- By using AutoConfiguration classes present in
spring-boot-autoconfigurepackage
- These classes contain ConditionalOn.. classes that scan classpath resources and register beans, according to starters and dependencies that user introduced in POMs
- Spring Boot checks for the presence of a
META-INF/spring.factoriesfile within your published jar. The file should list your configuration classes under theEnableAutoConfigurationkey, as shown in the following example: org.springframework.boot.autoconfigure.EnableAutoConfiguration=\ com.code.in.packets.autoconf.SomeAutoConfiguration1,\ com.code.in.packets.autoconf.SomeAutoConfiguration2
- See all ConditionalOn classes
Using @Profile("some-profile") on a @Bean, and then load this profile
Using @Conditional("<some-class>") on a @Bean
What does @EnableAutoConfiguration do?
- Applied in Application/Main class of Spring application
- In Spring Boot application, already included in @SpringBootApplication annotation
- Responsible for kick-starting autoconfiguration process, by
- @AutoConfigurationPackage
- Importing AutoConfigurationPackages.Registrar, that registers a bean which provides Bean list for Spring boot to use internally, in
spring-boot-autoconfigurepackage
- @Import(EnableAutoConfigurationImportSelector.class)
- Imports AutoConfigurationImportSelector that aggregates all spring.factories
- Can exclude some autoconfiguration class, like: @EnableAutoConfiguration(exclude={DataSourceAutoConfiguration.class}) in our @Configuration class
What does @SpringBootApplication do?
- Applied only in root package class, and contains below annotations:
- @EnableAutoConfiguration: enables Spring Boot’s auto-configuration mechanism
- @ComponentScan: enable @Component scan on the package where the application is located
- @Configuration: allow to register extra beans in the context or import additional configuration classes
Does Spring Boot do component scanning? Where does it look by default?
- Yes, @ComponentScan or @SpringBootApplication enables component scanning.
- By default, it scans for components in whole package (and subpackages) of the class with @SpringBootApplication
- If no component scanning is set, it only scans the package of the main class
- @ComponentScan is also used to specify base packages and base package classes using thebasePackageClasses or basePackages attributes
How are DataSource and JdbcTemplate auto-configured?
- DataSource
- with DataSourceAutoConfiguration class from spring-boot-autoconfigure package.
- that retrieves properties spring.datasource.url, spring.datasource.username, spring.datasource.password from application.properties or yml file.
- JdbcTemplate
- with JdbcTemplateAutoConfiguration, that contains:
- @ConditionalOnClass(value={javax.sql.DataSource.class,org.springframework.jdbc.core.JdbcTemplate.class})
- If DataSource class exists in classpath, only then configure JdbcTemplate
- @ConditionalOnSingleCandidate(value=javax.sql.DataSource.class)
- We need only one candidate
- @AutoConfigureAfter(value=DataSourceAutoConfiguration.class)
- First configure DataSource, and only then JdbcTemplate
What is spring.factories file for?
- spring.factories contains auto-configuration classes needed for the AutoConfiguration mechanism of Spring
- @EnableAutoConfiguration eventually gathers all spring.factories files of all project modules/packages and aggregates them into one.
- A spring.factories contains an EnableAutoConfiguration key with values as the list of auto-configuration classes, and resides under META/INF directory
- spring.factories should be used when we want to set default components globally for our app. Thus the need for @Autowire-ing a dependency disappears. But of course, this default behavior can later be overwritten with some @Bean/@Configuration
How do you customize Spring Boot auto configuration?
- To create our own auto-configuration:
- add a spring.factories in META-INF, with content:
- org.springframework.boot.autoconfigure.EnableAutoConfiguration= com.myapp.package1.autoconfigure.MyDataSourceAutoConfiguration, com.myapp.package2.autoconfigure.MyJpaAutoConfiguration
- Implement above classes as @Configurations accordingly to specific needs
- Disable loading of auto-configuration class for DataSource loading: @EnableAutoConfiguration(exclude = {DataSourceAutoConfiguration.class})
What are the examples of @Conditional annotations? How are they used?
Conditional classes are used from within AutoConfiguration classes to load conditionally classpath resources :
ConditionalOnBean- Spring Bean must be registeredConditionalOnMissingBean- Spring Bean must not be registeredConditionalOnClass- Class must exist in classpathConditionalOnMissingClass- Class must not exist in classpathConditionalOnExpression- spEL expression must result to trueConditionalOnJava- Java JDK in certain version must existConditionalOnJndi - JNDI location should existConditionalOnProperty- property must existConditionalOnResource- resource must exist
