How can we configure the internal Jackson mapper when using RestTemplate?

Solution 1:

The default RestTemplate constructor registers a set of HttpMessageConverters:

this.messageConverters.add(new ByteArrayHttpMessageConverter());
this.messageConverters.add(new StringHttpMessageConverter());
this.messageConverters.add(new ResourceHttpMessageConverter());
this.messageConverters.add(new SourceHttpMessageConverter());
this.messageConverters.add(new XmlAwareFormHttpMessageConverter());
if (jaxb2Present) {
    this.messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
}
if (jacksonPresent) {
    this.messageConverters.add(new MappingJacksonHttpMessageConverter());
}
if (romePresent) {
    this.messageConverters.add(new AtomFeedHttpMessageConverter());
    this.messageConverters.add(new RssChannelHttpMessageConverter());
}

The MappingJacksonHttpMessageConverter in turns, creates ObjectMapper instance directly. You can either find this converter and replace ObjectMapper or register a new one before it. This should work:

@Bean
public RestOperations restOperations() {
    RestTemplate rest = new RestTemplate();
    //this is crucial!
    rest.getMessageConverters().add(0, mappingJacksonHttpMessageConverter());
    return rest;
}

@Bean
public MappingJacksonHttpMessageConverter mappingJacksonHttpMessageConverter() {
    MappingJacksonHttpMessageConverter converter = new MappingJacksonHttpMessageConverter();
    converter.setObjectMapper(myObjectMapper());
    return converter;
}

@Bean
public ObjectMapper myObjectMapper() {
    //your custom ObjectMapper here
}

In XML it is something along these lines:

<bean id="restOperations" class="org.springframework.web.client.RestTemplate">
    <property name="messageConverters">
        <util:list>
            <bean class="org.springframework.http.converter.ByteArrayHttpMessageConverter"/>
            <bean class="org.springframework.http.converter.StringHttpMessageConverter"/>
            <bean class="org.springframework.http.converter.ResourceHttpMessageConverter"/>
            <bean class="org.springframework.http.converter.xml.SourceHttpMessageConverter"/>
            <bean class="org.springframework.http.converter.xml.XmlAwareFormHttpMessageConverter"/>
            <bean class="org.springframework.http.converter.xml.Jaxb2RootElementHttpMessageConverter"/>
            <bean class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">
                <property name="objectMapper" ref="customObjectMapper"/>
            </bean>
        </util:list>
    </property>
</bean>

<bean id="customObjectMapper" class="org.codehaus.jackson.map.ObjectMapper"/>

Note that the transition isn't really 1:1 - I have to explicitly create messageConverters list in XML while with @Configuration approach I could reference existing one and simply modify it. But this should work.

Solution 2:

If you are not using Spring IOC, you can do something like this (Java 8):

ObjectMapper objectMapper = new ObjectMapper();
// configure your ObjectMapper here

RestTemplate restTemplate = new RestTemplate();    

MappingJackson2HttpMessageConverter messageConverter = new MappingJackson2HttpMessageConverter();
messageConverter.setPrettyPrint(false);
messageConverter.setObjectMapper(objectMapper);
restTemplate.getMessageConverters().removeIf(m -> m.getClass().getName().equals(MappingJackson2HttpMessageConverter.class.getName()));
restTemplate.getMessageConverters().add(messageConverter);