in mysql-connector-java-8.0.18 ,the GENERATED_KEY will be always set to MysqlType.BIGINT_UNSIGNED
in file mysql/mysql-connector-java/8.0.18/mysql-connector-java-8.0.18-sources.jar!/com/mysql/cj/jdbc/StatementImpl.java:1414
fields[0] = new Field("", "GENERATED_KEY", collationIndex, encoding, MysqlType.BIGINT_UNSIGNED, 20);