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().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; } 

Answer 2

It is possible to define a custom deserializer for the String type, overriding the standard String deserializer:

this.mapper = new ObjectMapper();  SimpleModule module = new SimpleModule();  module.addDeserializer(String.class, new StdDeserializer<String>(String.class) {      @Override     public String deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {         String result = StringDeserializer.instance.deserialize(p, ctxt);         if (StringUtils.isEmpty(result)) {             return null;         }         return result;     } });  mapper.registerModule(module); 

This way all String fields will behave the same way.