Chapter 2: Spring Beans & Dependency Injection

Spring's IoC container is the core engine managing component lifecycles, dependency graphs, and enterprise cross-cutting concerns. Mastering bean stereotypes, constructor injection, scoping rules, disambiguation, and runtime proxies empowers you to build robust, scalable architectures.

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🧭 Subtopics & Curriculum

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Component Discovery2.1

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2.1 Bean Stereotypes & Component Scan

• What is a Spring Bean?
• @Component vs @bean
  • @Component
  • @Bean
• Stereotype Annotations Hierarchy
  • @Component
  • @Service
  • @Repository
  • @Controller
  • @RestController
• @Configuration and @Bean Mechanics
• Component Scanning Mechanics

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Wiring & Inversion2.2

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2.2 Dependency Injection

• What is Dependency Injection?
• Without Dependency Injection
• With Dependency Injection
• The Three Forms of Injection
  • 1. Constructor Injection (Industry Best Practice) ✅
  • 2. Setter Injection (For Optional Dependencies)
  • 3. Field Injection (Discouraged / Anti-Pattern) ❌
• Why Field Injection is an Anti-Pattern
• Circular Dependencies & Resolution
  • What Happens in Spring Boot?
  • Production Fixes:
• DI vs Factory Pattern
  • Factory
  • Dependency Injection

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Wiring & Injection2.3

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2.3 Autowiring

• What is Autowiring?
  • What does @Autowired do?
  • Dependency Injection vs Autowiring
  • The Autowiring Process
• Types of Autowiring
  • 1. Constructor Injection (Recommended)
  • 2. Setter / Method Injection
  • 3. Field Injection (Discouraged Anti-Pattern)
• How Spring Resolves Autowired Beans Under the Hood
  • @Primary
  • @Qualifier
  • @Primary vs @Qualifier
• 4. Handling Optional Dependencies
  • Approach 1: @Autowired(required = false)
  • Approach 2: Java 8 java.util.Optional<T> (Recommended)
  • Approach 3: @Nullable
• 5. Legacy XML Autowiring Modes
• 6. @Autowired vs @Resource vs @Inject
  • Detailed Comparison Matrix
  • Example Using @Resource:
• 7. Real-World Gotchas & Interview Traps
  • 1. The new Operator Trap
  • 2. Autowiring static Fields Fails
  • 3. Missing Default Constructor with Multiple Constructors
• 8. Summary Checklist

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Lifecycle & Memory2.4

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2.4 Bean Scopes & Lifecycle

• What is Bean scope?
  • 1. Singleton Scope — Most Important
  • 2. Prototype Scope
  • 3. Request Scope
  • 4. Session Scope
  • 5. Application Scope
  • 6. WebSocket Scope
• Spring Bean Scopes Matrix
  • What happens when Prototype is injected into Singleton?
  • Is Singleton Bean Thread-Safe?
• Complete Bean Lifecycle Hook Sequence
  • @PostConstruct
  • @PreDestroy
• BeanPostProcessor Real-World Power

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Candidate Selection2.5

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2.5 Disambiguation & Qualifiers

• 1. The Disambiguation Problem
• 2. Solutions: @Primary vs @Qualifier
  • A. @Primary (Default Priority)
  • B. @Qualifier (Explicit Target Selection)
• 3. Custom Qualifier Annotations
• 4. Injecting Collections of Beans & @Order

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Cross-Cutting Concerns2.6

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2.6 Spring AOP & Proxies

• 1. Core AOP Terminology
• 2. Advice Types & Code Example
  • Advice Kinds:
• 3. CGLIB vs JDK Dynamic Proxies
  • The Self-Invocation Gotcha:

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