Spring Boot Autowiring

Overview

• Dependency Injection in Spring Boot
• Focus on Autowiring: Automating the connection of dependent objects

Example Scenario

• Developers need a machine (laptop/desktop) to compile and debug code
• Create a Laptop class with a compile method public void compile() { System.out.println("Compiling with 44 bugs"); }
• Instantiate Laptop object in main class to call compile method
• Issue: Direct instantiation using new keyword is not ideal

Spring Component and Autowiring

• Use @Component to mark classes for Spring to manage and instantiate
• Autowiring allows Spring Boot to automatically inject dependencies @Component public class Laptop { // methods }
• Autowire with @Autowired to automatically wire dependencies @Autowired private Laptop laptop;
• No need to manually create object instances

Types of Injection

1. Field Injection
   • @Autowired on fields
   • Typically less preferred
2. Constructor Injection
   • @Autowired on the constructor or omit since it's the default
   public Dev(@Autowired Laptop laptop) { this.laptop = laptop; }
3. Setter Injection
   • @Autowired on setter methods
   @Autowired public void setLaptop(Laptop laptop) { this.laptop = laptop; }

Interface-based Autowiring

• Create an interface Computer which Laptop and Desktop implement interface Computer { void compile(); }
• Use Computer type for dependencies instead of specific Laptop/Desktop @Autowired private Computer computer;
• Allows for loose coupling and flexibility in dependency types

Handling Multiple Implementations

• If multiple beans of the same type exist, specify which one to use
• Primary: Mark preferred bean with @Primary @Primary @Component public class Laptop implements Computer { // methods }
• Qualifier: Specify the exact bean to use with @Qualifier @Autowired @Qualifier("laptop") private Computer computer;

Conclusion

• Autowiring simplifies dependency management in Spring Boot
• Understand different types of injection and when to use them
• Managing multiple implementations using @Primary and @Qualifier

Additional Notes

• Dependency injection helps in writing modular, maintainable code
• Field injection is less preferred, Constructor and Setter injections are better
• Always code to an interface for flexibility and loose coupling

Practical Tips

• Use @Primary sparingly, only when dealing with multiple beans
• Prefer Constructor injection for mandatory dependencies
• Use @Qualifier for more specific bean resolution needs