Java ATM CLI Dev Log #2：转账，卡住了？

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Narrative

我从没想到构建这个功能会如此疯狂。开发过程中，一切都很顺利——直到函数在执行时开始挂起。

我使用 ChatGPT 和 Gemini 进行深入调查，以找出问题根源。在此过程中，我了解了 COMMIT、ROLLBACK，以及事务在数据库中为何至关重要——尤其是对我正在构建的金融应用而言。

事务 是一系列被视为单一逻辑单元的 SQL 语句。事务以数据更改操作开始，并以 COMMIT（成功）或 ROLLBACK（错误）结束。

初始实现

AccountService.java

public static void transferCash(Account sender, Account receiver, double amount) {
    if (sender.getBalance() > amount) {
        double newSenderBalance   = sender.getBalance() - amount;
        double newReceiverBalance = receiver.getBalance() + amount;

        boolean isTransactionSuccessful = AccountDAO.makeTransfer(
                sender.getId(),
                receiver.getId(),
                sender.getAccountType(),
                receiver.getAccountType(),
                newSenderBalance,
                newReceiverBalance
        );

        if (isTransactionSuccessful) {
            System.out.println("Transaction Successful!");
            sender.setBalance(newSenderBalance);
            System.out.println("Withdrawal successful. New balance: $" + sender.getBalance());
            sender.stringifyAccount();
        } else {
            System.out.println("Transaction Failed!");
        }
    } else {
        System.out.println("Insufficient funds.");
    }
}

AccountDAO.java

public static boolean makeTransfer(
        int senderId,
        int receiverId,
        String senderAccountType,
        String receiverAccountType,
        double newSenderBalance,
        double newReceiverBalance) {

    String senderQuery   = "UPDATE accounts SET balance = ? WHERE customerId = ? AND accountType = ?";
    String receiverQuery = "UPDATE accounts SET balance = ? WHERE customerId = ? AND accountType = ?";
    boolean isTransactionSuccessful = false;

    try (Connection conn = DBHelper.getConnection()) {
        conn.setAutoCommit(false);

        // ---- Sender ----
        try (PreparedStatement senderStmt = conn.prepareStatement(senderQuery)) {
            senderStmt.setDouble(1, newSenderBalance);
            senderStmt.setInt(2, senderId);
            senderStmt.setString(3, senderAccountType);
            int senderRowsAffected = senderStmt.executeUpdate();

            if (senderRowsAffected == 0) {
                conn.rollback();
                return false;
            }
        } catch (SQLException e) {
            e.printStackTrace();
            conn.rollback();
            return false;
        }

        // ---- Receiver ----
        try (PreparedStatement receiverStmt = conn.prepareStatement(receiverQuery)) {
            receiverStmt.setDouble(1, newReceiverBalance);
            receiverStmt.setInt(2, receiverId);
            receiverStmt.setString(3, receiverAccountType);
            int receiverRowsAffected = receiverStmt.executeUpdate();

            if (receiverRowsAffected == 0) {
                conn.rollback();
                return false;
            }
        } catch (SQLException e) {
            e.printStackTrace();
            conn.rollback();
            return false;
        }

        conn.commit();
        System.out.println("\nBalance Updated Successfully!");
        isTransactionSuccessful = true;

    } catch (Exception e) {
        e.printStackTrace();
        isTransactionSuccessful = false;
    }

    return isTransactionSuccessful;
}

Source: …

重构后根据高级工程师反馈

一位拥有银行 Java 经验的高级工程师指出，之前的做法是 不良实践。
服务层已拆分为两个更小、单一职责的操作：借记（debit） 和 贷记（credit）。

更新后的 AccountService.java

public class AccountService {

    public static void debitAccount(Account account, double amount) {
        if (account.getBalance() > amount) {
            double newBalance = account.getBalance() - amount;

            boolean isTransactionSuccessful = AccountDAO.changeBalance(
                    account.getId(),
                    account.getAccountType(),
                    newBalance
            );

            if (isTransactionSuccessful) {
                System.out.println("Transaction Successful!");
                account.setBalance(newBalance);
                System.out.println("Withdrawal successful. New balance: $" + account.getBalance());
                account.stringifyAccount();
            } else {
                System.out.println("Transaction Failed!");
            }
        } else {
            System.out.println("Insufficient funds.");
        }
    }

    public static void creditAccount(Account account, double amount) {
        double newBalance = account.getBalance() + amount;

        boolean isTransactionSuccessful = AccountDAO.changeBalance(
                account.getId(),
                account.getAccountType(),
                newBalance
        );

        if (isTransactionSuccessful) {
            System.out.println("Transaction Successful!");
            account.setBalance(newBalance);
            System.out.println("Deposit successful. New balance: $" + account.getBalance());
            account.stringifyAccount();
        } else {
            System.out.println("Transaction Failed!");
        }
    }
}

注意： AccountDAO.changeBalance 是一个新辅助方法，用于更新单个账户的余额。
现在每一次数据库操作都是原子性的，调用方可以在适当的事务范围内编排完整的转账流程（借记 → 贷记）。

当前状态

在完成所有这些更改后，我决定暂停此功能的开发，先完成应用程序的其余部分，然后再回来正确地修复转移逻辑。

随后，我应该已经掌握了在 Java 中正确处理事务的方法。

如果你想查看 GitHub 仓库，可以点击这里自行查看。

好了，暂时就这些。

下次提交再见 👋