How to properly configure jackson converters for JSON and plain text

Question

For a Spring MVC (not Spring Boot) I've had to change the configuration class that extended WebMvcConfigurationSupport to implement WebMvcConfigurer and add the @EnableWebMvc annotation. This causes problems with the conversion of the responses for several endpoints. The project defaults to application/json and it is used for most of the responses however, there are several endpoints which return application/xml and even a few that return text/plain. JSON responses are modified to remove fields containing null using the following Java config:

@Bean
public MappingJackson2HttpMessageConverter jsonConverter() {
  List<MediaType> supportedMediaTypes = new ArrayList<>();
  supportedMediaTypes.add(MediaType.APPLICATION_JSON);

  Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder();
  builder.serializationInclusion(JsonInclude.Include.NON_NULL);
  builder.timeZone(TimeZone.getTimeZone(timeZone));

  MappingJackson2HttpMessageConverter jsonConverter = new MappingJackson2HttpMessageConverter(
    builder.build()
  );
  jsonConverter.setSupportedMediaTypes(supportedMediaTypes);
  return jsonConverter;
}

This causes JSON responses to be returned correctly but results in an exception for the text/plain endpoints. They then produce an error:

No converter for [class java.lang.String] with preset Content-Type 'null'

The error can be resolved by adding the default string converter before the JSON converter:

@Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
  converters.add(new StringHttpMessageConverter());
  converters.add(jsonConverter());
}

However, this causes a problem specifically for endpoints that return JSON but in Java only have String as the return type. A string in between double quotes should be returned: "response", but they only return the string without quotes: response. This makes most clients to not recognise the response as valid JSON. Curiously POJOs are still converted to valid JSON.

How can I configure my Spring MVC (not Spring Boot) project using a configuration class that implements WebMvcConfigurer and is annotated with @EnableWebMvc to return JSON without null fields and single strings as valid JSON (e.g. with double quotes: "response") but also plain text?

You can force it using `consumes` property from `RequestMapping` annotation: [Spring RequestMapping for controllers that produce and consume JSON](https://stackoverflow.com/questions/35123835/spring-requestmapping-for-controllers-that-produce-and-consume-json) — Michał Ziober, Jul 10 '21 at 17:13

score 1 · Accepted Answer · answered Jul 14 '21 at 11:22

A suitable solution has been found to have the REST API return valid JSON responses even for methods which return a string (so with double quotes around the returned string) while also being able to return XML and plain text responses. In the configuration class that implements WebMvcConfigurer and is annotated with @EnableWebMvc we register the default converters by overriding the extendMessageConverters. The default MappingJackson2HttpMessageConverter is removed from the list and our custom JSON converter is added instead as the first converter in the list. By adding the JSON converter before the StringHttpMessageConverter methods that return a string have their responses converted to valid JSON (with double quotes) while application/xml and text/plain responses also work properly.

score 0 · Answer 2 · answered Jul 09 '21 at 13:31

0

What I can suggest is add one more bean for handling string responses as you added for json and add the convertor to converters.add(stringConverter());

answered Jul 09 '21 at 13:31

anish sharma

568
3
5

Creating a `StringHttpMessageConverter` with supported media types `text/plain` and `application/xml` resolves the problems I have. However, it introduces a new problem, the springdoc `v3/api-docs endpoint which returns JSON gets converted to one big string, making it invalid JSON. – Pieter Jul 12 '21 at 06:55
add the new convertor after json one converters.add(new StringHttpMessageConverter()); converters.add(jsonConverter());converters.add(stringConverter()); – anish sharma Jul 12 '21 at 07:26
I've tried that sadly it doesn't work. The order for loading the converters doesn't seem to effect the responses. Adding `application/json` to the supported media types for the string converter also doesn't seem to have any effect. At least not if it's added after `text/plain` and `application/xml`. – Pieter Jul 12 '21 at 14:50

How to properly configure jackson converters for JSON and plain text

2 Answers2