Spring Boot - Auto-configuration

Spring Boot's auto-configuration is one of its most powerful features, designed to reduce the effort required to set up and configure a Spring application. Auto-configuration attempts to automatically configure the Spring application based on the libraries present in the classpath.

Let's delve into how auto-configuration works and its key aspects:

Working Principle:

Check Dependencies: When a Spring Boot application starts, it examines the libraries in the classpath. Based on these libraries, it makes some educated guesses about what you're likely going to need.
Default Configurations: Spring Boot provides a plethora of default configurations, assuming you'd want a ready-to-run application.
Conditional Configuration: It doesn't just blindly apply all default configurations. Instead, Spring Boot checks a variety of conditions before applying a configuration. This is done through the @Conditional annotations or their variants (@ConditionalOnClass, @ConditionalOnBean, @ConditionalOnProperty, etc.).

Key Features:

Custom Starters: Spring Boot comes with a set of predefined project templates called "Starters". These starters are dependency descriptors that bring in a bunch of related libraries to help achieve a functionality (like web development, JPA-based data access, etc.). When you include a starter in your project, Spring Boot auto-configures components based on the libraries in that starter.
Properties Overriding: Even though Spring Boot provides many default configurations, it also offers the flexibility to override these defaults. You can use application.properties or application.yml files to customize these settings.
Minimal Configuration: One of the main goals of auto-configuration is to reduce the need for specifying beans in the configuration file. If Spring Boot's assumptions match your needs, you may not need to write any configuration at all!

Examples:

If you have spring-boot-starter-web on your classpath, Spring Boot assumes you're developing a web application and sets up Tomcat as an embedded server.
If you have JPA libraries and a database connection in the classpath, Spring Boot will set up the default configurations for a database, including a DataSource bean, EntityManager, and more. If you've defined your DataSource properties, it will use those.
If spring-boot-starter-data-jpa is in the classpath but there's no EntityManager bean defined, Spring Boot will auto-configure one for you. But if you define your custom EntityManager, Spring Boot will back off and won't create the default one.

Overriding Auto-configuration:

If you find that specific auto-configurations aren't useful, you can exclude them. Use the exclude attribute of @SpringBootApplication or @EnableAutoConfiguration:

@SpringBootApplication(exclude = {DataSourceAutoConfiguration.class})
public class MyApplication {
    // ...
}

Viewing Auto-configuration Report:

For debugging purposes or to better understand what auto-configurations have been applied, you can enable the auto-configuration report by setting the property debug=true in your application.properties.

Conclusion:

Auto-configuration significantly simplifies the setup and configuration process for Spring applications, allowing developers to focus on building the application's core functionality. However, it's always good to understand the underlying configurations and conventions to effectively troubleshoot or optimize when necessary.

How does auto-configuration work in Spring Boot?

Spring Boot automatically configures beans based on the project's dependencies and environment.

Example:

// Auto-configured DataSource bean
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ DataSource.class, EmbeddedDatabaseType.class })
@ConditionalOnMissingBean(DataSource.class)
@ConditionalOnProperty(name = "spring.datasource.type")
class DataSourceConfiguration {

    @Bean
    @ConfigurationProperties(prefix = "spring.datasource")
    DataSource dataSource() {
        return DataSourceBuilder.create().build();
    }
}

Customizing auto-configuration in Spring Boot:

Customize auto-configuration by providing your own configuration.

Example:

// Custom DataSource configuration
@Configuration
public class CustomDataSourceConfig {

    @Bean
    public DataSource customDataSource() {
        // Custom DataSource bean creation
        return new CustomDataSource();
    }
}

Conditional auto-configuration in Spring Boot:

Conditionally apply auto-configuration based on specific conditions.

Example:

// Conditional DataSource configuration
@ConditionalOnProperty(name = "custom.datasource.enabled", havingValue = "true")
@Configuration
public class ConditionalDataSourceConfig {

    @Bean
    public DataSource conditionalDataSource() {
        // Conditional DataSource bean creation
        return new ConditionalDataSource();
    }
}

Ordering and prioritizing auto-configurations in Spring Boot:

Use @AutoConfigureOrder or @Order to specify the order of auto-configurations.

Example:

// Custom Auto-configuration with order
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
public class CustomAutoConfiguration {

    // Auto-configuration logic
}

Excluding auto-configuration classes in Spring Boot:

Exclude specific auto-configurations using @SpringBootApplication annotation.

Example:

// Exclude DataSourceAutoConfiguration
@SpringBootApplication(exclude = DataSourceAutoConfiguration.class)
public class MyApplication {
    // Application code
}

Creating custom auto-configuration in Spring Boot:

Create a custom auto-configuration class with @Configuration and @Conditional annotations.

Example:

// Custom Auto-configuration
@Configuration
@ConditionalOnClass(MyService.class)
public class MyAutoConfiguration {

    @Bean
    public MyService myService() {
        // Custom service bean creation
        return new MyService();
    }
}

Troubleshooting auto-configuration issues in Spring Boot:
- Use logging and debugging to identify issues with auto-configuration.
- Check dependencies and configurations.
- Example:
```
# Enable debug logging for auto-configuration
logging:
  level:
    org.springframework.boot.autoconfigure: DEBUG
```