Autoconfiguration

Spring Boot’s autoconfiguration is what makes @SpringBootApplication feel like magic: add a dependency to your classpath, and the beans you’d normally have to configure by hand — a DataSource, a JdbcTemplate, a DispatcherServlet — simply appear, already wired into the same IoC container described in IoC Containers.

How it decides what to configure

Each auto-configuration class is annotated with conditions such as:

@Configuration
@ConditionalOnClass(DataSource.class)
@ConditionalOnMissingBean(DataSource.class)
public class DataSourceAutoConfiguration {
    @Bean
    public DataSource dataSource(DataSourceProperties properties) {
        return properties.initializeDataSourceBuilder().build();
    }
}

• @ConditionalOnClass — only apply if a given class is on the classpath (e.g. you added the JDBC driver dependency).
• @ConditionalOnMissingBean — only apply if you haven’t already defined your own bean of that type. Your explicit @Bean always wins.

Overriding it

Define your own bean of the same type, and Spring Boot’s autoconfiguration backs off automatically:

@Configuration
public class DataSourceConfig {
    @Bean
    public DataSource dataSource() {
        return DataSourceBuilder.create()
            .url("jdbc:postgresql://localhost/app")
            .build();
    }
}

Disabling it entirely

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

Seeing what got configured

Add the Actuator dependency and hit /actuator/conditions in a running app — it lists every auto-configuration class considered, and whether it matched or was excluded, which is invaluable when something isn’t wired the way you expect.