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Biju Kunjummen

Customizing HttpMessageConverters with Spring Boot and Spring MVC

Exposing a REST based endpoint for a Spring Boot application or for that matter a straight Spring MVC application is straightforward, the following is a controller exposing an endpoint to create an entity based on the content POST’ed to it:
 
 
 
 
 
 
 
 

@RestController
@RequestMapping("/rest/hotels")
public class RestHotelController {
        ....
 @RequestMapping(method=RequestMethod.POST)
 public Hotel create(@RequestBody @Valid Hotel hotel) {
  return this.hotelRepository.save(hotel);
 }
}

Internally Spring MVC uses a component called a HttpMessageConverter to convert the Http request to an object representation and back.

A set of default converters are automatically registered which supports a whole range of different resource representation formats – json, xml for instance.

Now, if there is a need to customize the message converters in some way, Spring Boot makes it simple. As an example consider if the POST method in the sample above needs to be little more flexible and should ignore properties which are not present in the Hotel entity – typically this can be done by configuring the Jackson ObjectMapper, all that needs to be done with Spring Boot is to create a new HttpMessageConverter bean and that would end up overriding all the default message converters, this way:

@Bean
 public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter() {
  MappingJackson2HttpMessageConverter jsonConverter = new MappingJackson2HttpMessageConverter();
  ObjectMapper objectMapper = new ObjectMapper();
  objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
  jsonConverter.setObjectMapper(objectMapper);
  return jsonConverter;
 }

This works well for a Spring-Boot application, however for straight Spring MVC applications which do not make use of Spring-Boot, configuring a custom converter is a little more complicated – the default converters are not registered by default and an end user has to be explicit about registering the defaults – the following is the relevant code for Spring 4 based applications:

@Configuration
public class WebConfig extends WebMvcConfigurationSupport {

 @Bean
 public MappingJackson2HttpMessageConverter customJackson2HttpMessageConverter() {
  MappingJackson2HttpMessageConverter jsonConverter = new MappingJackson2HttpMessageConverter();
  ObjectMapper objectMapper = new ObjectMapper();
  objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
  jsonConverter.setObjectMapper(objectMapper);
  return jsonConverter;
 }
 
 @Override
 public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
  converters.add(customJackson2HttpMessageConverter());
  super.addDefaultHttpMessageConverters();
 }
}

Here WebMvcConfigurationSupport provides a way to more finely tune the MVC tier configuration of a Spring based application. In the configureMessageConverters method, the custom converter is being registered and then an explicit call is being made to ensure that the defaults are registered also. A little more work than for a Spring-Boot based application.

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