bug: reject non-positive amounts in deposit/withdraw/transfer

[devin-ai-integration]

Summary

AccountService.withdraw only guarded against insufficient funds, never against a negative amount. balance.compareTo(-1000000) < 0 is false for any non-negative balance, so the guard passed and balance.subtract(negative) increased the balance — an authenticated user could mint unlimited funds via POST /withdraw?amount=-1000000 (and then spend them through legitimate transfers). deposit and transferAmount had the same missing validation; a negative transfer would pull money out of the recipient's account.

Fix: a single validateAmount guard runs before any balance mutation in all three service methods.

private static void validateAmount(BigDecimal amount) {
    if (amount == null || amount.compareTo(BigDecimal.ZERO) <= 0) {
        throw new RuntimeException("Amount must be greater than zero");
    }
}

RuntimeException matches the existing convention in this service/controller pair. BankController.deposit had no try/catch (unlike withdraw/transfer), so a rejected amount would have surfaced as a 500 — it now renders the dashboard with the error attribute like the other two endpoints.

New AccountServiceTest is a plain Mockito unit test (no Spring context — the existing BankappApplicationTests requires a real MySQL) covering negative/zero/null rejection with balance unchanged and no repository interaction, plus the happy paths.

Note: this environment has no cached Maven artifacts and Maven Central is unreachable from it, so ./mvnw compile/test could not be run locally; CI is authoritative.

Written by Devin

Devin-Org: engineering