How to add Jdk8Module on ObjectMapper for Jackson

Question

I am using Jackson 2.9.6. I have a controller in which I am trying to send an Optional as one of my fields. When I receive a response from the controller I always get it in this format {"field":{"present":true}} (as shown in this question).

Basically I have a RestTemplate bean configured as such:

<bean id="restTemplate" class="org.springframework.web.client.RestTemplate">
    <property name="messageConverters">
        <list>
            <bean class="org.springframework.http.converter.ByteArrayHttpMessageConverter"/>
            <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter"/>
        </list>
    </property>
</bean>

And I would like to add the Jdk8Module as part of the serialzation process. I noticed that there's a MappingJackson2HttpMessageConverter(ObjectMapper mapper) constructor which takes in an ObjectMapper and I'm thinking of creating an ObjectMapper bean which will have the Jdk8Module registered to it (using a public method called registerModule(Module module) found in the ObjectMapper class) so I can pass that module as such:

<bean id="restTemplate" class="org.springframework.web.client.RestTemplate">
    <property name="messageConverters">
        <list>
            <bean class="org.springframework.http.converter.ByteArrayHttpMessageConverter"/>
            <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
                <constructor-arg name="objectMapper" ref="ObjectMapperWithJDK8Bean"></constructor-arg>
            </bean>
        </list>
    </property>
</bean>

<bean id="ObjectMapperWithJDK8Bean" class="com.fasterxml.jackson.databind.ObjectMapper">
* Pass in com.fasterxml.jackson.datatype.jdk8.Jdk8Module here via the method *
</bean>

But the issue im current facing is how to call the registModule method from the xml file when creating the bean? I am using Spring 4.1. I'm new to Spring so this is pretty challenging!

According to this the jdk8 module should be registered automatically when found on the class path. — Marvin
– Marvin, Commented Oct 11, 2019 at 0:12
But it says that was added as of 4.2... I'm currently using 4.1 :/ I can't really upgrade to 4.2 right now — eghe
– eghe, Commented Oct 11, 2019 at 0:14

chrylis -cautiouslyoptimistic- · Accepted Answer · 2019-10-11 14:17:56Z

You could create a bean with java config instead of xml like :

@Configuration
public class GeneralConfiguration { 
    @Bean
    public ObjectMapper objectMapper() {
        ObjectMapper mapper = new ObjectMapper()
           .registerModule(new ParameterNamesModule())
           .registerModule(new Jdk8Module())
           .registerModule(new JavaTimeModule());
        return mapper;
    }
}

I believe that both the @Configuration and @Bean annotations are available since spring 3.0 , so you could use them without any issues. If you still wish to continue with xml based configuration then :

<bean id="objectMapper"
    class="org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean">
    <property name="modulesToInstall"
        value="
        com.fasterxml.jackson.datatype.jdk8.Jdk8Module,
        com.fasterxml.jackson.datatype.jsr310.JavaTimeModule,
        com.fasterxml.jackson.module.paramnames.ParameterNamesModule" />
</bean>

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