How to deserialize a blank JSON string value to null for java.lang.String?

Question

I am trying a simple JSON to de-serialize in to java object. I am however, getting empty String values for java.lang.String property values. In rest of the properties, blank values are converting to null values(which is what I want).

My JSON and related Java class are listed below.

JSON string:

{
  "eventId" : 1,
  "title" : "sample event",
  "location" : "" 
}

EventBean class POJO:

public class EventBean {

    public Long eventId;
    public String title;
    public String location;

}

My main class code:

ObjectMapper mapper = new ObjectMapper();
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
mapper.enable(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);

try {
    File file = new   File(JsonTest.class.getClassLoader().getResource("event.txt").getFile());

    JsonNode root = mapper.readTree(file);
    // find out the applicationId

    EventBean e = mapper.treeToValue(root, EventBean.class);
    System.out.println("It is " + e.location);
}

I was expecting print "It is null". Instead, I am getting "It is ". Obviously, Jackson is not treating blank String values as NULL while converting to my String object type.

I read somewhere that it is expected. However, this is something I want to avoid for java.lang.String too. Is there a simple way?

Answer 1

Jackson will give you null for other objects, but for String it will give empty String.

But you can use a Custom JsonDeserializer to do this:

class CustomDeserializer extends JsonDeserializer<String> {

    @Override
    public String deserialize(JsonParser jsonParser, DeserializationContext context) throws IOException, JsonProcessingException {
        JsonNode node = jsonParser.readValueAsTree();
        if (node.asText().isBlank()) {
            return null;
        }
        return node.toString();
    }

}

In class you have to use it for location field:

class EventBean {
    public Long eventId;
    public String title;

    @JsonDeserialize(using = CustomDeserializer.class)
    public String location;
}