Spring @Lazy 注解快速指南

1. Overview

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.

In this quick tutorial, we’re going to discuss Spring’s @Lazy annotation.

2. Lazy Initialization

The @Lazy annotation has been present since Spring version 3.0. There’re several ways to tell the IoC container to initialize a bean lazily.

2.1. @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.

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();
    }
}

Let’s now test the functionality:

@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

To apply this to only a specific bean, let’s remove the @Lazy from a class.

Then we add it to the config of the desired bean:

@Bean
@Lazy(true)
public Region getRegion(){
    return new Region();
}

2.2. With @Autowired

Before going ahead, check out these guides for @Autowired and @Component annotations.

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.

With the @Component annotation on the City class and while referencing it with @Autowired:

@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.

3. Conclusion

In this quick tutorial, we learned the basics of Spring’s @Lazy annotation. We examined several ways to configure and use it.

As usual, the complete code for this guide is available on GitHub.