Don't return property when it's an empty list with jackson

Question

I have a class need to be dessrialized using jackson and the class has an collection property. The collection is empty, but not null. Question: How to desersialise the class without empty collection. Sample code below:

class Person {     String name;     List<Event> events;     //....getter, setter } 

if

person.list = new List<Event>();  persion.name = "hello"; 

then excepted the json would be:

{name: "hello"} 

not

{name: "hello", events:[]} 

How to make it? Thank you~~

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I have solved this by n1ckolas's advice. Thank you first. My jackson version is 2.1.1, and Spring-3.2.2 import better support for this verson of jackson. Also, this works for both arrays and collection. Below is my configuraion:

<!-- Enables the Spring MVC @Controller programming model --> <mvc:annotation-driven>     <mvc:message-converters>         <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">             <property name="objectMapper" ref="objectMapper"/>         </bean>     </mvc:message-converters>         </mvc:annotation-driven>  <!--Json Mapper--> <bean name="objectMapper" class="org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean" autowire="no">     <property name="featuresToDisable">         <list>             <!--not to return empty colletion-->             <value type="com.fasterxml.jackson.databind.SerializationFeature">WRITE_EMPTY_JSON_ARRAYS</value>         </list>     </property> </bean> 

Answer 1

You can use @JsonInclude(JsonInclude.Include.NON_EMPTY) annotation on your class or filed.

import com.fasterxml.jackson.annotation.JsonInclude; @JsonInclude(JsonInclude.Include.NON_EMPTY) class Person {     String name;     List<Event> events;     //....getter, setter } 

Answer 2

If you can change your original model with Jackson annotations, here is the way how to achieve this.

Jackson has WRITE_EMPTY_JSON_ARRAYS. And you may turn off it with:

objectMapper.configure(SerializationConfig.Feature.WRITE_EMPTY_JSON_ARRAYS, false); 

But it works only for Arrays, but not for Collections. But you may combine @JsonProperty with @JsonIgnore, something like the following:

//....getter, setter of Person class @JsonIgnore public List<Event> getEvents() {     return events; }  @JsonProperty("events") public Event[] getEventsArr() {     return events.toArray(new Event[0]); } 

And afterwards you'll have output, as you expected.

EDIT: If you are using SpringMVC, then you can configure your actual ObjectMapper with explicit reference in mvc:annotation-driven:

<!-- Configures the @Controller programming model --> <mvc:annotation-driven>     <mvc:message-converters>         <bean class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">             <property name="objectMapper" ref="objectMapper"/>         </bean>     </mvc:message-converters> </mvc:annotation-driven>  <!--custom Json Mapper configuration--> <bean name="objectMapper" class="org.springframework.http.converter.json.JacksonObjectMapperFactoryBean" autowire="no">     <property name="featuresToDisable">         <list>             <!--Ignore unknown properties-->             <value type="org.codehaus.jackson.map.SerializationConfig.Feature">WRITE_EMPTY_JSON_ARRAYS</value>         </list>     </property> </bean> 

Offtop: usually it's quite useful to specify explicit instance of ObjectMapper, because:

• you can configure it in the way you want
• you can use its reference through @Autowired