Spring @Lazy

淡淡的烟草味﹌ 2024-03-25 20:09 18阅读 0赞

By default, `Spring creates all singleton beans eagerly at the startup/bootstrapping of the application context`. The reason behind this is simple: to avoid and detect all possible errors `immediately` rather than at runtime.

However, there’re cases `when we need to create a bean, not at the application context startup, but when we request it`.

`@Lazy` `has an argument named value with the default value of true`. It is useful to override the default behavior.

### Lazy Initialization ###

The *`@Lazy`* annotation has been present since Spring version 3.0.

This annotation behaves differently depending on where we exactly place it.  
1、a *`@Bean`* annotated bean factory method, to delay the method call (hence the bean creation)  
2、a *`@Configuration`* class and all contained *`@Bean`* methods will be affected  
3、a *`@Component`* class, which is not a *`@Configuration`* class, this bean will be initialized lazily  
4、a *`@Autowired`* `constructor`, `setter`, or `field`, to load the dependency itself lazily (via proxy)

--------------------

### @Configuration Class ###

When 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.

@Lazy
    @Configuration
    @ComponentScan(basePackages = "com.lazy")
    public class AppConfig {
        
    
        @Bean
        public Region getRegion(){
        
            return new Region();
        }
    
        @Bean
        public Country getCountry(){
        
            return new Country();
        }
    }
    
    @Test
    public void givenLazyAnnotation_whenConfigClass_thenLazyAll() {
        
        AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
        ctx.register(AppConfig.class);
        ctx.refresh();
        ctx.getBean(Region.class);
        ctx.getBean(Country.class);
    }

As we see, all beans are created only when we request them for the first time:

Bean factory for ...AnnotationConfigApplicationContext: 
    ...DefaultListableBeanFactory: [...];
    // application context started
    Region bean initialized
    Country bean initialized

### @Bean method ###

# To apply this to only a specific bean
    @Configuration
    @ComponentScan(basePackages = "com.lazy")
    public class AppConfig {
        
    
        @Bean
        @Lazy(true)
        public Region getRegion(){
        
            return new Region();
        }
    
        @Bean
        public Country getCountry(){
        
            return new Country();
        }
    }

### With @Autowired ###

# The bean that we want to load lazily
    @Lazy
    @Component
    public class City {
        
        public City() {
        
            System.out.println("City bean initialized");
        }
    }
    
    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 `n both places
    @Test
    public void givenLazyAnnotation_whenAutowire_thenLazyBean() {
        
        // load up ctx appication context
        Region region = ctx.getBean(Region.class);
        region.getCityInstance();
    }

Here, `the City bean is initialized only when we call the getCityInstance() method`.

**参考：[A Quick Guide to the Spring @Lazy Annotation][A Quick Guide to the Spring _Lazy Annotation]**

[A Quick Guide to the Spring _Lazy Annotation]: https://www.baeldung.com/spring-lazy-annotation