Mayur Mayur - 7 months ago 10
Java Question

how to deserialize a blank json string value to null for java.lang.String

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 Java class are listed below:

Json string:

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


Java code:

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 " Is is null". Instead, I am getting "It is". Obviously, jackson is not treating blank String values as Null while converting to my java.lang.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

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().isEmpty()) {
            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;
}
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