spring 引导中的 ObjectMapper 可以读取字符串，但在解析标头时不起作用

Question

I am trying to give an enum value as a header parameter to my rest endpoint in a spring boot @RestController . To that end I put the jackson libraries in my build.gradle file since the autogenerated enum used jackson annotations. I cannot change the enum code (it is autogenerated from a openapi specification). It looks like this:

public enum DocumentTypes {

  APPLICATION_PDF("application/pdf"),

  APPLICATION_RTF("application/rtf"),

  APPLICATION_VND_OASIS_OPENDOCUMENT_TEXT("application/vnd.oasis.opendocument.text"),

  APPLICATION_VND_OPENXMLFORMATS_OFFICEDOCUMENT_WORDPROCESSINGML_DOCUMENT("application/vnd.openxmlformats-officedocument.wordprocessingml.document"),

  APPLICATION_VND_MS_WORD("application/vnd.ms-word"),

  TEXT_HTML("text/html"),

  TEXT_PLAIN("text/plain");

  private String value;

  DocumentTypes(String value) {
    this.value = value;
  }

  @Override
  @JsonValue
  public String toString() {
    return String.valueOf(value);
  }

  @JsonCreator
  public static DocumentTypes fromValue(String text) {
    for (DocumentTypes b : DocumentTypes.values()) {
      if (String.valueOf(b.value).equals(text)) {
        return b;
      }
    }
    throw new IllegalArgumentException("Unexpected value '" + text + "'");
  }
}

The restcontroller I am using to test looks like this:

@RestController
@RequestMapping("/test")
public class TestController {

    @Autowired
    private ObjectMapper objectMapper;

    @RequestMapping(path = "", method = RequestMethod.GET)
    public void test(@RequestHeader(value = "Accept", required = false) DocumentTypes targetFormat) throws IOException {
        DocumentTypes value = objectMapper.readValue("\"application/pdf\"", DocumentTypes.class);
    }

}

If I don't supply the Accept header and just let break inside the code I can see that the first line of the code works fine, the application/pdf String is transformed into value so the ObjectMapper did its job using the @JsonCreator method.

However if I pass Accept=application/pdf header along with the request I get an error:

Failed to convert value of type 'java.lang.String' to required type 'de.some.namespace.model.DocumentTypes'; 
nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [@org.springframework.web.bind.annotation.RequestHeader de.some.namespace.model.DocumentTypes] for value 'application/pdf'; 
nested exception is java.lang.IllegalArgumentException: No enum constant de.some.namespace.model.DocumentTypes.application/pdf"

This looks to me as if spring is not using the Jackson provided ObjectMapper , thus ignoring the @JsonCreator method and just trying to resolve the enum by default by looking if there is a key with that provided name.

This to me does not make sense, becuase I also only @Autowire the ObjectMapper ,... isn't that the one that spring should also use, how can I force spring to use the correct one for parsing the arguments? I tried putting it into a @Configuration and making it a @Bean and @Primary with the same results.

I have a workaround by implementing a converter:

@Component
public class StringToDocumentTypesConverter implements Converter<String, DocumentTypes> {

    @Autowired
    private ObjectMapper mapper;

    @Override
    public DocumentTypes convert(String s) {
        try {
            return mapper.readValue(String.format("\"%s\"", s), DocumentTypes.class);
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }
}

But I don't understand why this would be necessary, normally spring automatically puts arguments through the ObjectMapper .

Answer 1

I think this is working as designed. Spring only uses the Jackson ObjectMapper for conversion of message bodies (using a registered HttpMessageConverter , specifically the MappingJackson2HttpMessageConverter ).

This is documented at https://docs.spring.io/spring/docs/current/spring-framework-reference/web.html#mvc-ann-typeconversion :

Some annotated controller method arguments that represent String-based request input (such as @RequestParam , @RequestHeader , @PathVariable , @MatrixVariable , and @CookieValue ) can require type conversion if the argument is declared as something other than String.

For such cases, type conversion is automatically applied based on the configured converters

And https://docs.spring.io/spring/docs/current/spring-framework-reference/web.html#mvc-ann-requestbody :

You can use the @RequestBody annotation to have the request body read and deserialized into an Object through an HttpMessageConverter