try(Connection conn = connPool.getConnection()) {
    try { 
        conn.setAutoCommit(false);
        conn.setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);
        // do my things
        conn.commit();
        System.out.println("commit");
    } catch (Exception e) {
         e.printStackTrace();
         conn.rollback(); 
         throw e; 
    }
}
System.out.println("conn closed");

commit
2021-10-21 11:56:01,577 WARN  [AgroalEventLoggingListener.java:76] (executor-thread-0) Datasource '<default>': Transaction isolation level -1 not supported.
conn closed