Spring Boot2 @SpringBootApplication Auto Configuration

 Spring boot is very easy to use and it does a lot of things under the hood, you might not be aware of. In future, a good developer will be who will know exactly what is going on behind spring boot auto configuration, how to use it in your favor and how to disable certain sections which you do not want into your project.

To understand most basic things behind spring boot, we will create a minimum boot application with single dependency and single launch class file. We will then analyze the startup logs to get the insights.

Create Spring boot application with launch class

1. Create a new maven project in eclipse with archetype “maven-archetype-quickstart“.
2. Update pom.xml file with spring-boot-starter-web dependency and plugin information.

   <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd;     <modelVersion>4.0.0</modelVersion>       <groupId>com.howtodoinjava</groupId>     <artifactId>springbootdemo</artifactId>     <version>0.0.1-SNAPSHOT</version>     <packaging>jar</packaging>       <name>springbootdemo</name>     <url>http://maven.apache.org</url>       <parent>         <groupId>org.springframework.boot</groupId>         <artifactId>spring-boot-starter-parent</artifactId>         <version>2.0.0.RELEASE</version>     </parent>       <properties>         <java.version>1.8</java.version>         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>     </properties>       <dependencies>         <dependency>             <groupId>org.springframework.boot</groupId>             <artifactId>spring-boot-starter-web</artifactId>         </dependency>     </dependencies>       <build>         <plugins>             <plugin>                 <groupId>org.springframework.boot</groupId>                 <artifactId>spring-boot-maven-plugin</artifactId>             </plugin>         </plugins>     </build>       <repositories>         <repository>             <id>repository.spring.release</id>             <name>Spring GA Repository</name>             <url>http://repo.spring.io/release</url>         </repository>     </repositories> </project>

3. Create launch application.

   package com.howtodoinjava.demo;   import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; import org.springframework.context.ApplicationContext;   @SpringBootApplication public class App {     public static void main(String[] args)     {         ApplicationContext ctx = SpringApplication.run(App.class, args);     } }

   What this launch class does?

   Above class is called spring boot application launch class. It used to Bootstrap and launch a Spring application from a Java main() method. It typically does following things –

   • Create an instance of Spring’s ApplicationContext.
   • Enable the functionality to accept command-line arguments and expose them as Spring properties.
   • Load all the Spring beans as per the configuration. You can do other operations as well as per project need arises.

@SpringBootApplication Annotation

This annotation is a shortcut of applying 3 annotations in one statement –

1. @SpringBootConfiguration

   @SpringBootConfiguration is new annotation in Spring boot 2. Previously, we have been using @Configuration annotation. You can use @Configuration in place of this. Both are same thing.

   It indicates that a class provides Spring Boot application @Configuration. It simply means that annotated class is a configuration class and shall be scanned for further configurations and bean definitions.

2. @EnableAutoConfiguration

   This annotation is used to 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.

   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 using two methods –

   i) Use excludeName()
   ii) Using the spring.autoconfigure.exclude property in properties file. e.g.

   @EnableAutoConfiguration(excludeName = {"multipartResolver","mbeanServer"})

   Auto-configuration is always applied after user-defined beans have been registered.

3. @ComponentScan

   This annotation provides support parallel with Spring XML’s context:component-scan element.

   Either basePackageClasses() or basePackages() may be specified to define specific packages to scan. If specific packages are not defined, scanning will occur from the package of the class that declares this annotation.

Run the launch application and check logs

Let’s start running it with the simplest option–running as a Java application. In your IDE, right-click on the application class and run it as Java Application. For getting insight of registered beans, I have added modified the launch application as below.

@SpringBootApplication public class App {     public static void main(String[] args)     {         ApplicationContext ctx = SpringApplication.run(App.class, args);                   String[] beanNames = ctx.getBeanDefinitionNames();                   Arrays.sort(beanNames);           for (String beanName : beanNames) {             System.out.println(beanName);         }     } }

Now see the logs –

.   ____          _            __ _ _  /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \ ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \  \\/  ___)| |_)| | | | | || (_| |  ) ) ) )   '  |____| .__|_| |_|_| |_\__, | / / / /  =========|_|==============|___/=/_/_/_/  :: Spring Boot ::        (v2.0.0.RELEASE)   2018-04-02 13:09:41.100  INFO 11452 --- [           main] com.howtodoinjava.demo.App               : Starting App on FFC15B4E9C5AA with PID 11452 (C:\Users\zkpkhua\IDPPaymentTransfers_Integrated\springbootdemo\target\classes started by zkpkhua in C:\Users\zkpkhua\IDPPaymentTransfers_Integrated\springbootdemo) 2018-04-02 13:09:41.108  INFO 11452 --- [           main] com.howtodoinjava.demo.App               : No active profile set, falling back to default profiles: default 2018-04-02 13:09:41.222  INFO 11452 --- [           main] ConfigServletWebServerApplicationContext : Refreshing org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@4450d156: startup date [Mon Apr 02 13:09:41 IST 2018]; root of context hierarchy 2018-04-02 13:09:43.474  INFO 11452 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http) 2018-04-02 13:09:43.526  INFO 11452 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat] 2018-04-02 13:09:43.526  INFO 11452 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/8.5.28 2018-04-02 13:09:43.748  INFO 11452 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext 2018-04-02 13:09:43.748  INFO 11452 --- [ost-startStop-1] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 2531 ms 2018-04-02 13:09:43.964  INFO 11452 --- [ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean  : Servlet dispatcherServlet mapped to [/] 2018-04-02 13:09:43.969  INFO 11452 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'characterEncodingFilter' to: [/*] 2018-04-02 13:09:43.970  INFO 11452 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'hiddenHttpMethodFilter' to: [/*] 2018-04-02 13:09:43.970  INFO 11452 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'httpPutFormContentFilter' to: [/*] 2018-04-02 13:09:43.970  INFO 11452 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'requestContextFilter' to: [/*] 2018-04-02 13:09:44.480  INFO 11452 --- [           main] s.w.s.m.m.a.RequestMappingHandlerAdapter : Looking for @ControllerAdvice: org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@4450d156: startup date [Mon Apr 02 13:09:41 IST 2018]; root of context hierarchy 2018-04-02 13:09:44.627  INFO 11452 --- [           main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error]}" onto public org.springframework.http.ResponseEntity<java.util.Map<java.lang.String, java.lang.Object>> org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest) 2018-04-02 13:09:44.630  INFO 11452 --- [           main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error],produces=}" onto public org.springframework.web.servlet.ModelAndView org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.errorHtml(javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse) 2018-04-02 13:09:44.681  INFO 11452 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/webjars/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler] 2018-04-02 13:09:44.682  INFO 11452 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler] 2018-04-02 13:09:44.747  INFO 11452 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/**/favicon.ico] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler] 2018-04-02 13:09:45.002  INFO 11452 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup 2018-04-02 13:09:45.070  INFO 11452 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path '' 2018-04-02 13:09:45.076  INFO 11452 --- [           main] com.howtodoinjava.demo.App               : Started App in 4.609 seconds (JVM running for 5.263) app basicErrorController beanNameHandlerMapping beanNameViewResolver characterEncodingFilter conventionErrorViewResolver defaultServletHandlerMapping defaultValidator defaultViewResolver dispatcherServlet dispatcherServletRegistration error errorAttributes errorPageCustomizer errorPageRegistrarBeanPostProcessor faviconHandlerMapping faviconRequestHandler handlerExceptionResolver hiddenHttpMethodFilter httpPutFormContentFilter httpRequestHandlerAdapter jacksonCodecCustomizer jacksonObjectMapper jacksonObjectMapperBuilder jsonComponentModule localeCharsetMappingsCustomizer mappingJackson2HttpMessageConverter mbeanExporter mbeanServer messageConverters methodValidationPostProcessor multipartConfigElement multipartResolver mvcContentNegotiationManager mvcConversionService mvcHandlerMappingIntrospector mvcPathMatcher mvcResourceUrlProvider mvcUriComponentsContributor mvcUrlPathHelper mvcValidator mvcViewResolver objectNamingStrategy org.springframework.boot.autoconfigure.AutoConfigurationPackages org.springframework.boot.autoconfigure.condition.BeanTypeRegistry org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration$StringHttpMessageConverterConfiguration org.springframework.boot.autoconfigure.http.JacksonHttpMessageConvertersConfiguration org.springframework.boot.autoconfigure.http.JacksonHttpMessageConvertersConfiguration$MappingJackson2HttpMessageConverterConfiguration org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration$JacksonCodecConfiguration org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration org.springframework.boot.autoconfigure.internalCachingMetadataReaderFactory org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$Jackson2ObjectMapperBuilderCustomizerConfiguration org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$JacksonObjectMapperBuilderConfiguration org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$JacksonObjectMapperConfiguration org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$ParameterNamesModuleConfiguration org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration$TomcatWebServerFactoryCustomizerConfiguration org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration$DispatcherServletConfiguration org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration$DispatcherServletRegistrationConfiguration org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryConfiguration$EmbeddedTomcat org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$EnableWebMvcConfiguration org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$WebMvcAutoConfigurationAdapter org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$WebMvcAutoConfigurationAdapter$FaviconConfiguration org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration$DefaultErrorViewResolverConfiguration org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration$WhitelabelErrorViewConfiguration org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration$TomcatWebSocketConfiguration org.springframework.boot.context.properties.ConfigurationBeanFactoryMetadata org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor org.springframework.context.annotation.internalAutowiredAnnotationProcessor org.springframework.context.annotation.internalCommonAnnotationProcessor org.springframework.context.annotation.internalConfigurationAnnotationProcessor org.springframework.context.annotation.internalRequiredAnnotationProcessor org.springframework.context.event.internalEventListenerFactory org.springframework.context.event.internalEventListenerProcessor parameterNamesModule preserveErrorControllerTargetClassPostProcessor propertySourcesPlaceholderConfigurer requestContextFilter requestMappingHandlerAdapter requestMappingHandlerMapping resourceHandlerMapping restTemplateBuilder server-org.springframework.boot.autoconfigure.web.ServerProperties servletWebServerFactoryCustomizer simpleControllerHandlerAdapter spring.http.encoding-org.springframework.boot.autoconfigure.http.HttpEncodingProperties spring.info-org.springframework.boot.autoconfigure.info.ProjectInfoProperties spring.jackson-org.springframework.boot.autoconfigure.jackson.JacksonProperties spring.mvc-org.springframework.boot.autoconfigure.web.servlet.WebMvcProperties spring.resources-org.springframework.boot.autoconfigure.web.ResourceProperties spring.security-org.springframework.boot.autoconfigure.security.SecurityProperties spring.servlet.multipart-org.springframework.boot.autoconfigure.web.servlet.MultipartProperties standardJacksonObjectMapperBuilderCustomizer stringHttpMessageConverter tomcatServletWebServerFactory tomcatServletWebServerFactoryCustomizer tomcatWebServerFactoryCustomizer viewControllerHandlerMapping viewResolver webServerFactoryCustomizerBeanPostProcessor websocketContainerCustomizer welcomePageHandlerMapping

You see how many beans got registered automatically. That’s beauty of spring boot. If you want to dig deeper into why any particular bean got registered? You can see that by putting a debug flag at application startup.

Simply pass -Ddebug=true as VM argument.

Now when you run the application, you will get lots of debug logs having similar information :

CodecsAutoConfiguration.JacksonCodecConfiguration matched:   - @ConditionalOnClass found required class 'com.fasterxml.jackson.databind.ObjectMapper'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)   CodecsAutoConfiguration.JacksonCodecConfiguration#jacksonCodecCustomizer matched:   - @ConditionalOnBean (types: com.fasterxml.jackson.databind.ObjectMapper; SearchStrategy: all) found bean 'jacksonObjectMapper' (OnBeanCondition)   DispatcherServletAutoConfiguration.DispatcherServletConfiguration matched:   - @ConditionalOnClass found required class 'javax.servlet.ServletRegistration'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)   - Default DispatcherServlet did not find dispatcher servlet beans (DispatcherServletAutoConfiguration.DefaultDispatcherServletCondition)   DispatcherServletAutoConfiguration.DispatcherServletRegistrationConfiguration matched:   - @ConditionalOnClass found required class 'javax.servlet.ServletRegistration'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)   - DispatcherServlet Registration did not find servlet registration bean (DispatcherServletAutoConfiguration.DispatcherServletRegistrationCondition)     ...   ...   ...

Above logs tell why a particular bean was registered into spring context. This information is very useful when you debug the issues with auto configutation.

Similarily, everytime we add a new dependency to a Spring Boot project, Spring Boot auto-configuration automatically tries to configure the beans based on the dependency.

I hope that information discussed above will help you in future while debugging spring boot related issues.