Deep Dive into SpringBoot Auto‑Configuration and Startup Process

SpringBoot dramatically simplifies Java backend development by providing zero‑configuration, auto‑configured components; this article explains how the auto‑configuration works and how the application starts.

1. Frequently Used SpringBoot Annotations

@Value injects literal values or placeholders from configuration files, equivalent to XML <property> entries.

@Component
public class Person {
    @Value("i am name")
    private String name;
}

@ConfigurationProperties binds a group of related properties to a POJO, avoiding repetitive @Value usage.

# application.properties
person.name=kundy
person.age=13
person.sex=male

@Component
@ConfigurationProperties(prefix = "person")
public class Person {
    private String name;
    private Integer age;
    private String sex;
}

@Import can import regular Java classes, custom ImportSelector, or ImportBeanDefinitionRegistrar implementations, turning them into beans.

// Direct import example
@Import({Circle.class})
@Configuration
public class MainConfig { }

// ImportSelector example
public class MyImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata metadata) {
        return new String[]{"com.example.Triangle"};
    }
}

@Import({Circle.class, MyImportSelector.class})
@Configuration
public class MainConfigTwo { }

@Conditional activates a configuration only when a custom condition evaluates to true.

@Configuration
@Conditional(MyCondition.class)
public class ConditionConfig {
    @Bean
    public ConditionBean conditionBean() { return new ConditionBean(); }
}

2. SpringBoot Startup Process

The SpringApplication.run() method creates a SpringApplication instance and executes the run() method, which performs environment preparation, banner printing, application‑context creation, context preparation, refresh, and finally invokes ApplicationRunner and CommandLineRunner beans.

public ConfigurableApplicationContext run(String... args) {
    // ...prepare environment, create context, refresh, call runners...
}

3. Auto‑Configuration Principle

The core annotation @SpringBootApplication combines @SpringBootConfiguration, @EnableAutoConfiguration, and @ComponentScan. @EnableAutoConfiguration imports AutoConfigurationImportSelector, which reads META‑INF/spring.factories to obtain a list of auto‑configuration classes.

Each auto‑configuration class is typically annotated with conditions such as @ConditionalOnWebApplication, @ConditionalOnClass, and @ConditionalOnProperty, and may enable configuration properties via @EnableConfigurationProperties.

@Configuration
@EnableConfigurationProperties(HttpProperties.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass(CharacterEncodingFilter.class)
@ConditionalOnProperty(prefix = "spring.http.encoding", name = "enabled", matchIfMissing = true)
public class HttpEncodingAutoConfiguration { }

The corresponding HttpProperties class is bound to spring.http.* properties using @ConfigurationProperties.

@ConfigurationProperties(prefix = "spring.http")
public class HttpProperties { }

Conclusion

SpringBoot’s auto‑configuration loads many candidate classes from META‑INF/spring.factories, filters them based on the current environment and classpath, and registers beans automatically, enabling developers to focus on business logic without manual wiring.