并发预订是指多个用户同时尝试预订同一资源，如酒店房间、航班座位等。为了解决并发预订的问题，可以使用以下方法之一：

1. 悲观锁：使用数据库的锁机制，确保同时只有一个用户可以访问和修改资源。这可以通过数据库的事务来实现。

// 使用悲观锁实现并发预订
public void bookRoom(String roomId, String userId) {
    // 获取数据库连接
    Connection connection = getConnection();
    try {
        // 开始事务
        connection.setAutoCommit(false);
        
        // 加锁，阻塞其他用户预订该房间
        String sql = "SELECT * FROM rooms WHERE room_id = ? FOR UPDATE";
        PreparedStatement statement = connection.prepareStatement(sql);
        statement.setString(1, roomId);
        statement.executeQuery();
        
        // 检查房间是否已被预订
        String checkSql = "SELECT * FROM bookings WHERE room_id = ? AND status = 'BOOKED'";
        PreparedStatement checkStatement = connection.prepareStatement(checkSql);
        checkStatement.setString(1, roomId);
        ResultSet resultSet = checkStatement.executeQuery();
        
        if (resultSet.next()) {
            // 房间已被预订
            throw new Exception("该房间已被预订");
        } else {
            // 预订房间
            String insertSql = "INSERT INTO bookings (room_id, user_id, status) VALUES (?, ?, 'BOOKED')";
            PreparedStatement insertStatement = connection.prepareStatement(insertSql);
            insertStatement.setString(1, roomId);
            insertStatement.setString(2, userId);
            insertStatement.executeUpdate();
            
            // 提交事务
            connection.commit();
        }
    } catch (Exception e) {
        // 发生异常时回滚事务
        connection.rollback();
        throw e;
    } finally {
        // 关闭数据库连接
        connection.close();
    }
}

2. 乐观锁：通过版本控制来避免并发冲突。每个资源都有一个版本号，当多个用户尝试同时修改资源时，只有一个用户能够成功。其他用户需要重新获取最新的版本，并重新尝试修改。

// 使用乐观锁实现并发预订
public void bookRoom(String roomId, String userId) {
    // 获取数据库连接
    Connection connection = getConnection();
    try {
        // 开始事务
        connection.setAutoCommit(false);
        
        // 查询房间信息
        String selectSql = "SELECT * FROM rooms WHERE room_id = ?";
        PreparedStatement selectStatement = connection.prepareStatement(selectSql);
        selectStatement.setString(1, roomId);
        ResultSet resultSet = selectStatement.executeQuery();
        
        if (resultSet.next()) {
            int version = resultSet.getInt("version");
            
            // 检查房间是否已被预订
            String checkSql = "SELECT * FROM bookings WHERE room_id = ? AND status = 'BOOKED'";
            PreparedStatement checkStatement = connection.prepareStatement(checkSql);
            checkStatement.setString(1, roomId);
            ResultSet checkResultSet = checkStatement.executeQuery();
            
            if (checkResultSet.next()) {
                // 房间已被预订
                throw new Exception("该房间已被预订");
            } else {
                // 更新房间预订信息
                String updateSql = "UPDATE rooms SET version = ?, status = 'BOOKED' WHERE room_id = ? AND version = ?";
                PreparedStatement updateStatement = connection.prepareStatement(updateSql);
                updateStatement.setInt(1, version + 1);
                updateStatement.setString(2, roomId);
                updateStatement.setInt(3, version);
                int affectedRows = updateStatement.executeUpdate();
                
                if (affectedRows == 0) {
                    // 更新失败，说明房间版本已变化，需要重新获取最新版本并尝试预订
                    throw new Exception("房间已被其他用户预订");
                } else {
                    // 预订成功
                    String insertSql = "INSERT INTO bookings (room_id, user_id, status) VALUES (?, ?, 'BOOKED')";
                    PreparedStatement insertStatement = connection.prepareStatement(insertSql);
                    insertStatement.setString(1, roomId);
                    insertStatement.setString(2, userId);
                    insertStatement.executeUpdate();
                    
                    // 提交事务
                    connection.commit();
                }
            }
        } else {