One of the great things about RestTemplate is its simplicity. You simply instantiate it like this... RestTemplate restTemplate = new RestTemplate(); and off you go. Under the hood, Spring automatically creates and registers a number of message converters to handle various data formats for requests and responses.

A MappingJackson2HttpMessageConverter uses Jackson to map POJOs to JSON and vice versa. When the MappingJackson2HttpMessageConverter is created it's given a new instance of ObjectMapper. A default instance of ObjectMapper is fine is many cases, but there are times when you might want to customize the ObjectMapper used.

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For example, I ran into an issue recently where I needed to configure Jackson to accept case insensitive properties like this.

private ObjectMapper createObjectMapper() {

     ObjectMapper objectMapper = new ObjectMapper();
     objectMapper.configure(MapperFeature.ACCEPT_CASE_INSENSITIVE_PROPERTIES, true);

     return objectMapper;
}

To make the custom ObjectMapper available to MappingJackson2HttpMessageConverter, simply create a new MappingJackson2HttpMessageConverter and pass in the ObjectMapper instance.

private MappingJackson2HttpMessageConverter createMappingJacksonHttpMessageConverter() {

    MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
    converter.setObjectMapper(createObjectMapper());
    return converter;
}

Then, you can create a RestTemplate and add your custom MappingJackson2HttpMessageConverter to its list of message converters.

@Bean
public RestTemplate createRestTemplate() {

     RestTemplate restTemplate = new RestTemplate();
     restTemplate.getMessageConverters().add(0, createMappingJacksonHttpMessageConverter());
}

Note that you should add it to the beginning of the list so that it takes precedence over the default MappingJackson2HttpMessageConverter that Spring has already registered.

Further Reading

How to Use Spring RESTTemplate to Post Data to a Web Service

Processing JSON With Jackson