请教一个关于数据库事务的问题

两个线程同时查一行记录，然后更新记录。

class MyRunner implements Runnable {
        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName() + ":连接数据库...");
            Connection connection = null;
            try {
                connection = DriverManager.getConnection(RepeatableRead.DB_URL, RepeatableRead.USER, RepeatableRead.PASS);
                connection.setAutoCommit(false);
                connection.setTransactionIsolation(Connection.TRANSACTION_REPEATABLE_READ);
                String querySql = "select * from account where id=?";

                PreparedStatement queryStmt = connection.prepareStatement(querySql);
                queryStmt.setInt(1, 1);
                ResultSet set = queryStmt.executeQuery();
                set.next();
                int amount = set.getInt("amount");
                System.out.println(Thread.currentThread().getName() + ":Amount=" + amount);
                Thread.sleep(1000);
                // 读取了数据，未提交事务。
                synchronized (this) {
                    String updateSql = "update account set amount=? where id=1";
                    PreparedStatement updateStmt = connection.prepareStatement(updateSql);
                    updateStmt.setInt(1, amount + 100);
                    int count = updateStmt.executeUpdate();
                    if (count > 0) {
                        set = queryStmt.executeQuery();
                        set.next();
                        amount = set.getInt("amount");
                        System.out.println(Thread.currentThread().getName() + ":更新成功...|" + amount);
                        connection.commit();
                    }
                }
            } catch (SQLException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    @Test
    public void testMyRunner() throws ClassNotFoundException, SQLException, InterruptedException {
        MyRunner myRunner = new MyRunner();
        Thread t1 = new Thread(myRunner, "T1");
        Thread t2 = new Thread(myRunner, "T2");
        t1.start();
        t2.start();
        t1.join();
        t2.join();
    }

/* 
输出：
T1:连接数据库...
T2:连接数据库...
T2:Amount=277
T1:Amount=277
T2:更新成功...|377
T1:更新成功...|277

T1 为什么还是 277 ？
*/

由于隔离级别是 RR，T1 不应该也输出 377 吗？

另外要怎么才能保证最后 amount 得到正确的结果 447 呢？