Lazy initialization

Definition

By default Spring creates every singleton at startup, so mistakes show up immediately. A @Lazy bean is created only when it is first needed. It speeds up startup but moves any failure to run time.

Example code

When each bean is created

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Lazy;

class Report {
    Report(String name) { System.out.println("creating " + name); }
}

@Configuration
class AppConfig {
    @Bean
    Report eager() { return new Report("eager report"); }       // created at startup

    @Bean
    @Lazy
    Report lazy() { return new Report("lazy report"); }         // created on first use
}

class Demo {
    public static void main(String[] args) {
        try (var context = new AnnotationConfigApplicationContext(AppConfig.class)) {
            System.out.println("context started");
            context.getBean("lazy");                           // first use: now it is created
        }
    }
}

// Output:
// creating eager report
// context started
// creating lazy report