Davranışsal Kalıplar — State, Chain of Responsibility, Command

1
// One link: decide, or hand the request to the rest of the chain.
2
public interface TransferCheck {
3

4
    Decision check(TransferRequest request, TransferCheckChain next);
5

6
    sealed interface Decision {
7
        record Approved() implements Decision {}
8
        record Rejected(String reason) implements Decision {}
9
    }
10
}

1
@Component
2
class DailyLimitCheck implements TransferCheck {
3

4
    private static final Currency TRY = Currency.getInstance("TRY");
5

6
    private final DailyUsage usage;
7
    private final Money dailyLimit;
8

9
    DailyLimitCheck(DailyUsage usage, @Value("${transfers.fast.daily-limit-try}") BigDecimal limit) {
10
        this.usage = usage;
11
        this.dailyLimit = new Money(limit, TRY);   // FAST moves TRY only
12
    }
13

14
    @Override
15
    public Decision check(TransferRequest request, TransferCheckChain next) {
16
        Money afterThis = usage.sentToday(request.customerNo()).plus(request.amount());
17
        if (afterThis.isGreaterThan(dailyLimit)) {
18
            return new Decision.Rejected("daily FAST limit exceeded");
19
        }
20
        return next.proceed(request);
21
    }
22
}

1
// Immutable: every link receives a chain that starts one step further on.
2
public final class TransferCheckChain {
3

4
    private final List<TransferCheck> checks;
5
    private final int position;
6

7
    public TransferCheckChain(List<TransferCheck> checks) {
8
        this(List.copyOf(checks), 0);
9
    }
10

11
    private TransferCheckChain(List<TransferCheck> checks, int position) {
12
        this.checks = checks;
13
        this.position = position;
14
    }
15

16
    public TransferCheck.Decision proceed(TransferRequest request) {
17
        if (position == checks.size()) {
18
            // The end is explicit: a request that passed every link is approved.
19
            return new TransferCheck.Decision.Approved();
20
        }
21
        return checks.get(position).check(request, new TransferCheckChain(checks, position + 1));
22
    }
23
}

1
@Configuration
2
class TransferChecksConfig {
3

4
    // The order is a decision, written down once. Sanctions screening goes first:
5
    // a listed beneficiary must be caught and reported whatever the amount,
6
    // not turned away early as "over the limit".
7
    @Bean
8
    TransferCheckChain fastTransferChecks(SanctionsScreeningCheck sanctions,
9
                                          DailyLimitCheck dailyLimit,
10
                                          FraudScoreCheck fraud) {
11
        return new TransferCheckChain(List.of(sanctions, dailyLimit, fraud));
12
    }
13
}