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

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

I could get this by following configuration.

final ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT, true);

Answer 3

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.

Answer 4

it is possible to use JsonCreator annotation. It worked for me

public class Foo {
private String field;

 @JsonCreator
 public Foo(
   @JsonProrerty("field") String field) {
     this.field = StringUtils.EMPTY.equals(field) ? null : field ;
}
}

Answer 5

A simple work around would be to replace "" with null before deserializing the json string.

.replace("\"\"", "null")

This way all blank string will be deserialized as null

Answer 6

You might first like to see if there has been any progress on the Github issue requesting this exact feature.

For those using Spring Boot: The answer from jgesser was the most helpful to me, but I spent a while trying to work out the best way to configure it in Spring Boot.

Actually, the documentation says:

Any beans of type com.fasterxml.jackson.databind.Module are automatically registered with the auto-configured Jackson2ObjectMapperBuilder and are applied to any ObjectMapper instances that it creates.

So here's jgesser's answer expanded into something you can copy-paste into a new class in a Spring Boot application

@Configuration
public class EmptyStringAsNullJacksonConfiguration {

  @Bean
  SimpleModule emptyStringAsNullModule() {
    SimpleModule module = new SimpleModule();

    module.addDeserializer(
        String.class,
        new StdDeserializer<String>(String.class) {

          @Override
          public String deserialize(JsonParser parser, DeserializationContext context)
              throws IOException {
            String result = StringDeserializer.instance.deserialize(parser, context);
            if (StringUtils.isEmpty(result)) {
              return null;
            }
            return result;
          }
        });

    return module;
  }
}