AOP, Proxy ve Self-Invocation Tuzağı

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@Target(ElementType.METHOD)
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@Retention(RetentionPolicy.RUNTIME)
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public @interface Measured {
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    /** Metric name, e.g. "loan.scoring". */
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    String value();
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}

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@Aspect
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@Component
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class MeasuredAspect {
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    private final MeterRegistry meters;
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    MeasuredAspect(MeterRegistry meters) {
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        this.meters = meters;
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    }
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    // Runs only when the call enters through the proxy: a call from another bean.
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    // Private, final and self-invoked methods never reach this advice.
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    @Around("@annotation(measured)")
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    Object time(ProceedingJoinPoint call, Measured measured) throws Throwable {
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        Timer.Sample sample = Timer.start(meters);
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        try {
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            return call.proceed();
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        } finally {
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            sample.stop(meters.timer(measured.value()));
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        }
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    }
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}

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// BEFORE: looks right, measures nothing. The dashboard shows no scoring latency,
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// so nobody notices when the credit bureau call slows down.
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@Service
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class LoanApplicationServiceBefore {
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    private final CreditBureauClient bureau;
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    LoanApplicationServiceBefore(CreditBureauClient bureau) {
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        this.bureau = bureau;
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    }
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    public Decision apply(LoanApplication application) {
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        CreditScore score = this.score(application);   // `this` is the raw object, not the proxy
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        return Decision.of(application, score);
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    }
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    @Measured("loan.scoring")
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    @Transactional(readOnly = true)                    // skipped for the same reason
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    public CreditScore score(LoanApplication application) {
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        return bureau.fetchScore(application.nationalId());
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    }
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}

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// AFTER: the measured work lives in its own bean.
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@Service
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class LoanApplicationService {
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    private final LoanApplicationRepository applications;
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    private final CreditScoring scoring;     // injected: this reference IS the proxy
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    LoanApplicationService(LoanApplicationRepository applications, CreditScoring scoring) {
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        this.applications = applications;
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        this.scoring = scoring;
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    }
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    public Decision apply(long applicationId) {
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        LoanApplication application = applications.findById(applicationId).orElseThrow();
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        CreditScore score = scoring.score(application);   // through the proxy: timed
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        return Decision.of(application, score);
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    }
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}
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@Component
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class CreditScoring {
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    private final CreditBureauClient bureau;
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    CreditScoring(CreditBureauClient bureau) {
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        this.bureau = bureau;
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    }
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    @Measured("loan.scoring")
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    public CreditScore score(LoanApplication application) {
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        return bureau.fetchScore(application.nationalId());
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    }
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}

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@SpringBootTest
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class CreditScoringProxyTest {
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    @Autowired CreditScoring scoring;
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    @Autowired LoanApplicationService service;
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    @Autowired MeterRegistry meters;
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    @MockitoBean LoanApplicationRepository applications;
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    @MockitoBean CreditBureauClient bureau;     // never call the real bureau from a test
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    @Test
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    void theInjectedBeanIsAProxy() {
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        assertThat(AopUtils.isAopProxy(scoring)).isTrue();   // CGLIB subclass: CreditScoring$$SpringCGLIB$$0
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    }
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    @Test
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    void scoringIsTimed() {
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        given(applications.findById(1L)).willReturn(Optional.of(LoanApplications.sample()));
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        given(bureau.fetchScore(any())).willReturn(new CreditScore(1450));
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        service.apply(1L);
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        assertThat(meters.timer("loan.scoring").count()).isEqualTo(1);
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    }
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}