custom enum mapping

1. May 2024

How do I use custom names for OpenAPI enum values with openapi-processor?

For example having an enum in the OpenAPI description like this

# ...

      type: string
        - "A1"
        - "A2"

openapi-processor will generate the enum as
public enum AnEnum {

    // ...

but we may like to use more descriptive names for the enum values in the source code.

the unsupported solution …​

Other generators use an OpenAPI x-tension keyword to provide better names for the enum values:

    - "nicer"
    - "names"

Using x-tension keywords like this is a bit against the philosophy of openapi-processor.

openapi-processor philosophy

I would like to keep tooling specific stuff out of the OpenAPI description.

Let’s assume we have multiple consumers of the api. Each consumer may use another programming language and different tooling. Now we would have to add the x-enumNames equivalent of each tooling to the OpenAPI…​ Things get more difficult when we don’t have control over the OpenAPI description. We would have to add some pre-processing to add the enum names to the api.

The typical openapi-processor way would be to create some kind of mapping. That would be possible, but I fear this may be cumbersome to work with. Not sure.

It probably depends on how many enums I need to map and how often I have to change them.

Maybe this is a future feature. :-)

Now let’s see how we can solve this with the current openapi-processor version.

a bit code & type mapping to the rescue

openapi-processor has no explicit support for mapping OpenAPI enum values to custom names in the generated source code.

With a little bit of code and a simple type mapping we can still get custom enum vale names.

custom enum

First, we create the custom enum. It uses nearly the same code as the generated enum. We just change the enum value names to more descriptive ones:

package io.openapiprocessor.samples

import com.fasterxml.jackson.annotation.JsonCreator
import com.fasterxml.jackson.annotation.JsonValue

enum class NamedEnum(@JsonValue /* <2> */ val value: String) {
    FOO("A1"), (1)
    BAR("A2"); (1)

    companion object {
        @JsonCreator (2)
        fun fromValue(value: String): NamedEnum {
            for (v in NamedEnum.entries) {
                if (v.value == value) {
                    return v
            throw IllegalArgumentException(value)
1 the custom enum value names we want to use in the source code.
2 jackson annotations for serialization.

type mapping

Second, we tell the processor to use our custom enum class instead of generating it.

openapi-processor-mapping: v7

  # ...

    - type: AnEnum => io.openapiprocessor.samples.NamedEnum

The processor will now use our custom Enum at all places in the generated source code where AnEnum is used in the OpenAPI description, and we can use our more descriptive enum value names.

request body

This solution will work for request payloads with enum properties. Our custom enum uses two jackson annotations to help jackson serialize/deserialize the enum when it finds an enum in the payload.

If the enum is not used as query parameter we are done here.

query parameter

If the enum is used as query parameter we need a little bit more code to help Spring by providing an enum converter for proper deserialization.

The converter is quite simple, it just delegates to the enums fromValue() method to convert the string value to the matching enum value.

package io.openapiprocessor.samples

import org.springframework.core.convert.converter.Converter

class NamedEnumConverter: Converter<String, NamedEnum> {
    override fun convert(source: String): NamedEnum {
        return NamedEnum.fromValue(source)

The last step is to register the converter using a WebMvcConfigurer:

package io.openapiprocessor.samples

import org.springframework.context.annotation.Configuration
import org.springframework.format.FormatterRegistry
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer

open class WebConfig : WebMvcConfigurer {
    override fun addFormatters(registry: FormatterRegistry) {

Spring is now able to deserialize an enum query parameter to our custom enum class.


This article describes how to use custom value names for an OpenAPI enum. This can be achieved with a little bit of custom code. It also explains why openapi-processor doesn’t use an x-tension keyword.