A short presentation that explains the internals of the Spring Boot framework. Includes an example of a starter and an autoconfig artifac, and a github repo: https://github.com/MichelSchudel/springbootunderdahood

3. The ApplicationContext
@Config
classes
ApplicationContext ctx = new AnnotationConfigApplicationContext(basePackage);
Fully
Configured
Application
Example
Bean
ctx.getBean(ExampleBean.class)
Xml
Files

4. ApplicationContext

5. What is Spring Boot doing for us?

6. SpringApplication.run(MyClass.class, args);
1. Creates an Spring applicationContext based on
1. the context information of the class passed in the runner (in this case,
MyClass.class)
2. Type of context (FE webappContext) based on classes in the classpath
2. Deduces the “main” class (for runnable jar packaging - > java –jar myApp.jar)
3. Enriches the applicationContext if AutoConfiguration annotation is present (more on this
in a moment)
4. Returns the applicationContext to the user / caller

7. @Component
@Service
@Bean
@Qualifier
@Controller
@RestController
@Repository
@PropertySource
@Configuration
@Import
@GetMapping, @PostMapping etc
All part of the Spring framework, NOT Spring Boot

8. SpringApplication class

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

10. @ComponentScan
@SpringBootApplication
component
component
component
component
Spring bean
@SpringBootConfiguration ~ @Configuration
@EnableAutoConfiguration

11. @Import(AutoConfigurationImportSelector.class)
@EnableAutoConfiguration
AutoConfigurationImportSelector implements ImportSelector
Interface (base Spring framework) to be implemented by types
that determine which @Configuration class(es) should be
imported
@AutoConfigurationPackage
Stores client side configuration package list for use by:

12. AutoConfigurationImportSelector
• Loads configuration classes using each META-INF/spring.factories it can find,
using SpringFactoriesLoader class
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveRestClientAutoConfiguration,
org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,
META-INF/spring.factories

13. JpaRepositoriesAutoConfiguration
@ConditionalOnBean({DataSource.class})
@ConditionalOnClass({JpaRepository.class})
@ConditionalOnMissingBean({JpaRepositoryFactoryBean.class, JpaRepositoryConfigExtension.class})
@ConditionalOnProperty(
prefix = "spring.data.jpa.repositories",
name = {"enabled"},
havingValue = "true",
matchIfMissing = true
)
• Loads if DataSource bean is present
• AND the JpaRepository class is on the classpath
• AND no custom JpaRepositoryFactoryBean is present
• AND property condition matches
Spring Backs out based on your own
Application context

15. How is the embedded webserver (Tomcat)
started?
…Just another AutoConfig!
• If applicationContext type is SERVLET (Servlet class is on the
classpath),
EmbeddedWebServerFactoryCustomizerAutoConfiguration
is loaded
• If Tomcat class is on the classpath,
TomcatWebServerFactoryCustomizer is initialized, which
configures and starts Tomcat server with a DispatcherServlet that
delegates requests to the applicationContext (normal SpringWebMvc
behaviour)

16. How to build your own autoconfig!
• SomeLibrary – existing library that you want to support in SB
• SomeLibraryAutoConfig – contains conditionals and bean factories
• SomeLibraryStarter – just a POM that bundles as dependencies:
• SomeLibrary
• SomeLibraryAutoConfig
• Your Application – imports SomeLibraryStarter as a dependency

17. Artifact dependency tree
Your SB Application
SomeLibrary
Starter
SomeLibrary
AutoConfiguration
SomeLibrary
Depends on
Depends on
Depends on
Depends on
Spring factories
@Conditional configs
Just a pom
FlyWay, H2

18. Let’s build our own starter!

19. Summary –Spring Boot at its core
• Provide utility class to load application context
• Introduce autoconfig mechanism through @Conditional annotations
https://github.com/MichelSchudel/springbootunderdahood