By default, openapi-processor creates a java Enum from an OpenAPI schema that’s using the enum keyword.Something like this:

OpenAPI enum
      type: string
        - one
        - two
generated Java enum
package io.openapiprocessor.openapi2.model;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonValue;
import io.openapiprocessor.openapi2.support.Generated;

@Generated(value = "openapi-processor-spring", version = "latest")
public enum Type {

    private final String value;

    Type(String value) {
        this.value = value;

    public String getValue() {
        return this.value;

    public static Type fromValue(String value) {
        for (Type val : Type.values()) {
            if (val.value.equals(value)) {
                return val;
        throw new IllegalArgumentException(value);

This works without issues if used as part of a request payload.

Unfortunately it may cause an error, like the following if the enum is used as a query parameter:

Failed to convert value of type 'java.lang.String' to required type 'io.openapiprocessor.openapi.model.Type'; Failed to convert from type [java.lang.String] to type [@org.springframework.web.bind.annotation.RequestParam io.openapiprocessor.openapi.model.Type] for value [one]

The reason is, that Spring uses org.springframework.core.convert.converter.Converter implementations to deserialize parameters and the default enum deserialization expects the incoming string value to exactly match an enum value.

That is, to successfully convert to the enum value ONE the incoming value string has to be ONE. It will not accept the lowercase one.

The converter doesn’t use jackson, so it won’t use the @JsonCreator method to convert from the incoming lowercase value to the corresponding enum value.

To handle this issue we can either use the enum-type string or framework.


Do not create a Java enum classes for OpenAPI enums and simply use java.lang.String.

openapi-processor-mapping: v5

  enum-type: string

This is an alternative to generating enum classes. It will pass the enum value string as given in the api request to avoid the issue described in the default section.

api interface
public interface FooApi {

    @PostMapping(path = "/foo", produces = {"application/json"})
    Foo postFoo(@RequestParam(name = "enum", required = false) String aEnum);


In this simple form it doesn’t provide any help to make sure that the incoming values is a valid value as described in the OpenAPI.

By enabling bean-validation the processor will generate and use a custom validation annotation to check that the incoming string is an allowed value.

openapi-processor-mapping: v5

  bean-validation: jakarta
  enum-type: string
api interface with validation
public interface FooApi {

    @PostMapping(path = "/foo", produces = {"application/json"})
    Foo postFoo(@RequestParam(name = "enum", required = false) @Values(values = {"one", "two"}) String aEnum);


make sure you annotate the controller with @Validated to run the @Values check.

api interface with validation
public class ApiController implements FooApi {
    // ...


This is another alternative to the default enum classes to avoid the issue described above.

It creates Java enum classes and a Spring ConverterFactory with the name +{package-name}+.spring.StringToEnumConverterFactory that does create enum converters for all generated enums.The enum converters convert incoming strings to their enum by comparing with the OpenAPI enum values.

openapi-processor-mapping: v5

  enum-type: framework

To enable the converter factory use a WebMvcConfigurer (or WebFluxConfigurer) like the code below:

enable enum converter factory
package io.openapiprocessor.samples;

import io.openapiprocessor.openapi.spring.StringToEnumConverterFactory;
import org.springframework.context.annotation.Configuration;
import org.springframework.format.FormatterRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

public class WebConfig implements WebMvcConfigurer {

    public void addFormatters(FormatterRegistry registry) {
        registry.addConverterFactory(new StringToEnumConverterFactory());