Reflection & Annotations — Interview Questions (72+)

See `pkg17metaprogramming`.

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Detailed Questions

1. What is reflection?

• Short: Inspect and invoke classes/methods/fields at runtime via java.lang.reflect.
• Detailed: Class.forName, getDeclaredMethods, setAccessible(true), Method.invoke. Used by frameworks (Spring, Hibernate, Jackson) for dependency injection, ORM mapping, JSON binding. Bypasses compile-time checks.
• Example: obj.getClass().getMethod("foo", int.class).invoke(obj, 42);

2. How do you obtain a `Class` object?

• Short: .class, obj.getClass(), Class.forName(name).
• Detailed: Primitives have int.class. forName loads class (runs static init). Class objects are singletons per loader.
• Example: String.class == "hi".getClass().

3. getMethod vs getDeclaredMethod?

• Short: getMethod includes inherited public; getDeclared all declared in class (any visibility).
• Detailed: For private methods use getDeclaredMethod + setAccessible(true). getMethods returns only public including inherited.
• Example: Access private field in test via reflection (careful in production).

4. setAccessible and module system?

• Short: Java 9+ strong encapsulation; --add-opens may be required for JDK internals.
• Detailed: Deep reflection on non-exported packages illegal by default. Frameworks use opens in module-info or JVM flags. Prefer public API / MethodHandles over breaking encapsulation.
• Example: Hibernate opens entity packages for bytecode access.

5. Annotations — retention and target?

• Short: @Retention(RUNTIME|CLASS|SOURCE) and @Target (TYPE, METHOD, FIELD…).
• Detailed: SOURCE = compile-time only (e.g. @Override). CLASS = bytecode, not runtime. RUNTIME = visible to reflection (@Entity, @Autowired). @Inherited on class annotations propagates to subclasses.
• Example: @Retention(RetentionPolicy.RUNTIME) @interface MyTag {}

6. Annotation processing — compile-time vs runtime?

• Short: APT generates code at compile (Lombok, MapStruct); runtime reflection reads annotations.
• Detailed: JSR 269 annotation processors inspect AST during javac. Cleaner than runtime reflection for codegen. MapStruct generates mapper impls at compile time.
• Example: @Mapper interface → generated UserMapperImpl.

7. MethodHandles vs reflection?

• Short: MethodHandles are JVM-aware, faster, type-safe; preferred for repeated invocation.
• Detailed: MethodHandles.lookup().findVirtual(...). LambdaMetafactory uses handles for lambdas. Reflection easier for ad-hoc tools; handles for performance-critical paths.
• Example: metaprogramming5ReflectionVsHandles.java.

8. Dynamic proxies?

• Short: Proxy.newProxyInstance creates interface impl at runtime invoking InvocationHandler.
• Detailed: Only interfaces, not classes (use ByteBuddy/CGLIB for classes). Used for mocking, transactions (@Transactional), logging.
• Example: metaprogramming1DynamicProxy.java.

9. VarHandles (Java 9+)?

• Short: Typed, atomic field/array access without boxing.
• Detailed: Replacement for some sun.misc.Unsafe uses. Support compare-and-set on fields. Used in concurrent collections internals.
• Example: Low-level lock-free structures.

10. Common annotation examples in enterprise?

• Short: @Override, @Deprecated, @SuppressWarnings, @FunctionalInterface, JPA @Entity, Spring @Component.
• Detailed: JPA maps object model to tables. Spring stereotypes (@Service, @Repository) enable component scan. Jackson @JsonProperty controls JSON.
• Example: Stereotype meta-annotated with @Component.

11. Repeatable annotations?

• Short: @Repeatable wraps multiple same annotation in container.
• Detailed: @Schedules({@Schedule(...), @Schedule(...)}) or repeated form Java 8+. Container annotation holds array.
• Example: Multiple @AttributeOverride in JPA.

12. Reflection performance cost?

• Short: First invoke slow (security checks); cache Method/Field objects; prefer handles or codegen.
• Detailed: setAccessible reduces checks. Frameworks cache metadata at startup. Don't reflect in tight loops without caching.
• Example: Spring caches bean metadata at context refresh.

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Rapid-Fire (Q → A)

1. Reflect private field? → getDeclaredField + setAccessible.
2. Field.get object arg? → Instance field needs target; static null.
3. invoke static method? → null as first arg to invoke.
4. Constructor.newInstance? → Creates instance; exception wrapper.
5. getConstructors vs declared? → Public only vs all.
6. isAssignableFrom? → Subtype check.
7. cast with Class? → class.cast(obj).
8. instanceof with Class? → class.isInstance(obj).
9. Array reflection? → Array.newInstance(component, len).
10. Generic type erasure reflection? → Type interface for ParameterizedType.
11. getGenericSuperclass? → For type tokens.
12. Annotation on parameter? → getParameterAnnotations.
13. Default annotation values? → annotation.defaultMember().
14. Marker annotation? → No methods (@Deprecated style).
15. Single-value annotation? → value() shorthand.
16. @Documented? → Appears in javadoc.
17. @Inherited class only? → Yes, not methods.
18. @Repeatable container? → Holds annotation array.
19. @Native annotation? → Native method marker.
20. @SafeVarargs on? → Private/static/final varargs only.
21. FunctionalInterface check? → Compiler + runtime optional.
22. Proxy requires? → Interfaces only.
23. InvocationHandler? → invoke(proxy, method, args).
24. CGLIB proxy? → Subclass-based (Spring).
25. ByteBuddy? → Modern bytecode generation.
26. ASM library? → Low-level bytecode.
27. javassist? → Bytecode editing.
28. ClassLoader loadClass? → vs Class.forName static init.
29. defineClass? → Custom class loaders.
30. SPI ServiceLoader? → META-INF/services.
31. double-checked reflection? → Cache lookup + Method object.
32. SecurityManager reflection? → Deprecated/removed modern JDK.
33. InaccessibleObjectException? → Module blocks access.
34. --add-opens syntax? → --add-opens module/pkg=target-module.
35. records reflection? → isRecord, getRecordComponents.
36. sealed permits reflection? → getPermittedSubclasses.
37. enum reflection? → getEnumConstants.
38. AnnotationMirror compile? → Processor API.
39. Element vs TypeElement? → AST model.
40. Filer in processor? → Generate source files.
41. Messager printMessage? → Compiler errors/warnings.
42. Lombok how works? → Annotation processor codegen.
43. MapStruct how? → Processor generates mappers.
44. Hibernate bytecode? → ByteBuddy enhancement.
45. Jackson annotations runtime? → RUNTIME retention.
46. JAX-RS annotations? → Runtime for REST mapping.
47. Validation @NotNull? → Bean Validation runtime.
48. ConstraintValidator? → Custom validation logic.
49. CDI @Inject? → Jakarta EE DI.
50. Qualifier annotation? → Disambiguate beans.
51. @Primary Spring? → Default bean choice.
52. @ConditionalOnProperty? → Boot auto-config condition.
53. Meta-annotation? → Annotation on annotation.
54. @AliasFor? → Attribute alias in Spring.
55. Kotlin reflection separate? → kotlin.reflect.
56. GraalVM native reflection? → Registration config needed.
57. reachability metadata? → Native image reflect config.
58. serialization reflect? → Reflective access to private ctor.
59. Unsafe status? → Internal; use VarHandle/Foreign API.
60. Foreign Function API? → Panama native interop.