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.

Happy Learning !!

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