Jackson MismatchedInputException异常：用户组POST请求问题排查

Looks like you're hitting a common snag when working with Spring Data REST's HAL (Hypertext Application Language) format alongside a custom OAuthRestTemplate setup. Let's break down the fixes step by step:

1. Complete Your ObjectMapper HAL Configuration #

Just registering Jackson2HalModule isn't enough—you need to add the HalHandlerInstantiator to properly parse HAL-specific fields like _links and _embedded. Here's the full, working configuration:

ObjectMapper objectMapper = new ObjectMapper();
Jackson2HalModule halModule = new Jackson2HalModule();
objectMapper.registerModule(halModule);

// Critical: This handles HAL's special properties during serialization/deserialization
objectMapper.setHandlerInstantiator(new Jackson2HalModule.HalHandlerInstantiator(
    new DefaultRelProvider(), null, null
));

// Disable failing on unknown properties (HAL adds fields like _links that aren't in your UserGroup class)
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

2. Bind Your Custom ObjectMapper to OAuthRestTemplate #

A frequent mistake is that OAuthRestTemplate uses its default ObjectMapper instead of your HAL-configured one. Replace its default message converter with your HAL-aware version:

// Create a message converter that uses your custom ObjectMapper
MappingJackson2HttpMessageConverter halConverter = new MappingJackson2HttpMessageConverter();
halConverter.setObjectMapper(objectMapper);
// Explicitly support the HAL JSON media type
halConverter.setSupportedMediaTypes(Collections.singletonList(MediaTypes.HAL_JSON));

// Swap out the default converter in your OAuthRestTemplate
List<HttpMessageConverter<?>> converters = oauthRestTemplate.getMessageConverters();
converters.removeIf(c -> c instanceof MappingJackson2HttpMessageConverter);
converters.add(halConverter);

3. Ensure Your POST Request Uses HAL Media Type #

When sending the request, set the Content-Type to application/hal+json so the server understands the format, and your client can correctly parse the HAL response:

public UserGroup createUserGroup(UserGroup userGroup) { 
    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaTypes.HAL_JSON);
    HttpEntity<UserGroup> requestEntity = new HttpEntity<>(userGroup, headers);

    ResponseEntity<UserGroup> userGroupResponseEntity = oauthRestTemplate 
        .postForEntity(GROUPS_ENDPOINT, requestEntity, UserGroup.class); 
    return userGroupResponseEntity.getBody(); 
}

4. Adjust Your UserGroup Class for HAL Compatibility #

Add @JsonIgnoreProperties to your UserGroup class to ignore any HAL-specific fields that don't map to your entity's properties:

@JsonIgnoreProperties(ignoreUnknown = true)
public class UserGroup {
    // Your existing fields, constructors, and methods
}

These changes ensure both your request and response are properly serialized/deserialized using HAL format, which should resolve the MismatchedInputException you're seeing.

内容的提问来源于stack exchange，提问作者Stefan Falk