Springboot2教程：@SpringBootApplication自动配置

SpringBoot非常容易使用，它可以做很多事情，你可能没有意识到：将来，一个优秀的开发人员必须得知道spring boot自动配置的具体内容，如何使用它对您有利，以及如何禁用您不希望进入项目的某些部分。

要了解spring boot的大多数基本知识，我们将创建一个具有单一依赖关系和单个启动类文件的最小启动应用程序。然后，我们将分析启动日志来帮助了解它。

使用启动类创建Springboot应用程序

1、使用archtype“ maven-archetype-quickstart” 在eclipse中创建一个新的maven项目。

2、使用spring-boot-starter-web依赖项和插件信息更新pom.xml文件。

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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd;
4.0.0

com.howtodoinjava
springbootdemo
0.0.1-SNAPSHOT
jar

springbootdemo
http://maven.apache.org


    org.springframework.boot
    spring-boot-starter-parent
    2.0.0.RELEASE



    1.8
    UTF-8



    
        org.springframework.boot
        spring-boot-starter-web
    



    
        
            org.springframework.boot
            spring-boot-maven-plugin
        
    



    
        repository.spring.release
        Spring GA Repository
        http://repo.spring.io/release

3、创建启动应用程序

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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);
}
}
这个启动类的作用是什么？

上面的类称为spring boot应用程序启动类。它曾用于Bootstrap并从Java main()方法启动Spring应用程序。它通常会做以下事情：

创建Spring的ApplicationContext实例。
启用该功能用来接受命令行参数并将它们设置为Spring属性。
根据配置加载所有Spring bean。您可以根据项目需要执行其他操作。
@SpringBootApplication Annotation

此注释是在一个语句中应用3个注释的快捷方式：

@SpringBootConfiguration

@SpringBootConfiguration是Spring boot 2中的新注释。之前，我们一直在使用@Configuration注释。你可以用@Configuration它代替。两者都是一回事。

它表明一个类提供了Spring Boot应用程序@Configuration。它只是意味着带注释的类是一个配置类，应扫描其他配置和bean定义。

@EnableAutoConfiguration

此注释用于启用Spring Application Context的自动配置，尝试猜测和配置您可能需要的bean。Auto-configuration 类通常基于您的类路径以及您定义的bean来应用。

Auto-configuration尝试尽可能智能，并在您定义更多自己的配置时进行后退。您始终可以使用两种方法手动排除任何您不想应用的配置：

i）使用excludeName（）
ii）使用spring.autoconfigure.exclude属性文件中的属性。例如

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

在注册用户定义的bean之后，始终应用自动配置。

@ComponentScan

此批注提供了与Spring XML context:component-scan元素并行的支持。

无论是basePackageClasses()或basePackages()可以指定特定的软件包进行扫描。如果未定义特定包，则将从声明此注释的类的包进行扫描。

运行application应用程序并检查日志

让我们开始使用最简单的选项运行它：作为Java应用程序运行。

在IDE中，右键单击应用程序类并将其作为Java Application运行。为了深入了解已注册的bean，我添加了修改启动应用程序，如下所示。

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@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);
    }
}

}
现在看日志

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. ____ _ __ _ _
/\ / ' __ _ () __ __ _ \ \ \
( ( )__ | '_ | '| | ' / ` | \ \ \
\/ )| |)| | | | | || (| | ) ) ) )
' |____| .|| ||| |_, | / / / /
=========||==============|/=////
:: 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> 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.HttpMessageConvertersAutoConfigurationMappingJackson2HttpMessageConverterConfiguration
org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration
org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfigurationJackson2ObjectMapperBuilderCustomizerConfiguration
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfigurationJacksonObjectMapperConfiguration
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.DispatcherServletAutoConfigurationDispatcherServletRegistrationConfiguration
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.ServletWebServerFactoryConfigurationEnableWebMvcConfiguration
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfigurationWebMvcAutoConfigurationAdapterDefaultErrorViewResolverConfiguration
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfigurationTomcatWebSocketConfiguration
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
您会看到自动注册了多少个bean，这是springboot的魅力。如果你想更深入地了解任何一个部分如何注册？您可以通过在应用程序启动时放置调试标志来查看。

简单地传递-Ddebug=true为VM参数。

现在，当您运行应用程序时，您将获得许多具有类似信息的调试日志：

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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)

...
...
...
以上日志说明了为什么特定bean已注册到spring上下文中。使用自动配置调试问题时，此信息非常有用。

类似地，每次我们向Spring Boot项目添加新的依赖项时，Spring Boot自动配置会自动尝试根据依赖项配置bean。

我希望上面讨论的信息将在调试spring boot相关问题时帮助你。

Springboot2教程：@SpringBootApplication自动配置

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