Spring Boot之自动配置

1、原理

(1)、SpringBoot启动的时候会加载主配置类，主配置类中已经开启了自动配置功能**@SpringBootApplication注解中包含了@EnableAutoConfiguration**

(2)、@EnableAutoConfiguration的作用

利用EnableAutoConfigurationImportSelector给容器中导入符合条件的组件信息
查看EnableAutoConfigurationImportSelector类中的selectImports()方法的内容信息

List configurations = getCandidateConfigurations(metadata,attributes);获取候选的配置

SpringFactoriesLoader.loadFactoryNames()
扫描所有jar包类路径下 META‐INF/spring.factories
把扫描到的这些文件的内容包装成properties对象
从properties中获取到EnableAutoConfiguration.class类（类名）对应的值，然后把他们添加在容器中

将类路径下META-INF/spring.factories里面配置的所有EnableAutoConfiguration的值加入到容器中：

# 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.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration,\
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.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.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.jms.hornetq.HornetQAutoConfiguration,\
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.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.velocity.VelocityAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
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

每一个XXXAutoConfiguration类都是容器中的一个组件，都加入到容器中；使用他们实现具体功能的自动配置

(3)、每一个这样的自动配置类进行自动配置功能；

(4)、以某个自动配置类来说明自动配置的原理，以HttpEncodingAutoConfiguration（Http编码自动配置）为例：

@Configuration //表示这是一个配置类，跟以前编写的配置文件类似，也可以给容器中添加组件
@EnableConfigurationProperties(HttpEncodingProperties.class)//启动指定类的ConfigurationProperties功能；将配置文件中对应的值和HttpEncodingProperties绑定起来；并把HttpEncodingProperties加入到IOC容器中
@ConditionalOnWebApplication // Spring底层@Conditional注解，根据不同的条件，如果满足指定的条件，整个配置类里面的配置就会生效； 该注解的意思是判断当前应用是否是web应用，如果是，当前配置类生效@ConditionalOnClass(CharacterEncodingFilter.class)//判断当前项目有没有这个类CharacterEncodingFilter；SpringMVC中进行乱码解决的过滤器
@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)//判断配置文件中是否存在某个配置，spring.http.encoding.enabled；如果不存在，判断也是成立的；即使我们的配置文件中不配置spring.http.encoding.ebabled=true，也是默认生效的public class HttpEncodingAutoConfiguration {//已经和SpringBoot的配置文件进行了映射private final HttpEncodingProperties properties;//只有一个有参构造方法的情况下，参数的值就会从容器中获取public HttpEncodingAutoConfiguration(HttpEncodingProperties properties) {this.properties = properties;}@Bean//给容器中添加一个组件，这个组件的某些值需要从properties中获取@ConditionalOnMissingBean(CharacterEncodingFilter.class)//判断容器中没有这个组件（OnMissing）public CharacterEncodingFilter characterEncodingFilter() {CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();filter.setEncoding(this.properties.getCharset().name());filter.setForceRequestEncoding(this.properties.shouldForce(Type.REQUEST));filter.setForceResponseEncoding(this.properties.shouldForce(Type.RESPONSE));return filter;}@Beanpublic LocaleCharsetMappingsCustomizer localeCharsetMappingsCustomizer() {return new LocaleCharsetMappingsCustomizer(this.properties);}private static class LocaleCharsetMappingsCustomizer implements EmbeddedServletContainerCustomizer, Ordered {private final HttpEncodingProperties properties;LocaleCharsetMappingsCustomizer(HttpEncodingProperties properties) {this.properties = properties;}@Overridepublic void customize(ConfigurableEmbeddedServletContainer container) {if (this.properties.getMapping() != null) {container.setLocaleCharsetMappings(this.properties.getMapping());}}@Overridepublic int getOrder() {return 0;}}
}

根据当前不同的条件判断，决定这个配置类是否生效？

一旦这个配置类生效；这个配置类就会给容器中添加各种组件；这些组件的属性是从对应的properties类中获取的，这些类里面的每一个属性又跟配置文件绑定的。

(5)、xxxProperties类中封装了所有能在配置文件中能配置的属性；配置文件能配置什么就可以参照某个功能对应的这个属性类

@ConfigurationProperties(prefix = "spring.http.encoding")//从配置文件中获取指定的值和bean的属性进行绑定
public class HttpEncodingProperties {public static final Charset DEFAULT_CHARSET = Charset.forName("UTF-8");private Charset charset = DEFAULT_CHARSET;private Boolean force;private Boolean forceRequest;private Boolean forceResponse;private Map<Locale, Charset> mapping;
}

(6)、配置文件application-dev.properties中关于Http编码的配置信息如下：

spring.http.encoding.charset=utf-8
spring.http.encoding.enabled=true
spring.http.encoding.force=true
spring.http.encoding.force-request=true
spring.http.encoding.force-response=true

2、@Conditional派生注解

@Conditional相关注解的作用是：必须是@Conditional指定的条件成立，才给容器中添加组件，配置类里面的所有内容才生效

@Conditional扩展注解	作用（判断是否满足当前指定条件）
@ConditionalOnJava	系统地java版本是否符合要求
@ConditionalOnBean	容器中存在指定的Bean
@ConditionalOnMissingBean	容器中不存在指定的Bean
@ConditionalOnExpression	满足SpEL表达式指定
@ConditionalOnClass	系统中有指定的类
@ConditionalOnMissingClass	系统中没有指定的类
@ConditionalOnSingleCandidate	容器中只有一个指定的Bean，或者这个Bean是首选Bean
@ConditionalOnProperty	系统中指定的属性是否有指定的值
@ConditionalOnResource	类路径下是否存在指定资源文件
@ConditionalOnWebApplication	当前是Web环境
@ConditionalOnNotWebApplication	当前不是Web环境
@ConditionalOnJndi	JNDI存在指定项

自动配置类必须在一定的条件下才能生效

3、默认自动配置项

如何查看SpringBoot工程默认是哪些自动配置类生效呢？
可以通过启用debug=true属性，来让控制台打印自动配置报告的内容。
默认自动配置报告如下所示：

============================
CONDITIONS EVALUATION REPORT
============================Positive matches:（自动配置类启用的）
-----------------AopAutoConfiguration matched:- @ConditionalOnProperty (spring.aop.auto=true) matched (OnPropertyCondition)AopAutoConfiguration.ClassProxyingConfiguration matched:- @ConditionalOnMissingClass did not find unwanted class 'org.aspectj.weaver.Advice' (OnClassCondition)- @ConditionalOnProperty (spring.aop.proxy-target-class=true) matched (OnPropertyCondition)GenericCacheConfiguration matched:- Cache org.springframework.boot.autoconfigure.cache.GenericCacheConfiguration automatic cache type (CacheCondition)JmxAutoConfiguration matched:- @ConditionalOnClass found required class 'org.springframework.jmx.export.MBeanExporter' (OnClassCondition)- @ConditionalOnProperty (spring.jmx.enabled=true) matched (OnPropertyCondition)JmxAutoConfiguration#mbeanExporter matched:- @ConditionalOnMissingBean (types: org.springframework.jmx.export.MBeanExporter; SearchStrategy: current) did not find any beans (OnBeanCondition)JmxAutoConfiguration#mbeanServer matched:- @ConditionalOnMissingBean (types: javax.management.MBeanServer; SearchStrategy: all) did not find any beans (OnBeanCondition)……Negative matches:（没有启动，没有匹配成功的自动配置类）
-----------------ActiveMQAutoConfiguration:Did not match:- @ConditionalOnClass did not find required class 'javax.jms.ConnectionFactory' (OnClassCondition)AopAutoConfiguration.AspectJAutoProxyingConfiguration:Did not match:- @ConditionalOnClass did not find required class 'org.aspectj.weaver.Advice' (OnClassCondition)ArtemisAutoConfiguration:Did not match:- @ConditionalOnClass did not find required class 'javax.jms.ConnectionFactory' (OnClassCondition)BatchAutoConfiguration:Did not match:- @ConditionalOnClass did not find required class 'org.springframework.batch.core.launch.JobLauncher' (OnClassCondition)

4、小结

SpringBoot启动会加载大量的自动配置类
我们看到需要的功能有没有SpringBoot默认写好的自动配置类；
再来看这个自动配置类中到底配置了哪些组件；（只要我们计划要使用的组件有，我们就不需要再来配置了）
给容器中自动配置类添加组件的时候，会从properties类中获取某些属性。我们就可以在配置文件中指定这个属性的值；
xxxxAutoConfiguration：自动配置类；给容器中添加组件
xxxxProperties：封装配置文件中相关属性