Exception Yönetimi — Hata Yukarı Çıkarken Ne Kaybolur

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// abstract + sealed: a switch over it needs no default, the compiler knows every case.
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public abstract sealed class CollectionException extends RuntimeException
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        permits CollectionException.InstallmentNotFound, CollectionException.CoreBankingUnavailable {
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    CollectionException(String message, Throwable cause) {
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        super(message, cause);
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    }
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    public static final class InstallmentNotFound extends CollectionException {
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        public InstallmentNotFound(long installmentId) {
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            super("installment " + installmentId + " not found", null);
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        }
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    }
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    public static final class CoreBankingUnavailable extends CollectionException {
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        public CoreBankingUnavailable(String system, Throwable cause) {
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            super("core banking " + system + " unavailable", cause);   // keep the cause
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        }
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    }
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}

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public class CoreBankingClient {
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    private final HttpClient http = HttpClient.newHttpClient();
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    // Debits the customer's account for one loan installment.
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    public String debit(long installmentId, String iban, BigDecimal amount) {
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        var request = HttpRequest.newBuilder(URI.create("https://corebanking.internal/debits"))
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                .header("Idempotency-Key", "installment-" + installmentId)   // a retry never debits twice
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                .POST(HttpRequest.BodyPublishers.ofString("{\"iban\":\"" + iban + "\",\"amount\":\"" + amount + "\"}"))
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                .build();
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        try {
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            return http.send(request, HttpResponse.BodyHandlers.ofString()).body();
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        } catch (IOException ex) {
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            // Wrap, never swallow: the stack trace will show both layers.
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            throw new CollectionException.CoreBankingUnavailable("corebanking.internal", ex);
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        } catch (InterruptedException ex) {
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            Thread.currentThread().interrupt();       // restore the flag before leaving
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            throw new CollectionException.CoreBankingUnavailable("corebanking.internal", ex);
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        }
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    }
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}

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// The end-of-day collection report for the loans department.
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public class CollectionReport {
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    // Both resources are closed in reverse order on every path.
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    // If writing fails AND closing fails, the close error is attached as "suppressed".
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    public void export(Path target, List<String> lines) throws IOException {
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        try (var out = Files.newBufferedWriter(target);
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             var audit = new AuditLog(target.resolveSibling("collection-audit.log"))) {   // AuditLog implements AutoCloseable
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            for (String line : lines) {
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                out.write(line);
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                out.newLine();
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            }
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            audit.record("exported " + lines.size() + " lines");
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        }
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    }
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}

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public class Cli {
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    private static final System.Logger log = System.getLogger("collection");
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    public static void main(String[] args) {
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        int exitCode = run(args);
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        System.exit(exitCode);
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    }
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    // The one place where exceptions become user-facing results.
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    static int run(String[] args) {
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        try {
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            new CollectionService(new CoreBankingClient()).collect(Long.parseLong(args[0]));
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            return 0;
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        } catch (CollectionException ex) {
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            log.log(System.Logger.Level.ERROR, "collection failed", ex);   // full chain goes to the log
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            System.err.println(switch (ex) {
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                case CollectionException.InstallmentNotFound e -> "Taksit bulunamadı.";
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                case CollectionException.CoreBankingUnavailable e -> "Ana bankacılık sistemine ulaşılamadı; tahsilat sonra tekrar denenecek.";
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            });
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            return 2;
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        }
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    }
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}

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class ExceptionChainTest {
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    @Test
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    void providerFailureKeepsTheOriginalCause() {
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        var io = new IOException("connection reset");
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        var wrapped = new CollectionException.CoreBankingUnavailable("corebanking.internal", io);
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        assertThat(wrapped).hasCause(io);
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        assertThat(wrapped).hasRootCauseInstanceOf(IOException.class);
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    }
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    @Test
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    void closeFailureIsSuppressedNotLost() {
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        var writeError = new IllegalStateException("write failed");
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        var closeError = new IllegalStateException("close failed");
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        Throwable thrown = catchThrowable(() -> {
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            try (AutoCloseable resource = () -> { throw closeError; }) {
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                throw writeError;
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            }
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        });
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        assertThat(thrown).isSameAs(writeError);                 // the primary error wins
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        assertThat(thrown.getSuppressed()).containsExactly(closeError);   // the other is kept
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    }
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}