How Spring Boot Chooses Response Formats and Customizes JSON, XML, and YAML Output

Environment: Spring Boot 3.0.5

Message Format Conversion Principle

By default a @RestController returns JSON. Example controller:

<code>@RestController
@RequestMapping("/rmf")
public class ResponseMessageFormatController {
    @GetMapping("/index")
    public Users index() {
        return new Users(1, "张飒", 66, "男");
    }
}
</code>

When no Accept header is sent, Spring treats the request as accepting any media type ( */* ), iterates over all HttpMessageConverter s, and selects the first concrete MediaType that is not a wildcard. application/json is first, so JSON is returned via MappingJackson2HttpMessageConverter .

Core source code that performs the selection:

<code>public abstract class AbstractMessageConverterMethodProcessor {
    protected <T> void writeWithMessageConverters(...) {
        List<MediaType> acceptableTypes;
        try {
            // get Accept header, default */*
            acceptableTypes = getAcceptableMediaTypes(request);
        }
        List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);
        List<MediaType> compatibleMediaTypes = new ArrayList<>();
        determineCompatibleMediaTypes(acceptableTypes, producibleTypes, compatibleMediaTypes);
        for (MediaType mediaType : compatibleMediaTypes) {
            if (mediaType.isConcrete()) {
                selectedMediaType = mediaType;
                break;
            } else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {
                selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
                break;
            }
        }
        if (selectedMediaType != null) {
            selectedMediaType = selectedMediaType.removeQualityValue();
            for (HttpMessageConverter<?> converter : this.messageConverters) {
                GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter) converter : null);
                if (genericConverter != null ? ((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) : converter.canWrite(valueType, selectedMediaType)) {
                    body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType, (Class<? extends HttpMessageConverter<?>>) converter.getClass(), inputMessage, outputMessage);
                    ((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);
                }
            }
        }
    }
}
</code>

Returning XML format

To enable XML, add the Jackson XML dependency:

<code>&lt;dependency&gt;
    &lt;groupId&gt;com.fasterxml.jackson.dataformat&lt;/groupId&gt;
    &lt;artifactId&gt;jackson-dataformat-xml&lt;/artifactId&gt;
&lt;/dependency&gt;
</code>

After adding the dependency, setting Accept: application/xml makes Spring automatically use MappingJackson2XmlHttpMessageConverter to produce XML.

Custom Message Format (YAML)

To support YAML, add the Jackson YAML dependency:

<code>&lt;dependency&gt;
    &lt;groupId&gt;com.fasterxml.jackson.dataformat&lt;/groupId&gt;
    &lt;artifactId&gt;jackson-dataformat-yaml&lt;/artifactId&gt;
&lt;/dependency&gt;
</code>

Create a custom HttpMessageConverter for the application/yaml media type:

<code>@Component
public class YamlHttpMessageConverter extends AbstractHttpMessageConverter<Object> {

    public YamlHttpMessageConverter() {
        super(new MediaType("application", "yaml"));
    }

    @Override
    protected boolean supports(Class<?> clazz) {
        return true;
    }

    @Override
    protected Object readInternal(Class<? extends Object> clazz, HttpInputMessage inputMessage)
            throws IOException, HttpMessageNotReadableException {
        return null;
    }

    @Override
    protected void writeInternal(Object t, HttpOutputMessage outputMessage)
            throws IOException, HttpMessageNotWritableException {
        try (OutputStream os = outputMessage.getBody()) {
            YAMLFactory factory = new YAMLFactory();
            // Remove leading "---"
            factory.configure(Feature.WRITE_DOC_START_MARKER, false);
            ObjectMapper mapper = new ObjectMapper(factory);
            os.write(mapper.writeValueAsString(t).getBytes(StandardCharsets.UTF_8));
        }
    }
}
</code>

Testing with Postman and Accept: application/yaml returns the correct YAML representation.