Spring Certification: Spring Boot Auto Configuration

How does Spring Boot know what to configure?

• By using AutoConfiguration classes present in spring-boot-autoconfigure package

• 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.factories file within your published jar. The file should list your configuration classes under the EnableAutoConfiguration key, 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-autoconfigure package

• @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 registered
• ConditionalOnMissingBean - Spring Bean must not be registered
• ConditionalOnClass - Class must exist in classpath
• ConditionalOnMissingClass - Class must not exist in classpath
• ConditionalOnExpression - spEL expression must result to true
• ConditionalOnJava - Java JDK in certain version must exist
• ConditionalOnJndi - JNDI location should exist
• ConditionalOnProperty - property must exist
• ConditionalOnResource - resource must exist