Spring Boot Specific

About

Spring Boot provides several annotations that simplify configuration, auto-configuration, and conditional bean loading.

1. Core Spring Boot Annotations

These annotations help in configuring and bootstrapping a Spring Boot application.

@SpringBootApplication

• This is the main entry point for a Spring Boot application.

• It is a combination of:

  • @Configuration – Marks the class as a Spring configuration class.

  • @EnableAutoConfiguration – Enables automatic configuration based on dependencies.

  • @ComponentScan – Scans for components in the same package and sub-packages.

@SpringBootApplication
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

@EnableAutoConfiguration

• Enables Spring Boot’s auto-configuration feature, which automatically configures beans based on the classpath.

• It is included in @SpringBootApplication.

@Configuration
@EnableAutoConfiguration
public class MyConfig {
}

2. Conditional Annotations

These annotations conditionally enable or disable beans based on various criteria.

@ConditionalOnProperty

• Loads a bean only if a specified property exists in application.properties or application.yml.

// If feature.enabled=true, MyFeature bean is created.
@ConditionalOnProperty(name = "feature.enabled", havingValue = "true")
@Bean
public MyFeature myFeature() {
    return new MyFeature();
}

@ConditionalOnClass

• Loads a bean only if a specified class is present in the classpath.

// If ExternalLibrary is available, myService() is registered.
@ConditionalOnClass(name = "com.example.ExternalLibrary")
@Bean
public MyService myService() {
    return new MyService();
}

@ConditionalOnMissingBean

• Registers a bean only if no other bean of the same type exists.

// If MyService is already defined elsewhere, this bean won’t be created.
@Bean
@ConditionalOnMissingBean(MyService.class)
public MyService defaultService() {
    return new MyService();
}

@ConditionalOnBean

• Registers a bean only if another specified bean exists.

// MyService is created only if MyDependency is present.
@Bean
@ConditionalOnBean(MyDependency.class)
public MyService myService() {
    return new MyService();
}