How Does Spring Boot Auto-Configuration Work?

What Is It?

As its name implies, "autoconfiguration" helps you to configure your Spring Boot application automatically. More precisely, it automatically create beans for you! Strangely, it is a cool feature, but not applicable outside of Spring Boot. 

The Purpose and Method

The purpose is simple: to save you time when you create beans, which is usually done by yourself. It sounds like magic, but behind the scene, it's nothing fancy actually. 

It is simply a bunch of normal Spring configuration classes (like others with the same annotation @Configuration). 

The difference is that these configuration classes are using some different annotations:

•  @ConditionalOnClass: acts only if a given class is on a classpath
•  @ConditionalOnMissingBean: acts only if a given bean is not created
•  @Conditional: acts only if given condition is met

In one sentence, generally, if a class is found in a classpath (@ConditionalOnClass) , and a bean is requested (@Autowired), but no others is creating it ( @ConditionalOnMissingBean), it'll create that bean. 

Let's have a look at one example:

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@Configuration(proxyBeanMethods = false)

@ConditionalOnClass({ MongoClient.class, ReactiveMongoTemplate.class })

@ConditionalOnBean(MongoClient.class)

@EnableConfigurationProperties(MongoProperties.class)

@Import(MongoDataConfiguration.class)

@AutoConfigureAfter(MongoReactiveAutoConfiguration.class)

public class MongoReactiveDataAutoConfiguration {

    @Bean

    @ConditionalOnMissingBean(ReactiveMongoDatabaseFactory.class)

    public SimpleReactiveMongoDatabaseFactory reactiveMongoDatabaseFactory(MongoProperties properties,

            MongoClient mongo) {

        String database = properties.getMongoClientDatabase();

        return new SimpleReactiveMongoDatabaseFactory(mongo, database);

    }

What Does the Process Look?

You are asking for a bean, by using an annotation like  @Autowire, Spring BeanFactory tries to find the bean for you.

If you have declared a bean by yourself (i.e.  @Configuration, @Component, @Service, etc.), Spring will pick that up and instantiate it for you. Otherwise, Spring will look into those auto-configuration classes (still @Configuration but with a @Conditional annotation), to find the suitable bean and instantiate it for you.

If still nothing is found, an exception is thrown by Spring, like NoSuchBeanDefinitionException.

How Do I Use This Feature, and Is There Anything I Can Do?

It is used via the annotation @EnableAutoConfiguration, which is included in  @SpringBootApplication already.

Spring does, however, leave options for you to have some control:

1. You can tell Spring to disable certain auto-configuration classes.  (i.e.  @EnableAutoConfiguration(exclude={DataSourceAutoConfiguration.class}) )
2. You can tell Spring to include your own auto-configuration class. 
3. You can declare your own bean, which will be respected by Spring and auto-configuration class will be ignored. This is done via @ConditionalOnMissingBean.