// 에러 발생
// BookAlreadyExistsException.java
package com.polarbookshop.catalogservice.domain;
 
public class BookAlreadyExistsException extends RuntimeException {
 
    public BookAlreadyExistsException(String isbn) {
        super("A book with ISBN " + isbn + " already exists.");
    }
 
}
 
 
// 에러 처리
// BookControllerAdvice.java
@RestControllerAdvice
public class BookControllerAdvice {
 
    @ExceptionHandler(BookNotFoundException.class)
    @ResponseStatus(HttpStatus.NOT_FOUND)
    String bookNotFoundHandler(BookNotFoundException ex) {
        return ex.getMessage();
    }
    ...