This requieres FileCreator.java in order to do something

FileController.java

 @Controller
public class ArchivoApiController { 
  
  @Autowired
  private FileCreator fileCreator;
  
  @GetMapping("/report")
  public ResponseEntity<byte[]> generateReport(String filename) {
        HttpHeaders header = new HttpHeaders();
        header.setContentType("application/octet-stream");
        header.setContentLength(bytes.length);
        header.set(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + (filename + ".xlsx") + "\"");
        byte[] file = genericCall();
        return new ResponseEntity<byte[]>(file, headers, HttpStatus.OK);
    }

    private byte[] genericCall() {
        XSSFSheet hoja = fileCreator.aperturarWorkbook("hoja1");
        // le agregas filas y columnas a la "hoja" 
        return fileCreator.crearExcel(hoja.getWorkbook()); // pasas el worbook de "la hoja"
    }
  
}