通常我们启动一个springboot项目会在启动方法中增加@SpringBootApplicatoin注解，该注解中包含了@EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters={@Filter(type= FilterType.CUSTOM, classes = TypeExcludeFilter.class),@Filter(type= FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })public @interfaceSpringBootApplication {/*** Exclude specific auto-configuration classes such that they will never be applied.*@returnthe classes to exclude*/@AliasFor(annotation= EnableAutoConfiguration.class)Class<?>[] exclude() default{};/*** Exclude specific auto-configuration class names such that they will never be* applied.*@returnthe class names to exclude*@since1.3.0*/@AliasFor(annotation= EnableAutoConfiguration.class)String[] excludeName()default{};/*** Base packages to scan for annotated components. Use {@link#scanBasePackageClasses}* for a type-safe alternative to String-based package names.*@returnbase packages to scan*@since1.3.0*/@AliasFor(annotation= ComponentScan.class, attribute = "basePackages")String[] scanBasePackages()default{};/*** Type-safe alternative to {@link#scanBasePackages} for specifying the packages to* scan for annotated components. The package of each class specified will be scanned.* <p>* Consider creating a special no-op marker class or interface in each package that* serves no purpose other than being referenced by this attribute.*@returnbase packages to scan*@since1.3.0*/@AliasFor(annotation= ComponentScan.class, attribute = "basePackageClasses")Class<?>[] scanBasePackageClasses() default{};}

那么@EnableAutoConfiguration是如何自动起作用的呢。我们先来看看@EnableAutoConfiguration的定义

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)public @interfaceEnableAutoConfiguration {String ENABLED_OVERRIDE_PROPERTY= "spring.boot.enableautoconfiguration";/*** Exclude specific auto-configuration classes such that they will never be applied.*@returnthe classes to exclude*/Class<?>[] exclude() default{};/*** Exclude specific auto-configuration class names such that they will never be* applied.*@returnthe class names to exclude*@since1.3.0*/String[] excludeName()default{};}

我们看看EnableAutoConfiguration的注释定义:

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.

大概的意思是:容器尝试猜测你想配置的bean，然后自动加载。

很欣喜的发现了@EnableAutoConfiguration里面引用了@Import(AutoConfigurationImportSelector.class)，直觉看起来像这个AutoConfigurationImportSelector里面引入了我们那些自动配置(数据源、拦截器)的类。

我们来看看AutoConfigurationImportSelector的定义:

public classAutoConfigurationImportSelectorimplementsDeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
BeanFactoryAware, EnvironmentAware, Ordered

原来AutoConfigurationImportSelector类实现了org.springframework.context.annotation.DeferredImportSelector

那么实现了该类的public String[] selectImports(AnnotationMetadata annotationMetadata)后，spring在解析bean的时候，就会自动调用该方法来加载拓展类，也就是selectImports方法返回的名称列表来加载类。

很容易起想到selectImports里面就返回了AutoEnableConfiguration的那些数据源、拦截器配置的类。！

接下来看看selectImports的具体实现:

@OverridepublicString[] selectImports(AnnotationMetadata annotationMetadata) {if (!isEnabled(annotationMetadata)) {returnNO_IMPORTS;}        //从org.springframework.boot.autoconfigure.AutoConfigurationMetadataLoader的loadMetadata加载要自动配置的资源数据

AutoConfigurationMetadata autoConfigurationMetadata =AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);AutoConfigurationEntry autoConfigurationEntry=getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);returnStringUtils.toStringArray(autoConfigurationEntry.getConfigurations());}

看下AutoConfigurationMetadataLoader的源码看，可以看到默认加载的自动配置的路径为:/META-INF/spring-autoconfigure-metadata.properteis。

final classAutoConfigurationMetadataLoader {//默认加载自动配置的文件位置protected static final String PATH = "META-INF/"+ "spring-autoconfigure-metadata.properties";privateAutoConfigurationMetadataLoader() {}public staticAutoConfigurationMetadata loadMetadata(ClassLoader classLoader) {returnloadMetadata(classLoader, PATH);}staticAutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) {try{Enumeration<URL> urls = (classLoader != null) ?classLoader.getResources(path): ClassLoader.getSystemResources(path);Properties properties= newProperties();while(urls.hasMoreElements()) {properties.putAll(PropertiesLoaderUtils.loadProperties(newUrlResource(urls.nextElement())));}returnloadMetadata(properties);}catch(IOException ex) {throw newIllegalArgumentException("Unable to load @ConditionalOnClass location [" + path + "]", ex);}}staticAutoConfigurationMetadata loadMetadata(Properties properties) {return newPropertiesAutoConfigurationMetadata(properties);}/*** {@linkAutoConfigurationMetadata} implementation backed by a properties file.*/private static classPropertiesAutoConfigurationMetadataimplementsAutoConfigurationMetadata {private finalProperties properties;PropertiesAutoConfigurationMetadata(Properties properties) {this.properties =properties;}@Overridepublic booleanwasProcessed(String className) {return this.properties.containsKey(className);}@OverridepublicInteger getInteger(String className, String key) {return getInteger(className, key, null);}@OverridepublicInteger getInteger(String className, String key, Integer defaultValue) {String value=get(className, key);return (value != null) ?Integer.valueOf(value) : defaultValue;}@Overridepublic Set<String>getSet(String className, String key) {return getSet(className, key, null);}@Overridepublic Set<String>getSet(String className, String key,Set<String>defaultValue) {String value=get(className, key);return (value != null) ?StringUtils.commaDelimitedListToSet(value): defaultValue;}@OverridepublicString get(String className, String key) {return get(className, key, null);}@OverridepublicString get(String className, String key, String defaultValue) {String value= this.properties.getProperty(className + "." +key);return (value != null) ?value : defaultValue;}}}

可以看到配置的默认加载的配载类有如下,这些类都被@Confiration注解，并且都自动创建了一些@Bean来使用组件的特性，通过这些自动创建的bean来代替我们系统中手动声明bean,达到加快开发效率的效果。

[org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration, org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration, org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration, org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration, org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration, org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration, org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration, org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration, org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration, org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration, org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration, org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration, 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.servlet.DispatcherServletAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration, org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration]

至此，springboot自动加载配置的原理也分析完毕。