啟動SpringBoot，需要在入口函數所在的類上添加@SpringBootApplication注解

1 @SpringBootApplication 2 public class Application { 3 public static void main(String[] args) { 4 SpringApplication.run(Application.class, args); 5 } 6 }

我們來看一下@SpringBootApplication注解

1 @Target(ElementType.TYPE)2 @Retention(RetentionPolicy.RUNTIME)3 @Documented4 @Inherited5 @SpringBootConfiguration6 @EnableAutoConfiguration7 @ComponentScan(excludeFilters = {8 @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),9 @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) }) 10 public @interface SpringBootApplication { 11 @AliasFor(annotation = EnableAutoConfiguration.class, attribute = "exclude") 12 Class<?>[] exclude() default {}; 13 14 @AliasFor(annotation = EnableAutoConfiguration.class, attribute = "excludeName") 15 String[] excludeName() default {}; 16 17 @AliasFor(annotation = ComponentScan.class, attribute = "basePackages") 18 String[] scanBasePackages() default {}; 19 20 @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses") 21 Class<?>[] scanBasePackageClasses() default {}; 22 }

1 @Target(ElementType.TYPE) 2 @Retention(RetentionPolicy.RUNTIME) 3 @Documented 4 @Configuration 5 public @interface SpringBootConfiguration { 6 }

從上面的代碼可以看出@SpringBootApplication = @Configuration + @EnableAutoConfiguration + @ComponentScan

@Configuration注解(可以理解為xml里面的<beans>標簽)，一般和@Bean注解(可以理解為xml里面的<bean>標簽)搭配使用。使用這2個注解可以創建一個配置類，示例如下

1 @Configuration2 public class Config {3 @Bean4 public People people() {5 People people = new People();6 people.setAge(26);7 people.setName("Dylan");8 people.setGender(1);9 return people; 10 } 11 }

可以替代xml配置文件里

1 <beans> 2 <bean id="people" class="com.dylan.java.beans.People"> 3 <property name="age" value="26"></property> 4 <property name="name" value="Dylan"></property> 5 <property name="gender" value="1"></property> 6 </bean> 7 </beans>

@EnableAutoConfiguration注解自動載入應用程序所需要的所有Bean，這依賴于SpringBoot在類路徑中的查找

1 @SuppressWarnings("deprecation")2 @Target(ElementType.TYPE)3 @Retention(RetentionPolicy.RUNTIME)4 @Documented5 @Inherited6 @AutoConfigurationPackage7 @Import(EnableAutoConfigurationImportSelector.class)8 public @interface EnableAutoConfiguration {9 String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration"; 10 11 Class<?>[] exclude() default {}; 12 13 String[] excludeName() default {}; 14 }

@EnableAutoConfiguration注解使用@Import注解導入EnableAutoConfigurationImportSelector類，此類繼承了AutoConfigurationImportSelector類，而AutoConfigurationImportSelector類的selectImports方法就是關鍵所在

1 @Override2 public String[] selectImports(AnnotationMetadata annotationMetadata) {3 if (!isEnabled(annotationMetadata)) {4 return NO_IMPORTS;5 }6 try {7 AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader8 .loadMetadata(this.beanClassLoader);9 AnnotationAttributes attributes = getAttributes(annotationMetadata); 10 List<String> configurations = getCandidateConfigurations(annotationMetadata, 11 attributes); 12 configurations = removeDuplicates(configurations); 13 configurations = sort(configurations, autoConfigurationMetadata); 14 Set<String> exclusions = getExclusions(annotationMetadata, attributes); 15 checkExcludedClasses(configurations, exclusions); 16 configurations.removeAll(exclusions); 17 configurations = filter(configurations, autoConfigurationMetadata); 18 fireAutoConfigurationImportEvents(configurations, exclusions); 19 return configurations.toArray(new String[configurations.size()]); 20 } 21 catch (IOException ex) { 22 throw new IllegalStateException(ex); 23 } 24 }

1 protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, 2 AnnotationAttributes attributes) { 3 List<String> configurations = SpringFactoriesLoader.loadFactoryNames( 4 getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader()); 5 Assert.notEmpty(configurations, 6 "No auto configuration classes found in META-INF/spring.factories. If you " 7 + "are using a custom packaging, make sure that file is correct."); 8 return configurations; 9 }

1 protected Class<?> getSpringFactoriesLoaderFactoryClass() { 2 return EnableAutoConfiguration.class; 3 }

可以看出，該方法使用了Spring Core包的SpringFactoriesLoader類的loadFactoryNames方法，該方法會查詢classpath下的JAR文件中包含的/META/spring.factories文件，從文件中讀取配置文件名(這里是org.springframework.boot.autoconfigure.EnableAutoConfiguration)的屬性，例如

在jar:file:/C:/Users/guiqingqing/.m2/repository/org/springframework/boot/spring-boot-autoconfigure/1.5.6.RELEASE/spring-boot-autoconfigure-1.5.6.RELEASE.jar!/META-INF/spring.factories文件找到如下配置

# 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

獲取自動配置的類之后調用removeDuplicates方法先去除重復，然后調用sort方法進行排序，接下來調用getExclusions方法獲取配置的exclude的類，比如@SpringBootApplication(exclude = {DataSourceAutoConfiguration.class})，然后這些exclude的類也會被排除

然后是調用filter方法，該方法決定哪些bean自動配置，哪些不配置。去classpath下的JAR文件中包含的/META/spring.factories文件里查詢org.springframework.boot.autoconfigure.AutoConfigurationImportFilter對應的配置，這里找到的是org.springframework.boot.autoconfigure.condition.OnClassCondition，檢查規則如下

經過上面的去重，排序，排除，(這里會有2個線程分別對前一半和后一半做檢查)遍歷所有剩下的class

首先看是否有@ConditionalOnClass注解，如果沒有那么不過濾，表示需要配置這個bean

如果有@ConditionalOnClass注解，再看這個注解依賴的類是否為空，如果為空，那么也不過濾，表示需要配置這個bean

如果@ConditionalOnClass注解依賴的類不為空，那么再看classpath下能不能找到這個依賴的類，如果能找到，那么也不過濾，表示需要配置這個bean，否則過濾，表示不配置這個bean

例如FreeMarkerAutoConfiguration類

1 @Configuration 2 @ConditionalOnClass({ freemarker.template.Configuration.class, FreeMarkerConfigurationFactory.class }) 4 @AutoConfigureAfter(WebMvcAutoConfiguration.class) 5 @EnableConfigurationProperties(FreeMarkerProperties.class) 6 public class FreeMarkerAutoConfiguration { 7 }

該類的@ConditionalOnClass注解里有freemarker.template.Configuration.class和FreeMarkerConfigurationFactory.class類，如果classpath下能找到這2個類，那么就會自動加載FreeMarkerAutoConfiguration類

最后去classpath下的JAR文件中包含的/META/spring.factories文件里查詢org.springframework.boot.autoconfigure.AutoConfigurationImportListener對應的屬性作為監聽器，然后發布一個AutoConfigurationImportEvent事件

最后交代一下，selectImports方法的調用時機，是在SpringBoot啟動時，AbstractApplicationContext類refresh方法 ->?invokeBeanFactoryPostProcessors方法 ->?PostProcessorRegistrationDelegate類的invokeBeanFactoryPostProcessors方法 ->?invokeBeanDefinitionRegistryPostProcessors方法 ->?ConfigurationClassPostProcessor類的postProcessBeanDefinitionRegistry方法 ->?processConfigBeanDefinitions方法 ->?ConfigurationClassParser類的parse方法 ->?processDeferredImportSelectors方法

1 private void processDeferredImportSelectors() {2 List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;3 this.deferredImportSelectors = null;4 Collections.sort(deferredImports, DEFERRED_IMPORT_COMPARATOR);5 6 for (DeferredImportSelectorHolder deferredImport : deferredImports) {7 ConfigurationClass configClass = deferredImport.getConfigurationClass();8 try {9 String[] imports = deferredImport.getImportSelector().selectImports(configClass.getMetadata()); 10 processImports(configClass, asSourceClass(configClass), asSourceClasses(imports), false); 11 } 12 catch (BeanDefinitionStoreException ex) { 13 throw ex; 14 } 15 catch (Throwable ex) { 16 throw new BeanDefinitionStoreException( 17 "Failed to process import candidates for configuration class [" + 18 configClass.getMetadata().getClassName() + "]", ex); 19 } 20 } 21 }

@ComponentScan注解會自動掃描指定包(如果不指定，那么默認掃描當前類路徑以及子路徑下的所有類)下的全部標有@Component(包括@Service、@Repository、@Controller等子注解)的類，并注冊成bean

總結

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