NotFoundHandler.java

@ControllerAdvice
public class NotFoundHandler {
  @Value("${spa.default-file}")
  String defaultFile;

  @ExceptionHandler(NoHandlerFoundException.class)
  public ResponseEntity<String> renderDefaultPage() {
    try {
      File indexFile = ResourceUtils.getFile(defaultFile);
      FileInputStream inputStream = new FileInputStream(indexFile);
      String body = StreamUtils.copyToString(inputStream, Charset.defaultCharset());
      return ResponseEntity.ok().contentType(MediaType.TEXT_HTML).body(body);
    } catch (IOException e) {
      e.printStackTrace();
      return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body("There was an error completing the action.");
    }
  }
}