I was thinking more about @RequestParam validation eg.

    fun calc1(@RequestParam @NotNull @Size(max=10) @Pattern(regexp = "^\\d{4}-\\d{1,2}-\\d{1,2}$") date:String,
              @RequestParam @NotNull @Size(max=5) @Pattern(regexp = "^\\d{2}:\\d{2}$") time:String,
              @RequestParam @NotNull @Size(max=40) @Pattern(regexp = "^[-_'A-Za-z/\\d]{2,50}$") zone:String,
             ): Map<String, Any> {

Why stuff everything in the param list? No other framework I can think of proliferates annotations like this. Spring annotations being Java-based means I must suffer Java's ridiculous inability to handle regex metacharacters - even after introducing raw string literals in Java 13. Kotlin handles this perfectly but once I'm in Spring annotation-land Spring's touted Kotlin compatibility goes out the window.