Understanding Dependency Injection with Spring

Key Concepts

• Dependency Injection (DI): technique where an object receives other objects it depends on, promoting loose coupling.
• Unit Testing: DI simplifies creating mock implementations for testing.
• Spring Framework: robust tool to facilitate DI with annotations, application contexts, and more.

Maven Project Setup

• Simple Maven project without Spring Boot
• Dependencies: Add spring-context to pom.xml

Code Structure

• BackgroundVerificationService interface with two implementations
• LoanService: Depends on BackgroundVerificationService via constructor
• LoanRequest: Bean with some properties
• Main Application: Console application setup with Spring DI for loan approval/rejection

Applying Dependency Injection with Spring

1. Annotate Beans with @Component: Marks classes as Spring-managed components.
2. Create Application Context: Use AnnotationConfigApplicationContext specifying base packages.
3. Bean Lookup: Retrieve beans from application context with context.getBean(LoanService.class).

Steps Behind the Scenes

1. Scanning Components: Using Spring's ClassPathScanningCandidateComponentProvider to find classes with @Component annotation.
2. Identifying Dependencies: Using Java Reflection to inspect constructors and their parameter types.
3. Instantiating Beans: Creating instances via reflection and injecting dependencies recursively.

Practical Implementation and Example

• Scan Classes: Identify @Component annotated classes.
• Reflection for Dependencies: Inspect constructor types to understand dependencies.
• Create Instances: Use reflection to instantiate dependencies and assemble the final bean.

Benefits of Understanding DI Mechanics

• Core concept across Spring ecosystem (e.g., Spring Data, Security, MVC).
• Robust knowledge supports better utilization and troubleshooting of Spring's numerous modules.

Creating a Simple Custom Bean Factory

• Example: Manually define and instantiate beans without using Spring, for small applications.
• BeanFactory Class: A custom version of an application context.

Conclusion

• Spring DI: Fundamental tool for building scalable, testable applications
• Custom DI: Possible without frameworks for simpler projects.
• Future Learning: Spring offers more complex customization like prototype beans, lazy initialization, etc.
• Key takeaway: Flexibility in object instantiation through DI, promoting a more modular and maintainable codebase.