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MID-009

الأنواع العامة المتقدمة والانعكاس

  • 6 درس
  • 2.5 ساعة تدريب (تقدير)
ابدأ الوحدة
في هذه الصفحة 8 موضوعات

نص الدروس والتمارين باللغة الإنجليزية، بينما تبقى واجهة الموقع وعناوين الدروس بالعربية.

نظرة عامة

الموضوعات

  • محو الأنواع (Type Erasure)
  • الانعكاس (Reflection)
  • التعليقات التوضيحية (Annotations)
  • الوكلاء الديناميكيون (Dynamic Proxies)

تعلّم أولًا

2.5 ساعة تدريب (تقدير)

مخرجات التعلم

  1. Explain type erasure and its consequences (no new T(), bridge methods, heap pollution).
  2. Apply PECS to wildcard signatures.
  3. Inspect classes, methods, and annotations with reflection and write a custom annotation processed at runtime.
  4. Explain how Spring uses reflection and dynamic proxies (and why @Transactional self-invocation fails).

الدروس

  1. 01محو الأنواع
  2. 02الرموز البديلة وقاعدة PECS
  3. 03الانعكاس
  4. 04تعليقات توضيحية مخصصة
  5. 05الوكلاء الديناميكيون
  6. 06عرض عملي: بناء @Timed مصغّر باستخدام وكيل

المختبر والتمارين

Lab

MID-009-LAB1 · Audit annotation

Outcomes: 3–4 (with 1–2 in exercises) · Time: ~2.5 h Create an @Audited annotation and a proxy-based interceptor that logs calls to annotated methods (method name, arguments, duration). Apply it to a Shop service interface. Demonstrate the self-invocation limitation. Acceptance: works through an interface; self-invocation limitation demonstrated.

Exercises (assessment: exercises plus a proxy-behavior quiz)

IDOutcomeTask
MID-009-EX011Show that new ArrayList<String>().getClass() == new ArrayList<Integer>().getClass(); explain, then use a raw list to cause heap pollution and observe where the exception appears.
MID-009-EX021Write <T> T create(Class<T>) as the replacement for new T().
MID-009-EX032Write addAll(Collection<? extends T> src, Collection<? super T> dst); explain PECS with Integer, Number, Object.
MID-009-EX043Use reflection to list all methods annotated with a custom @Audited on a class.
MID-009-EX054Explain why @Transactional on b() does not work when called from a() in the same class, and give two fixes.
MID-009-EX063Time 1,000,000 direct vs reflective calls; report the difference and one way to reduce it.
أظهر الحلول
  • EX01 Both classes are ArrayList (erased). Adding an Integer through a raw reference compiles with a warning; ClassCastException appears later when a String is read.
  • EX02 return type.getDeclaredConstructor().newInstance();
  • EX03 Collection<Integer> can be copied into Collection<Number> or Collection<Object>: source extends, destination super.
  • EX04 Arrays.stream(c.getDeclaredMethods()).filter(m -> m.isAnnotationPresent(Audited.class)) with RUNTIME retention.
  • EX05 The call is on this, bypassing the Spring proxy. Move b() to another bean, or inject the proxy (self-injection with care) or restructure.
  • EX06 Reflection is slower, mostly warm-up-dependent; cache Method objects or use MethodHandle/direct calls.

الاختبار

Outcomes: O1 erasure · O2 PECS · O3 reflection and annotations · O4 proxies and Spring.

اختبار ذاتي لك: لا توجد درجات، ولا يُحفظ أو يُرسل أي شيء. اختر إجابة ثم تحقق منها، ويمكنك المحاولة مرة أخرى كما تشاء.

  1. 1At runtime List<String> is seen as:O1
  2. 2Which does not compile?O1
  3. 3Heap pollution typically shows up as:O1
  4. 4For a source you only read from, use:O2
  5. 5For a destination you add T to, use:O2
  6. 6An annotation readable by reflection needs:O3
  7. 7A fair concern about reflection is:O3
  8. 8A JDK dynamic proxy requires the target to implement:O4
  9. 9this.save() inside a Spring bean with @Transactional on save:O4
  10. 10Which gives annotation-driven behavior around calls in Spring?O4

الجزء العملي

Six exercises MID-009-EX01 to EX06 plus the lab. Pass: at least five exercises correct, at least eight of the ten quiz questions right, and the lab showing the self-invocation limitation.

أخطاء شائعة

Reflection in hot paths; raw types from erasure confusion.

التقييم

Exercises plus a proxy-behavior quiz.

الصلة بالمسار الوظيفي

Reading framework internals when debugging.

اكتب للبحث في المستويات والمراحل والوحدات والدروس.