[Spring] Lazy Initialization

Lazy Initialization 은 뭐지?

Initialization

• By default, when your application starts, all beans are initialized
  • @Component, etc..
• Spring will create an instance of each and make them available

Diagnostics: Add println to constructors

@Component
public class CricketCoach implements Coach{
	public CricketCoach(){
		System.out.println(
		"In constructor: " + getClass().getSimpleName()
		);
	}
}

@Component
public class BaseballCoach implements Coach{
	public BaseballCoach(){
		System.out.println(
		"In constructor: " + getClass().getSimpleName()
		);
	}
}

......

When we start the application

---

In constructor : CricketCoach In constructor : BaseballCoach In constructor : TennisCoach In constructor : TrackCoach -> By default, when your application starts, all beans are initialized. Spring will create instance of each and make them available. —

Lazy Initialization

• Instead of creating all beans up front, we can specify lazy initialization
• A been will only be initialized in the following cases:
  • It is needed for dependency injection
  • Or it is explicitly requested
• Add the @Lazy annotation to a given class

Lazy Initialization with @Lazy

@Component
@Lazy 
public class TrackCoach implements Coach {
	public TrackCoach() {
		System.out.println("In constructor: " + getClass().getSimpleName()
		)
		.....
	}
}

• Bean is only initialized if needed for dependency injection

@RestController
public class DemoController {
	private Coach myCoach;

	@Autowired
	public DemoController(@Qualifier("cricketCoach") Coach theCoach){
		myCoach = theCoach;
	}
}

• Inject cricketCoach

---console---
In constructor : CricketCoach
In constructor : BaseballCoach
In constructor : TennisCoach
---

• Since we are NOT injecting TrackCoach .. it’s not initialized

---

• To configure other beans for Lazy initialization
  • We could need to add @Lazy to each class
• Turns into tedious work for a large number of classes
• I wish we could set a global configuration property.. –> The answer is YES

Lazy Initialization - Global configuration

File: application.properties

Spring.main.lazy-initialization=true

• All beans are lazy…
• No beans are created until needed, Including our DemoController
• Once we access REST endpoint/dailywork
• Spring will determine dependencies for DemoController
• For dependency resolution, Spring creates instance of CricketCoach first…
• Then creates instance of DemoController and injects the CricketCoach

So Lazy?

• Advantages
  • Only create objects as needed
  • May help with faster startup time if you have large number of components
• Disadvantages
  • If you have web related components like @RestController, not created until requested
  • May not discover configuration issues until too late
  • Need to make sure you have enough memory for all beans once created
• Lazy initialization feature is disabled by defualt
• You should profile your application before configuring lazy initiailiation.
• Avoid the common pitfall of premature optimization

출처 : luv2code.com