Spring Boot how to create Auto configuration class

Question

I am beginner for Spring Boot. When I am using any dependencies in Spring Boot, they have the auto configuration default.

My questions are:

1. What is actually auto configuration class?
2. How does auto configuration work?
3. How to make out own auto configuration?

Please suggest me any blog that describes easy manner or please provide me any code snippet for better understanding.

Answer 1

The Spring Boot core package spring-boot-starter contains the spring-boot-autoconfigure package.

What does it do? (from the JavaDoc)

Enable auto-configuration of the Spring Application Context, attempting to guess and configure beans that you are likely to need. Auto-configuration classes are usually applied based on your classpath and what beans you have defined. For example, If you have tomcat-embedded.jar on your classpath you are likely to want a TomcatEmbeddedServletContainerFactory (unless you have defined your own EmbeddedServletContainerFactory bean).

Auto-configuration tries to be as intelligent as possible and will back-away as you define more of your own configuration. You can always manually exclude() any configuration that you never want to apply (use excludeName() if you don't have access to them). You can also exclude them via the spring.autoconfigure.exclude property. Auto-configuration is always applied after user-defined beans have been registered.

So each jar in your classpath that Spring can autoconfigure, Spring will autoconfigure for you to use in your application. Think about Hibernate, ThymeLeaf, Jackson etc.

How do you use it?

Simply add the @EnableAutoConfiguration in your application to make Spring autoconfigure your application (you possibly also need @SpringBootConfiguration).

@SpringBootConfiguration
@EnableAutoConfiguration
// Or just @SpringBootApplication instead of the 2 above
@Import(AppConfig.class)
public class App {
  public static void main(String[] args) {
    SpringApplication.run(App.class);
  }
}

And your good to go.

What can it configure for you? All of these tools below (got this from looking at the org.springframework.boot.autoconfigure package)

admin
amqp
aop
batch
cache
cassandra
cloud
condition
context
couchbase
dao
data
analyzer
domain
jest
flyway
freemarker
template
gson
h2
hateoas
hazelcast
info
integration
jackson
jdbc
jersey
jms
jmx
jooq
kafka
ldap
liquibase
logging
mail
mobile
mongo
mustache
jpa
reactor
security
sendgrid
session
social
solr
template
thymeleaf
transaction
validation
web
webservices
websocket

How to create your own configuration?

Don't know, never needed to do this. But this blog is a good starting point.