AutoConfiguration

What is Auto Configuration?

Instead of developer having to configure every aspect of the application explicitly spring boot follows Convention Over Configuration approach where if you follow certain conventions you don't need to configure everything explicitly, and you need to configure only if you are deviating from the default conventions.
So that way it will drastically simplify the need for more configuration.
For Example , if we are configuring our application properties we don't need to explicitly configure @PropertySource(value ={"classpath:application.properties"}). Spring boot automatically load from application.properties from the classpath and automatically register those beans.
Similarly, if you are adding a spring boot web starter it will automatically assume that you are going to build a web application or a REST api and automatically adds Tomcat as a servlet container because that is the most widely used.
Some Customization Supports.
for example the default port is 8080 but if we change the port you can configure server.port = 9090

CaseStudy: DataSource Auto Configuration

Use explicitly registered bean, if defined
Use properties from application.properties
Use in-memory JDBC driver if available on classpath
Spring Boot Auto Configuration mechanism works by conditionally registering the beans Conditional Bean Registration
@ConditionalOnClass
@ConditionalOnMissingClass
@ConditionalOnMissingBean
@ConditionalOnProperty
@ConditionalOnExpression