Spring Interview Related Concept - Lazy Initialization

Lazy Initialization:
By default in Spring, all the defined beans, and their dependencies, are created when the application context is created. The reason behind this is simple: to avoid and detect all possible errors immediately rather than at runtime.

In contrast, when we configure a bean with lazy initialization, the bean will only be created, and its dependencies injected, once they're needed.

Enable Lazy Initialization

Setting the property "spring.main.lazy-initialization" value to true means that all the beans in the application will use lazy initialization.

Let's configure the property in our application.yml configuration file:
          spring: 

          main:
          lazy-initialization: true

Or, if it's the case, in our application.properties file:

spring.main.lazy-initialization=true
This configuration affects all the beans in the context. So, if we want to configure lazy initialization for a specific bean, we can do it through the @Lazy approach.
Or in other words, all the defined beans will use lazy initialization, except for those that we explicitly configure with @Lazy(false).
Lazy Initialization With Annotation:@Configuration ClassWhen we put @Lazy annotation over the @Configuration class, it indicates that all the methods with @Bean annotation should be loaded lazily.

This is the equivalent for the XML based configuration's default-lazy-init=“true“ attribute.

Let's have a look here:
@Lazy
@Configuration
@ComponentScan(basePackages = "com.baeldung.lazy")
public class AppConfig {
 @Bean
 public Region getRegion(){
  return new Region();
 }

 @Bean
 public Country getCountry(){
  return new Country();
 }
}




Then we add it to the config of the desired bean:
@Bean
@Lazy(true)
public Region getRegion(){
 return new Region();
}

With @Autowired

Here, in order to initialize a lazy bean, we reference it from another one.

The bean that we want to load lazily:
@Lazy
@Component
public class City {
 public City() {
  System.out.println("City bean initialized");
 }
}

And it's reference:
public class Region {

@Lazy
@Autowired
private City city;

public Region() {
 System.out.println("Region bean initialized");
}

public City getCityInstance() {
 return city;
}
}



Note, that the @Lazy is mandatory in both places.