app.groovy

import groovy.transform.Canonical
import org.springframework.http.converter.json.*
import com.fasterxml.jackson.databind.*

@Canonical
class Bar {
    String fooBar
}

@RestController
@RequestMapping('/foo')
class Foo {
    @PutMapping(consumes = 'application/json', produces = 'application/json')
    def put(@RequestBody Bar bar) {
        println(bar)
        bar
    }
}

@Configuration
class Config {

    @Bean
    ObjectMapper objectMapper() {
        def mapper = new ObjectMapper()
        mapper.propertyNamingStrategy = PropertyNamingStrategy.SNAKE_CASE
        mapper
    }
}

run.sh

#!/bin/env bash

sdk i springboot 2.1.4.RELEASE

spring run app.groovy