Q: How does Spring Boot auto-configuration work? How do you debug it?

Answer:

Auto-configuration = Boot inspects the classpath + your config + existing beans, then conditionally registers default beans.

The Entry Point

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

@SpringBootApplication = @SpringBootConfiguration + @EnableAutoConfiguration + @ComponentScan.

@EnableAutoConfiguration

Triggers AutoConfigurationImportSelector, which loads class names from:

META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports

(Pre-2.7 used META-INF/spring.factories.)

Each class is @AutoConfiguration annotated — a @Configuration evaluated only if its conditions hold.

Conditions

@AutoConfiguration
@ConditionalOnClass({DataSource.class, EmbeddedDatabaseType.class})
@ConditionalOnMissingBean(DataSource.class)
@EnableConfigurationProperties(DataSourceProperties.class)
public class DataSourceAutoConfiguration {
    @Bean @ConditionalOnProperty(name = "spring.datasource.url")
    DataSource dataSource(DataSourceProperties p) { ... }
}

Common conditions:

Order Matters

• @AutoConfigureBefore / @AutoConfigureAfter / @AutoConfigureOrder.
• User @Configuration classes process before auto-config → user beans win via @ConditionalOnMissingBean.

Debugging — --debug Mode

Run with --debug or debug=true:

=========================
AUTO-CONFIGURATION REPORT
=========================

Positive matches:
-----------------
   DataSourceAutoConfiguration matched:
      - @ConditionalOnClass found required class 'javax.sql.DataSource'
      ...

Negative matches:
-----------------
   GsonAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'com.google.gson.Gson'

Exclusions:
-----------
   org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration

Unconditional classes:
----------------------
   ...

Actuator /actuator/conditions

Same data as a live JSON endpoint when actuator is enabled.

Override / Disable

Disable specific auto-configs

@SpringBootApplication(exclude = { SecurityAutoConfiguration.class })
// or via property
spring.autoconfigure.exclude=org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration

Override a bean

@Bean
public DataSource dataSource() { return myCustomDataSource(); }
// Boot's @ConditionalOnMissingBean → its DataSource doesn't register

@ConfigurationProperties Binding

Tunables for auto-configs come from application.yml:

spring:
  datasource:
    url: jdbc:postgresql://db/app
    username: app
    hikari:
      maximum-pool-size: 20

@ConfigurationProperties(prefix = "spring.datasource")
public class DataSourceProperties { ... }

Common Gotchas

• Bean conflicts: forgot @ConditionalOnMissingBean in a custom starter → user can't override.
• Component scan misses package: @SpringBootApplication scans from its own package down. Move it up the package tree if needed.
• Test slices (@WebMvcTest, @DataJpaTest) load subset of auto-config. Some beans missing in tests but present in prod.
• Order surprises: two auto-configs both register a bean of same type → first wins, others skip due to @ConditionalOnMissingBean.

Interview Soundbite

"Auto-configuration = conditional @Bean definitions, gated on classpath + properties + existing beans. Boot looks under META-INF/spring/...AutoConfiguration.imports, evaluates each class's conditions, registers what fits. User beans always win because they're processed first and conditions check @ConditionalOnMissingBean."