Regarding the issue of MappingInstantiationException caused by using r2dbcEntityTemplate in Kotlin, and the support for Unsigned types by CustomConverter.

[yeyonggui]

package com.example.demo

import kotlinx.coroutines.flow.toList
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
import org.springframework.context.annotation.Bean
import org.springframework.context.annotation.Configuration
import org.springframework.core.convert.converter.Converter
import org.springframework.data.annotation.Id
import org.springframework.data.convert.ReadingConverter
import org.springframework.data.r2dbc.convert.R2dbcCustomConversions
import org.springframework.data.r2dbc.core.*
import org.springframework.data.r2dbc.dialect.MySqlDialect
import org.springframework.data.relational.core.mapping.Column
import org.springframework.data.relational.core.mapping.Table
import org.springframework.stereotype.Component
import org.springframework.web.reactive.function.server.ServerResponse
import org.springframework.web.reactive.function.server.bodyValueAndAwait
import org.springframework.web.reactive.function.server.coRouter
import org.springframework.web.reactive.function.server.json


@SpringBootApplication
class Demo1Application

fun main(args: Array<String>) {
    runApplication<Demo1Application>(*args)
}

@Component
class TestController {
    @Bean
    fun testRouter(
        r2dbcEntityTemplate: R2dbcEntityTemplate
    ) = coRouter {
        GET("read") {
            val result = r2dbcEntityTemplate.select<Simple>().flow().toList()
            ServerResponse.ok().json().bodyValueAndAwait(result)
        }
    }
}




@Table(name = "customer")
data class Simple(
    @Id
    @Column("id")
    val id: UInt,
    @Column("name")
    val name: String,
    @Column("age")
    val age: UInt,
)

@Table(name = "customer")
data class PrivateConstructor private constructor(
    @Id
    @Column("id")
    val id: UInt,
    @Column("name")
    val name: String,
    @Column("age")
    val age: UInt,
)

@Table(name = "customer")
data class DefaultId private constructor(
    @Id
    @Column("id")
    val id: UInt=0u,
    @Column("name")
    val name: String,
    @Column("age")
    val age: UInt,
)

@ReadingConverter
object LongToUInt : Converter<Long, UInt> {
    override fun convert(source: Long): UInt {
        println("LongToUInt $source")
        return source.toUInt()
    }
}
@ReadingConverter
object LongToInt : Converter<Long, Int> {
    override fun convert(source: Long): Int {
        println("LongToInt $source")
        return source.toInt()
    }
}
@Configuration
class Config{
    @Bean
    fun customConversions(): R2dbcCustomConversions =
        R2dbcCustomConversions.of(MySqlDialect.INSTANCE, listOf(LongToUInt,LongToInt))
}

CREATE TABLE `customer` (
  `id` int unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(255) COLLATE utf8mb4_general_ci NOT NULL,
  `age` int unsigned DEFAULT '0',
  PRIMARY KEY (`id`),
  UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=55 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;

mysql 8.0.24
kotlin 2.0.20
spring-boot 3.3.3

1.When using r2dbcEntityTemplate to query the database, if the Entity constructor is public, everything runs normally regardless of whether the attribute corresponding to the primary Key has a default value. But when the Entity of the constructor is private, if primary key corresponding attributes don't declare a default value, will lead to MappingInstantiationException.

2.Hope ReadingConverter, WriteConverter can support kotlin unsigned type, code only LongToInt will work, but LongToUInt will not.