Null values as empty strings when using @ResponseBody annotation

Question

Is there a way when using @ResponseBody annotation to have null values mapped to empty strings?

Answer 1

You will have to write a custom Jackson Serializer - a good example is here - http://wiki.fasterxml.com/JacksonHowToCustomSerializers (there is a specific example of how to convert null values to empty Strings that you can use)

Here are all the steps(for Jackson < 2.0):

Write your custom null Serializer:

import java.io.IOException;

import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.JsonProcessingException;
import org.codehaus.jackson.map.JsonSerializer;
import org.codehaus.jackson.map.SerializerProvider;

public class NullSerializer extends JsonSerializer<Object> {

    @Override
    public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
        jgen.writeString("");
    }
}

Register this with Jackson Objectmapper:

import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.map.ser.StdSerializerProvider;


public class CustomObjectMapper extends ObjectMapper{
    public CustomObjectMapper(){
        StdSerializerProvider sp = new StdSerializerProvider();
        sp.setNullValueSerializer(new NullSerializer());
        this.setSerializerProvider(sp);
    }

}

Register this objectmapper with Spring MVC:

<mvc:annotation-driven> 
   <mvc:message-converters register-defaults="true">
       <bean class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">
           <property name="objectMapper">
               <bean class="CustomObjectMapper"/>
           </property>
       </bean>
   </mvc:message-converters>
</mvc:annotation-driven>

Answer 2

I have also faced the same problem in my project and I have therefore quickly come up with a solution for the same. This post will surely help all those who have been struggling with the same issue.

Step 1:- Create your Custom Null Handler Serializer.

public class NullSerializer extends StdSerializer<Object> {

    public NullSerializer(Class<Object> t) {
       super(t);
    }

    public NullSerializer() {
      this(null);
    }

    @Override
    public void serialize(Object o, com.fasterxml.jackson.core.JsonGenerator jsonGenerator, com.fasterxml.jackson.databind.SerializerProvider serializerProvider) throws IOException {
       jsonGenerator.writeString("");
    }
   }

Step 2:- Create a bean of MappingJackson2HttpMessageConverter.

@Bean
@Primary
public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter() {

    MappingJackson2HttpMessageConverter jsonConverter = new MappingJackson2HttpMessageConverter();
    ObjectMapper mapper = new ObjectMapper();
    mapper.setPropertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE);
    mapper.getSerializerProvider().setNullValueSerializer(new NullSerializer());
    jsonConverter.setObjectMapper(mapper);
    return jsonConverter;

} 

Thank you for taking some time out to read this post. I hope that this was able to resolve your queries to some extent.