enum mapping in lowercase fails by default

[beeondev]

I have a legacy API that takes an enumeration as argument, here is a yaml fragment of OpenAPI spec :

...
components 
    schemas:
       Mode:
          type: string
          enum: 
            - a
            - b

Such enumeration should be used as string in API, but is translated to Enum with processor (why not, this is not the problem) as :

@Generated(value = "openapi-processor-spring", version = "2023.4", date = "2023-10-05T14:30:44.211810700+02:00")
public enum Mode {
    A("a"),
    B("b");
...

Internally spring use StringToEnumConverterFactory to convert the string to enumeration:

Daemon Thread [parallel-3] (Suspended)	
	owns: FluxConcatMapNoPrefetch$FluxConcatMapNoPrefetchSubscriber<T,R>  (id=741)	
	owns: FluxConcatMapNoPrefetch$FluxConcatMapNoPrefetchSubscriber<T,R>  (id=742)	
	Enum<E>.valueOf(Class<T>, String) line: 268	
	StringToEnumConverterFactory$StringToEnum<T>.convert(String) line: 54	
	StringToEnumConverterFactory$StringToEnum<T>.convert(Object) line: 39	
	GenericConversionService$ConverterFactoryAdapter.convert(Object, TypeDescriptor, TypeDescriptor) line: 436	
	ConversionUtils.invokeConverter(GenericConverter, Object, TypeDescriptor, TypeDescriptor) line: 41	
	DefaultFormattingConversionService(GenericConversionService).convert(Object, TypeDescriptor, TypeDescriptor) line: 192	
	TypeConverterDelegate.convertIfNecessary(String, Object, Object, Class<T>, TypeDescriptor) line: 129	
	SimpleTypeConverter(TypeConverterSupport).convertIfNecessary(Object, Class<T>, TypeDescriptor) line: 73	
	SimpleTypeConverter(TypeConverterSupport).convertIfNecessary(Object, Class<T>, MethodParameter) line: 53	
	BindingContext$ExtendedWebExchangeDataBinder(DataBinder).convertIfNecessary(Object, Class<T>, MethodParameter) line: 729	
	RequestParamMethodArgumentResolver(AbstractNamedValueArgumentResolver).applyConversion(Object, AbstractNamedValueArgumentResolver$NamedValueInfo, MethodParameter, BindingContext, ServerWebExchange) line: 193

and it fail, because Enum.valueOf(Mode.class, "a") delegates to internal enumConstantDirectory which contains only uppercase values "A" and "B" !

So the only workaround is to inject custom converter, but this is not desirable.

[beeondev]

say the same shortly :

 // processor-spring impl
 Mode.fromValue(source);

is not the same as

Mode.valueOf(source);

[hauner]

yes, this is annoying.

the question is how to improve it.

• generate lowercase enums

  • would need some switch to be backward compatible
  • maybe configure by enum
  • will not work for enum values that contain illegal characters (not allowed in java identifier)

• provide/generate converter

  • StringToEnumConverter implements Converter<String, MyEnum>

any other idea?

[beeondev]

• I would like specify what is annoying :

1. the need to add specific spring behavior
2. per enum spring behavior (converter) injection

Second point is far more annoying, because each enum must have its converter registered. Having a special interface on the enum i.e like "Supplier<String>" would allow :
- not break the backward compatibility
- reduce '2)' to '1)' by :
* allowing to identitfy that enum must converted another way.
* avoid to depends on specific openapi-processor type

• If breaking backward compatibily is an option, then perhaphs, because initial openapi spec declare a string type, we can just keep it as a string. enum declaration in spec could be interpreted a special validator constraint with something like :

    public Mono<String> someEndpoint(@RequestBody @DictionaryValue(allowedValues = {"a", "b"}) String mode)

with :

    import javax.validation.Constraint;
    import javax.validation.Payload;
    import java.lang.annotation.*;
    
    @Target({ElementType.FIELD, ElementType.PARAMETER})
    @Retention(RetentionPolicy.RUNTIME)
    @Constraint(validatedBy = DictionaryValueValidator.class)
    @Documented
    public @interface DictionaryValue {
        String message() default "Invalid value. Please choose from the dictionary.";
        Class<?>[] groups() default {};
        Class<? extends Payload>[] payload() default {};
        String[] allowedValues() default {};
    }

and

    public class DictionaryValueValidator implements ConstraintValidator<DictionaryValue, String> {
    
        private String[] allowedValues;
    
        @Override
        public void initialize(DictionaryValue constraintAnnotation) {
            this.allowedValues = constraintAnnotation.allowedValues();
        }
    
        @Override
        public boolean isValid(String value, ConstraintValidatorContext context) {
            return value != null && Arrays.asList(allowedValues).contains(value);
        }
    }

This is only suggestions.
Best regards

[hauner]

I have successfully created a prototype ConverterFactory that creates enum converters for generated enums that implement Supplier<>. Not much code which is nice :-)

import org.springframework.core.convert.converter.Converter;
import org.springframework.core.convert.converter.ConverterFactory;
import org.springframework.stereotype.Component;

import java.util.EnumSet;
import java.util.function.Supplier;

public class StringToEnumConverterFactory<T extends Enum<T> & Supplier<String>> implements ConverterFactory<String, T> {

    @Override
    @SuppressWarnings({"unchecked", "rawtypes"})
    public <E extends T> Converter<String, E> getConverter(Class<E> targetType) {
        return new StringToEnumConverter(targetType);
    }

    static class StringToEnumConverter<T extends Enum<T> & Supplier<String>> implements Converter<String, T> {

        private final Class<T> enumType;

        public StringToEnumConverter(Class<T> enumType) {
            this.enumType = enumType;
        }

        public T convert(String source) {
            String sourceValue = source.trim();

            for (T e : EnumSet.allOf(enumType)) {
                if (e.get().equals(sourceValue)) {
                    return e;
                }
            }

            throw new IllegalArgumentException(
                    String.format("No enum constant of %s has the value %s", enumType.getCanonicalName(), sourceValue));
        }
    }
}

It requires a one time registration using Web[Mvc|Flux]Configurer.addFormatters().

Your String version is also something I'm considering. I'm thinking about a global setting to switch between enum and String. enum would also generate the converter factory and it can be registered (or not if it is not needed).

Thanks for your input :-)