I have classes like this :

public class Root {
    public List<Collect> collect;
}

public class Collect {
    public Flag flag;
}

public class Flag {
    public String name;
}

now i want to deserialize json string to root object :

import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.IOException;

public class MainClass {
    private static final ObjectMapper om = new ObjectMapper();

    public static void main(String[] args) throws IOException {
        om.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        om.configure(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT, false);
        om.configure(DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT, false);
        om.configure(DeserializationFeature.READ_UNKNOWN_ENUM_VALUES_USING_DEFAULT_VALUE, false);
        om.setSerializationInclusion(JsonInclude.Include.NON_EMPTY);
        om.setSerializationInclusion(JsonInclude.Include.NON_NULL);

        //language=JSON
        String str = "{"collect": [{"flag": {"name": "ali"}},{"item": {"value": "test"}}]}";
        Root root = om.readValue(str, Root.class);

        String output = om.writeValueAsString(root);
        System.out.println(output);
    }
}

my problem is :
i have unknown key in json named item and jackson does not ignore unknown properties.
this is serialization result :

{"collect":[{"flag":{"name":"ali"}},{}]}

Why jackson does not ignore null objects ?
I want to do it globally without using annotations .