参考： https://blog.csdn.net/Dongguabai/article/details/80865599。如有侵权，请联系本人删除！

入口：

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.netflix.eureka.server.EnableEurekaServer;

@SpringBootApplication
@EnableEurekaServer
public class SpringCloundEurekaServerApplication {

public static void main(String\[\] args) {  
    SpringApplication.run(SpringCloundEurekaServerApplication.class, args);  
}

这里有个很重要的注解：@SpringBootApplication。这是一个组合注解，点击进入可以看到详情如下：

@Target(ElementType.TYPE) //作用于类上面
@Retention(RetentionPolicy.RUNTIME) //作用域运行时
@Documented //doc文档
@Inherited //被继承
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
……
}

上面的组合注解也太多特别的，前面4个注解，就是4个元注解而已。主要关注其中的两个标红的注解。

先分析@SpringBootConfiguration，这也是一个组合注解，点击进去看到详情如下：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {

}

从上面可以看出，也就是组合了@Configuration，标志是配置类而已。

再看看@EnableAutoConfiguration，点击进入看到详情是：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

String ENABLED\_OVERRIDE\_PROPERTY = "spring.boot.enableautoconfiguration";

/\*\*  
 \* Exclude specific auto-configuration classes such that they will never be applied.  
 \* @return the classes to exclude  
 \*/  
Class<?>\[\] exclude() default {};

/\*\*  
 \* Exclude specific auto-configuration class names such that they will never be  
 \* applied.  
 \* @return the class names to exclude  
 \* @since 1.3.0  
 \*/  
String\[\] excludeName() default {};

}

其中，@AutoConfigurationPackage，这个注解组合情况如下：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

}

AutoConfigurationPackages.Registrar.class 这个类很关键。在我以前的博客Spring的@Enable*注解的工作原理，也写到，关于@Enablexxx 注解的三种实现，其中一种就是用到了这个类似于Registrar.class的

类。该类重写了两个方法，如下：

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

    @Override  
    public void registerBeanDefinitions(AnnotationMetadata metadata,            //spring初始化的时候，这个BeandEfinitions用得非常多  
            BeanDefinitionRegistry registry) {  
        register(registry, new PackageImport(metadata).getPackageName());  
    }

    @Override  
    public Set<Object> determineImports(AnnotationMetadata metadata) {  
        return Collections.singleton(new PackageImport(metadata));  
    }

}

单纯从代码还看不出什么，debug的时候，可以看到这个metadata 就是一个我们的启动类，截图如下：

 也就是说，这个注解的作用，就是扫描_com.drafire.springcloundeurekaserver._SpringCloundEurekaServerApplication 启动类所在的包以及子包，并注入到spring容器中
-------------------------------------------------------------------------------------------------------------------
下面再看下：AutoConfigurationImportSelector.class，源码如下：

、public class AutoConfigurationImportSelector
implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
BeanFactoryAware, EnvironmentAware, Ordered {

private static final AutoConfigurationEntry EMPTY\_ENTRY = new AutoConfigurationEntry();

private static final String\[\] NO\_IMPORTS = {};

private static final Log logger = LogFactory  
        .getLog(AutoConfigurationImportSelector.class);

private static final String PROPERTY\_NAME\_AUTOCONFIGURE\_EXCLUDE = "spring.autoconfigure.exclude";

private ConfigurableListableBeanFactory beanFactory;

private Environment environment;

private ClassLoader beanClassLoader;

private ResourceLoader resourceLoader;

@Override  
public String\[\] selectImports(AnnotationMetadata annotationMetadata) {  
    if (!isEnabled(annotationMetadata)) {  
        return NO\_IMPORTS;  
    }  
    AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader  
            .loadMetadata(this.beanClassLoader);  
    AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(  
            autoConfigurationMetadata, annotationMetadata);  
    return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());  
}

/\*\*  
 \* Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}  
 \* of the importing {@link Configuration @Configuration} class.  
 \* @param autoConfigurationMetadata the auto-configuration metadata  
 \* @param annotationMetadata the annotation metadata of the configuration class  
 \* @return the auto-configurations that should be imported  
 \*/  
protected AutoConfigurationEntry getAutoConfigurationEntry(  
        AutoConfigurationMetadata autoConfigurationMetadata,  
        AnnotationMetadata annotationMetadata) {  
    if (!isEnabled(annotationMetadata)) {  
        return EMPTY\_ENTRY;  
    }  
    AnnotationAttributes attributes = getAttributes(annotationMetadata);  
    List<String> configurations = getCandidateConfigurations(annotationMetadata,  
            attributes);  
    configurations = removeDuplicates(configurations);  
    Set<String> exclusions = getExclusions(annotationMetadata, attributes);  
    checkExcludedClasses(configurations, exclusions);  
    configurations.removeAll(exclusions);  
    configurations = filter(configurations, autoConfigurationMetadata);  
    fireAutoConfigurationImportEvents(configurations, exclusions);  
    return new AutoConfigurationEntry(configurations, exclusions);  
}  

………
}

这里有个非常重要的重写方法：selectImports，其实这个也是@Enablexxx的三种实现方法之一，具体参考我以前的博客Spring的@Enable*注解的工作原理。

一步一步跟踪，可以看到，最终是调用 getCandidateConfigurations(annotationMetadata,attributes)，跟踪进入可以看到，源代码如下：

protected List getCandidateConfigurations(AnnotationMetadata metadata,
AnnotationAttributes attributes) {
List configurations = SpringFactoriesLoader.loadFactoryNames(
getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
Assert.notEmpty(configurations,
"No auto configuration classes found in META-INF/spring.factories. If you "
+ "are using a custom packaging, make sure that file is correct.");
return configurations;
}

public static List loadFactoryNames(Class factoryClass, @Nullable ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}

private static Map> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap result = (MultiValueMap)cache.get(classLoader);
if (result != null) {
return result;
} else {
try {
Enumeration urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
LinkedMultiValueMap result = new LinkedMultiValueMap();

            while(urls.hasMoreElements()) {  
                URL url = (URL)urls.nextElement();  
                UrlResource resource = new UrlResource(url);  
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);  
                Iterator var6 = properties.entrySet().iterator();

                while(var6.hasNext()) {  
                    Entry<?, ?> entry = (Entry)var6.next();  
                    String factoryClassName = ((String)entry.getKey()).trim();  
                    String\[\] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());  
                    int var10 = var9.length;

                    for(int var11 = 0; var11 < var10; ++var11) {  
                        String factoryName = var9\[var11\];  
                        result.add(factoryClassName, factoryName.trim());  
                    }  
                }  
            }

            cache.put(classLoader, result);  
            return result;  
        } catch (IOException var13) {  
            throw new IllegalArgumentException("Unable to load factories from location \[META-INF/spring.factories\]", var13);  
        }  
    }  
}

从上面明显可以看出，classLoader最终是从 META-INF/spring.factories 中循环读取各种配置，并注入到spring 容器中。META-INF/spring.factories 在 spring-boot-autoconfigure-1.5.19.RELEASE.jar（各个版本号不一样）。看看spring.factories的详细内容，如下：

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

Application Listeners

org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

Auto Configuration Import Listeners

org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

Auto Configuration Import Filters

org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnClassCondition

Auto Configure

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\
org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\
org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\
org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

Failure analyzers

org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer

Template availability providers

org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.web.JspTemplateAvailabilityProvider

上面不仅仅是字符串，其实都是各个配置类的链接来的，可以按住ctrl+鼠标左键进入查看该配置类的详情。

这里面的配置类，包含很多，如果redis、rabbitmq、webmvc、OnClassCondition 等。通过这个配置，自动导入很多配置并生效。

------------------------------------------------------------------------------------------------------------------------
回顾整体流程，Springboot的启动，主要创建了配置环境(environment)、事件监听(listeners)、应用上下文(applicationContext)，并基于以上条件，
在容器中开始实例化我们需要的Bean，至此，通过SpringBoot启动的程序已经构造完成
而自动注入，则是通过Import(AutoConfigurationImportSelector.class)注解，通过一系列的操作，包括loadClassName、loadFactoryNames，将META-INF/spring.factories 的类信息，加载到容器（一个ConcurrentHashMap，以BeanDefineTion的封装形式）里面