following objectMapper so that non null values are not updated to null :

        Simple simple = new Simple();
		simple.setA("0a");
		simple.setSubSimple(new SubSimple());
		simple.getSubSimple().setA("00a");

		String json = "{\"b\" : \"b\", \"subSimple\" : {\"b\" : \"b\"}}";
		ObjectMapper objectMapper = new ObjectMapper();
		objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
		objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES,false);


		simple = objectMapper.readerForUpdating(simple).readValue(json);

		System.out.println("simple : " + simple);

Current output :

> simple : Simple{a='0a', b='b', subSimple=SubSimple{a='null', b='b'}}

Required :

> simple : Simple{a='0a', b='b', subSimple=SubSimple{a='00a', b='b'}}

Try to use com.fasterxml.jackson.annotation.JsonMerge annotation:

class Simple {

    String a;
    String b;

    @JsonMerge
    SubSimple subSimple;
}

or enable it on com.fasterxml.jackson.databind.ObjectMapper instance:

objectMapper.setDefaultMergeable(Boolean.TRUE);