spring boot 自动装配的原理
Posted drafire
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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
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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<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { List<String> 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<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) { String factoryClassName = factoryClass.getName(); return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList()); }
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) { MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader); if (result != null) { return result; } else { try { Enumeration<URL> 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。
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