[REQ] [Kotlin] Default optional parameters to null

[rupert-jung-mw]

Is your feature request related to a problem? Please describe.

While non-required properties of generatered DTOs are defaulted to null this is not true to non-required parameters.

Example:

Describe the solution you'd like

= null added to optional parameters

Additional context

Generator: kotlin-server
Library: jaxrs-spec

Options:

<configOptions>
    <library>jaxrs-spec</library>
    <!-- Required for Quarkus 3.x (Jakarta EE 10): uses jakarta.* instead of javax.* imports -->
    <useJakartaEe>true</useJakartaEe>
    <useSwaggerAnnotations>false</useSwaggerAnnotations>
    <sourceFolder>src/main/java</sourceFolder>
    <dateLibrary>java8</dateLibrary>
    <serializationLibrary>jackson</serializationLibrary>
    <useBeanValidation>true</useBeanValidation>
    <openApiNullable>false</openApiNullable>
    <sortModelPropertiesByRequiredFlag>true</sortModelPropertiesByRequiredFlag>
    <sortParamsByRequiredFlag>true</sortParamsByRequiredFlag>
    <interfaceOnly>true</interfaceOnly>
</configOptions>

I got it to work by adding a custom Mustache template querParams.mustache to my template directory:

{{#isQueryParam}}@QueryParam("{{#lambda.escapeDollar}}{{baseName}}{{/lambda.escapeDollar}}"){{#useBeanValidation}}{{>beanValidationQueryParams}}{{/useBeanValidation}}{{^isContainer}}{{#defaultValue}} @DefaultValue({{#lambda.doublequote}}{{{.}}}{{/lambda.doublequote}}){{/defaultValue}}{{/isContainer}} {{paramName}}: {{{dataType}}}{{#isNullable}}?{{/isNullable}}{{^isNullable}}{{^defaultValue}}{{^required}}? = null{{/required}}{{/defaultValue}}{{/isNullable}}{{/isQueryParam}}

(Code above has been generated by Claude Sonnet 5)

Maybe this could be validated by some experienced dev and included in one of the next releases :)

Thank you!

[Picazsoo]

Hi, @rupert-jung-mw,
Did you encounter any practical issues when using the generated methods without an explicit = null default for optional nullable parameters?

AFAIK at runtime, JAX-RS binds the HTTP query parameter to the method argument using @QueryParam. If the parameter is omitted from the request and no @DefaultValue is specified, a nullable reference type such as String? should be passed to the resource method as null. The Kotlin = null value only affects how callers invoke the generated method at compile time. It allows them to omit the argument. It should not change the runtime request-binding behavior.

But this is an assumption on my part.

[rupert-jung-mw]

Hello @Picazsoo and thanks for your quick reaction. It's more about a consistency and QoL thing. When calling the generated client method, you now have to explicitly pass null for all unused parameters. This is not only a little bit cumbersome but breaks existing code as soon as new (optional/defaulted) properties are added to the API.

This does not happen for the DTOs itself as here the null default is set currently which greatly helps for back-compat.

[Picazsoo]

Ah, so you're talking about the generated client, not the server interfaces. That makes sense, since = null defaults really only help the calling side.

[rupert-jung-mw]

Correct yes. Basically, the generated methods should default optional parameters with = null, same as when calling a constructor of a generated DTO with optional properties.

[Mattias-Sehlstedt]

It might be of interest to implement useSingleRequestParameter #19827 that exist for the Java generator? It enables a shorter definition of a talkative endpoint (with regard to queries), and it also does not move if the underlying api is expanded.