Change JSON value from null to empty String

im making a rest controller which return json. i get this data from database, mapping in to java class using setter getter.

{
  "example": null,
  "this": null,
  "is": null,
  "only": null,
  "example": null,
  "foo": null,
  "bar": null,
  "blabla": null,
  "lala": null
} 

its because the data in database not present. but i want the data to be like this:

{
      "example": ""    ,
      "this": "",
      "is": "",
      "only": "",
      "example": "",
      "foo": "",
      "bar": "",
      "blabla": "",
      "lala": ""
 }

i have more than 100 fields. so i think its not good to use if else in every variable.

You can override default ObjectMapper (provided by Spring Boot auto-configuration) and configure globally format to use for properties of type String.

@Configuration
public class JacksonConfiguration {

    @Bean
    ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
        final var objectMapper = builder.createXmlMapper(false).build();
        objectMapper.configOverride(String.class).setSetterInfo(JsonSetter.Value.forValueNulls(Nulls.AS_EMPTY));
        return objectMapper;
    }

}

DefaultSerializerProvider.Impl sp = new DefaultSerializerProvider.Impl();
sp.setNullValueSerializer(new NullSerializer());
new ObjectMapper().setSerializerProvider(sp)...

Where

public class NullSerializer extends JsonSerializer<Object> {
    @Override
    public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
        jsonGenerator.writeString("");
    }
}

You can solve it more easily by handling json string:

jsonString = jsonString.replaceAll("null,", "'',");