Support Kotlin parameter default values in HTTP handler methods [SPR-16598]

[spring-projects-issues]

Take Weiland opened SPR-16598 and commented

Support specifying defaults for e.g. @RequestMapping via Kotlin parameter default values like so:

@GetMapping
fun get(@RequestParam limit: Int = 20)

Such a method should behave equivalent to @RequestMapping with defaultValue, except that the value does not need to be processed by converters, etc.

This can be implemented in org.springframework.web.method.support.InvocableHandlerMethod using Kotlin Reflection (KCallable#callBy), which allows omitting parameters with default values.

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Affects: 5.0.4

Referenced from: pull request #1741

2 votes, 5 watchers

[spring-projects-issues]

Alex Rader commented

In Kotlin @RequestBody required=false when use default arguments
spring-projects/spring-boot#12691

[spring-projects-issues]

Sébastien Deleuze commented

As discussed in the PR, current Kotlin API force us to integrate that as a rather involved and not very elegant way, I will raise and discuss that point with Kotlin team. As a consequence, I think it is more reasonable to target 5.2 for that one.

[AlexTrotsenko]

Is there any updates on this issue?

[cmdjulian]

I'm also interested in this feature.

[jlous]

The pull-request was closed in August 2020.

[jlous]

Resolving the value in Spring (like a defaultValue) is not feasible, because Kotlin's default is not a simple value but a piece of essentially arbitrary object-internal kotlin code, which Spring has no access to.

An alternative approach would be to use @JvmOverloads to have kotlin generate explicit permutations of the method signature. Spring would need to select the right one depending on which params are present, in stead of rejecting them all as ambiguous endpoint mappings like it does now. One would still need to define a sane course of action if that annotation is missing. Bad request?

In any case: the situation today is that a kotlin endpoint with a default value will run without incident right up until a request without the parameter arrives, in which case it will just throw an NPE from way inside Spring's parameter mapping. That has got to be the wrong approach whichever way you look at it.