Spring boot Under Da Hood
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The document explains how Spring Boot initializes an application context and the role of various annotations like @SpringBootApplication and @EnableAutoConfiguration in this process. It delineates the steps involved in creating a Spring application context, discovering the main class, and loading necessary configuration classes based on conditions. It also discusses how to create custom auto-configuration and starters to support additional libraries within a Spring Boot application.
![public class SpringbootdemoApplication {
public static void main(String[] args) {
SpringApplication.run(SpringbootdemoApplication.class, args);
}
}
@SpringBootApplication](https://image.slidesharecdn.com/springbootunderdahood-200218113350/85/Spring-boot-Under-Da-Hood-9-320.jpg)
Spring boot Under Da Hood
- 3. The ApplicationContext @Config classes ApplicationContext ctx = new AnnotationConfigApplicationContext(basePackage); Fully Configured Application Example Bean ctx.getBean(ExampleBean.class) Xml Files
- 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
- 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