Spring Spring MVC: Unexpected bytes added to the HTTP response for kotlin.Unit return type in controller methods

Affects: Spring Boot 3.2.0-RC2, Spring Web 3.2.0-RC2, Kotlin 1.9.20

Issue

Using a Controller, with return type kotlin.Unit, which uses directly HttpServletResponse to write the response, adds unexpected bytes to the response. The following is a controller example where a file is read and written to the output directly:

    @GetMapping(path = ["public/file.txt"])
    fun getFile(
        httpServletResponse: HttpServletResponse
    ) {
        httpServletResponse.status = 200

        httpServletResponse.outputStream.write(Files.readAllBytes(Path.of("src/main/resources/file.txt")))
    }

The following shows the bytes expected vs current:

Current: [49, 50, 51, 52, 53, 54, 55, 56, 57, 123, 125]
Expected: [49, 50, 51, 52, 53, 54, 55, 56, 57]

123 and 125 bytes are { and }

How to reproduce the issue

This is a Spring Boot project with a test that reproduces the issue.

Debugging

My guess would be: there is a new converter which is taking kotlin.Unit and tries to serialize it, outputting { and }.

Comment From: quaff

It should be fixed at framework not boot side, I've created https://github.com/spring-projects/spring-framework/pull/31647 to handle this.

Comment From: quaff

@estigma88 You can add suspend modifier to the method as workaround, don't forget adding dependency org.jetbrains.kotlinx:kotlinx-coroutines-reactor

Comment From: estigma88

@quaff Thanks for the suggestion, but does that work with virtual threads? We are using virtual threads heavily in the project

Comment From: quaff

@quaff Thanks for the suggestion, but does that work with virtual threads? We are using virtual threads heavily in the project

I'm not expert of kotlin, but I don't think it will conflict with virtual threads.

Comment From: sdeleuze

Thanks for catching this regression, this should now be fixed.

Comment From: estigma88

I confirm it is working with Spring Boot 3.2.0