浅谈springboot自动配置原理

时间：2022-04-26 00:20:14

前言

springboot自动配置关键在于@SpringBootApplication注解，启动类之所以作为项目启动的入口，也是因为该注解，下面浅谈下这个注解的作用和实现原理

@SpringBootApplication注解

查看源码@SpringBootApplication,由以下组成

@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 @interface SpringBootApplication{

    ...

}

前四个注解：是元注解，用来修饰当前注解，没有实际功能，实际上重要的是剩余三个注解

@ComponentScan注解

@ComponentScan(excludeFilters = {

		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),

		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
  • @ComponentScan的功能其实就是自动扫描加载符合条件的组件（@Component,@Service,@Repository）或者用@Bean修饰加入到IOC容器中的组件
  • 我们可以通过basePackages等属性定制@ComponentScan自动扫描的范围，如果不指定，则默认会从启动类所在包开始扫描

@SpringBootConfiguration注解

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Configuration

public @interface SpringBootConfiguration {

    ...

}

起作用的只有@Configuration，所以@SpringBootConfiguration功能和@Configuration一样，将被注解类作为配置类，并交给spring容器管理

@EnableAutoConfiguration注解

这是我们关注的重点

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@AutoConfigurationPackage

@Import(AutoConfigurationImportSelector.class)

public @interface EnableAutoConfiguration {

 ...

}

出去元注解，还剩@AutoConfigurationPackage和@Import(AutoConfigurationImportSelector.class)

  • @AutoConfigurationPackage
@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@Import(AutoConfigurationPackages.Registrar.class)

public @interface AutoConfigurationPackage {

	/**

	 * Base packages that should be registered with {@link AutoConfigurationPackages}.

	 * <p>

	 * Use {@link #basePackageClasses} for a type-safe alternative to String-based package

	 * names.

	 * @return the back package names

	 * @since 2.3.0

	 */

	String[] basePackages() default {};

	/**

	 * Type-safe alternative to {@link #basePackages} for specifying the packages to be

	 * registered with {@link AutoConfigurationPackages}.

	 * <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.

	 * @return the base package classes

	 * @since 2.3.0

	 */

	Class<?>[] basePackageClasses() default {};

}

起作用的是@Import(AutoConfigurationPackages.Registrar.class)，继续查看源码

	static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

		@Override

		public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {

			register(registry, new PackageImports(metadata).getPackageNames().toArray(new String[0]));

		}

		@Override

		public Set<Object> determineImports(AnnotationMetadata metadata) {

			return Collections.singleton(new PackageImports(metadata));

		}

	}

回想下，我们是从启动类的@SpringBootApplication开始，所以元注解metadata中的数据可以定位到该启动类，而new PackageImports(metadata).getPackageNames()结果就是启动类所在的包，即在此包下springboot才会扫描各种文件，从而达到自动配置的目的。这也是为什么文件不能放在启动类上级目录中

  • @Import(AutoConfigurationImportSelector.class)

AutoConfigurationImportSelector从字面意思看是自动配置导入选择器，即批量选择自动导入的内容，猜想自动配置的实现是在这

1、getAutoConfigurationEntry(annotationMetadata);给容器中批量导入一些组件



    public String[] selectImports(AnnotationMetadata annotationMetadata) {

		if (!isEnabled(annotationMetadata)) {

			return NO_IMPORTS;

		}

		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);

		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());

	}

2、调用List configurations = getCandidateConfigurations(annotationMetadata, attributes)获取到所有需要导入到容器中的配置类



	protected AutoConfigurationEntry getAutoConfigurationEntry(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 = getConfigurationClassFilter().filter(configurations);

		fireAutoConfigurationImportEvents(configurations, exclusions);

		return new AutoConfigurationEntry(configurations, exclusions);

	}

3、利用工厂加载 Map<String, List> loadSpringFactories(@Nullable ClassLoader classLoader)；得到所有的组件

	private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {

		Map<String, List<String>> result = cache.get(classLoader);

		if (result != null) {

			return result;

		}

	result = new HashMap<>();

	try {

        //FACTORIES_RESOURCE_LOCATION: META-INF/spring.factories

		Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);

		while (urls.hasMoreElements()) {

			URL url = urls.nextElement();

			UrlResource resource = new UrlResource(url);

			Properties properties = PropertiesLoaderUtils.loadProperties(resource);

			for (Map.Entry<?, ?> entry : properties.entrySet()) {

				String factoryTypeName = ((String) entry.getKey()).trim();

				String[] factoryImplementationNames =

						StringUtils.commaDelimitedListToStringArray((String) entry.getValue());

				for (String factoryImplementationName : factoryImplementationNames) {

					result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())

							.add(factoryImplementationName.trim());

				}

			}

		}

		// Replace all lists with unmodifiable lists containing unique elements

		result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()

				.collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));

		cache.put(classLoader, result);

	}

4、从META-INF/spring.factories位置来加载一个文件。

默认扫描我们当前系统里面所有META-INF/spring.factories位置的文件

spring-boot-autoconfigure-2.3.4.RELEASE.jar包里面也有META-INF/spring.factories

文件里面写死了spring-boot一启动就要给容器中加载的所有配置类

#文件里面写死了spring-boot一启动就要给容器中加载的所有配置类

#spring-boot-autoconfigure-2.3.4.RELEASE.jar/META-INF/spring.factories

# 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.context.ConfigurationPropertiesAutoConfiguration,\

org.springframework.boot.autoconfigure.context.LifecycleAutoConfiguration,\

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.CassandraReactiveDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRestClientAutoConfiguration,\

org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\

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.r2dbc.R2dbcDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.r2dbc.R2dbcRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.r2dbc.R2dbcTransactionManagerAutoConfiguration,\

org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\

org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\

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.ElasticsearchRestClientAutoConfiguration,\

org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\

org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\

org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\

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.http.HttpMessageConvertersAutoConfiguration,\

org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\

org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\

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.jersey.JerseyAutoConfiguration,\

org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\

org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\

org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\

org.springframework.boot.autoconfigure.availability.ApplicationAvailabilityAutoConfiguration,\

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.mongo.embedded.EmbeddedMongoAutoConfiguration,\

org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\

org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\

org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\

org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\

org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\

org.springframework.boot.autoconfigure.r2dbc.R2dbcAutoConfiguration,\

org.springframework.boot.autoconfigure.rsocket.RSocketMessagingAutoConfiguration,\

org.springframework.boot.autoconfigure.rsocket.RSocketRequesterAutoConfiguration,\

org.springframework.boot.autoconfigure.rsocket.RSocketServerAutoConfiguration,\

org.springframework.boot.autoconfigure.rsocket.RSocketStrategiesAutoConfiguration,\

org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\

org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\

org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\

org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\

org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\

org.springframework.boot.autoconfigure.security.rsocket.RSocketSecurityAutoConfiguration,\

org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyAutoConfiguration,\

org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\

org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\

org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\

org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\

org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\

org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\

org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\

org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\

org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\

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.client.RestTemplateAutoConfiguration,\

org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\

org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\

org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\

org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\

org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\

org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\

org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\

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.reactive.WebSocketReactiveAutoConfiguration,\

org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\

org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\

org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\

org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

按需开启自动配置项

虽然我们127个场景的所有自动配置启动的时候默认全部加载。但xxxxAutoConfiguration按照条件装配规则（@Conditional），最终会按需配置。

需要注意的是，SpringBoot默认会在底层配好所有的组件。但是如果用户自己配置了以用户的优先

@Bean

@ConditionalOnMissingBean

public CharacterEncodingFilter characterEncodingFilter() {

}

总结:

  • SpringBoot先加载所有的自动配置类 xxxxxAutoConfiguration

  • 每个自动配置类按照条件进行生效，默认都会绑定配置文件指定的值。xxxxProperties里面拿。xxxProperties和配置文件进行了绑定

  • 生效的配置类就会给容器中装配很多组件，只要容器中有这些组件，这些功能就有了

  • 定制化配置

    • 用户直接自己@Bean替换底层的组件
    • 用户去看这个组件是获取的配置文件什么值就去修改。

自动配置类xxxxxAutoConfiguration ---> 组件 ---> xxxxProperties里面拿值 ----> application.properties

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