Auto-configuration is one of the most important features of Spring Boot. In this section, we will take a quick peek behind the scenes to understand how Spring Boot auto-configuration works.

Most of the Spring Boot auto-configuration magic comes from spring-boot-autoconfigure-{version}.jar. When we start any Spring Boot applications, a number of beans get auto-configured. How does this happen?

The following screenshot shows an extract from spring.factories from spring-boot-autoconfigure-{version}.jar. We have filtered out some of the configuration in the interest of space:

The preceding list of auto-configuration classes is run whenever a Spring Boot application is launched. Let's take a quick look at one of them:

org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration.

Here's a small snippet:

@Configuration
@ConditionalOnWebApplication
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class,
WebMvcConfigurerAdapter.class })
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter(DispatcherServletAutoConfiguration.class)
public class WebMvcAutoConfiguration {

Some of the important points to note are as follows:

• @ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurerAdapter.class }) : This auto-configuration is enabled if any of the mentioned classes are in the classpath. When we add a web starter project, we bring in dependencies with all these classes. Hence, this auto-configuration will be enabled.
• @ConditionalOnMissingBean(WebMvcConfigurationSupport.class): This auto-configuration is enabled only if the application does not explicitly declare a bean of the WebMvcConfigurationSupport.class class.
• @AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10): This specifies the precedence of this specific auto-configuration.

Let's look at another small snippet showing one of the methods from the same class:

    @Bean
    @ConditionalOnBean(ViewResolver.class)
    @ConditionalOnMissingBean(name = "viewResolver", 
    value = ContentNegotiatingViewResolver.class)
    public ContentNegotiatingViewResolver 
    viewResolver(BeanFactory beanFactory) {
      ContentNegotiatingViewResolver resolver = new 
      ContentNegotiatingViewResolver();
      resolver.setContentNegotiationManager
      (beanFactory.getBean(ContentNegotiationManager.class));
      resolver.setOrder(Ordered.HIGHEST_PRECEDENCE);
      return resolver;
     }

View resolvers are one of the beans configured by WebMvcAutoConfiguration class. The preceding snippet ensures that if a view resolver is not provided by the application, then Spring Boot auto-configures a default view resolver. Here are a few important points to note:

• @ConditionalOnBean(ViewResolver.class): Create this bean if ViewResolver.class is on the classpath
• @ConditionalOnMissingBean(name = "viewResolver", value = ContentNegotiatingViewResolver.class): Create this bean if there are no explicitly declared beans of the name viewResolver and of type ContentNegotiatingViewResolver.class
• The rest of the method is configured in the view resolver

To summarize, all the auto-configuration logic is executed at the start of a Spring Boot application. If a specific class (from a specific dependency or starter project) is available on the classpath, then the auto configuration classes are executed. These auto-configuration classes look at what beans are already configured. Based on the existing beans, they enable the creation of the default beans.