SpringBoot自动配置探究

@SpringBootApplication

• @SpringBootApplication表示SpringBoot应用，标注在某个类上说明这个类是SpringBoot的主配置类，SpringBoot就运行这个类的main方法来启动应用；
• @SpringBootApplication是组合注解

  @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 {
  ........
  }

  • @SpringBootConfiguration：表示是一个SpringBoot的配置类

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

    • @Configuration：Spring定义的注解，标注在类上表示这是个配置类，使用注解定义配置类来取代配置文件；配置类也是容器中的一个组件

      @Target({ElementType.TYPE})
      @Retention(RetentionPolicy.RUNTIME)
      @Documented
      @Component
      public @interface Configuration {
      @AliasFor(
      annotation = Component.class
      )
      String value() default "";
      }

  • @EnableAutoConfiguration：开启自动配置功能

    @AutoConfigurationPackage
    @Import(AutoConfigurationImportSelector.class)
    public @interface EnableAutoConfiguration {
    .......
    }

    • @AutoConfigurationPackage：自动配置包；Spring的底层注解@Import，表示给容器导入一个组件，

      @Import(AutoConfigurationPackages.Registrar.class)
      public @interface AutoConfigurationPackage {}

      • Registrar.class：new PackageImports(metadata).getPackageNames().toArray(new String[0])获得注解类@SpringBootApplication所在的包名，把包下面的所有类都加入到Spring容器中

        @Override
        public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
        register(registry, new PackageImports(metadata).getPackageNames().toArray(new String[0]));
        }

    • AutoConfigurationImportSelector.class：给容器导入组件的自动配置类，并配置好这些组件，有了自动配置类，就免去了手动编写配置注入功能组件等操作；

      @Override
      public String[] selectImports(AnnotationMetadata annotationMetadata) {
      if (!isEnabled(annotationMetadata)) {
      return NO_IMPORTS;
      }
      AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
      return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
      }
      protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
      if (!isEnabled(annotationMetadata)) {
      return EMPTY_ENTRY;
      }
      AnnotationAttributes attributes = getAttributes(annotationMetadata);
      //获得所有组件的自动配置类，包括mq的RabbitAutoConfiguration、redis的RedisAutoConfiguration等等
      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);
      }
      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 final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
      //loadFactoryNames调用了loadSpringFactories(),它加载了spring.factories配置的信息，根据Map键值对获取信息，然后导入到容器中，在spring-boot-autoconfigure-2.3.0.RELEASE.jar中定义了文件
      public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
      String factoryTypeName = factoryType.getName();
      return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
      }

      • 比如MVC的自动配置，就使用了WebMvcAutoConfiguration类，下面的代码就是给容器添加了视图解析器

        @Bean
        @ConditionalOnBean(View.class)
        @ConditionalOnMissingBean
        public BeanNameViewResolver beanNameViewResolver() {
        BeanNameViewResolver resolver = new BeanNameViewResolver();
        resolver.setOrder(Ordered.LOWEST_PRECEDENCE - 10);
        return resolver;
        }

@ConfigurationProperties

• 批量注入配置文件中的属性值，默认从全局配置文件中获取：在配置文件yml或properties定义值，使用该注解给配置类赋值

  //application.properties
  //可使用$占位符定义值，比如person.name=张三${random.int},person.maps.k2=${person.name}_v2；${person.name:hello}表示person_name没有定义，则使用默认值hello
  //结果name为张三12afa124,k2为张三12afa124_v2
  person.name=张三
  person.age=11
  person.maps.k1=v1
  person.maps.k2=v2
  person.list=a,b
  //定义配置类并加入容器中，将上面的值按照键值对一一赋值
  @Component
  @ConfigurationProperties(prefix = "person")
  public class Person {
  private String name;
  private Integer age;
  private Map<String,Object> maps;
  private List<String> list;
  }

• @ConfigurationProperties和@Value对比
  • @ConfigurationProperties批量注入文件的属性值；@Value一个个指定；
  • @ConfigurationProperties支持松散绑定(person.lastName；person.last_name；person.last-name；PERSON_LAST_NAME)；@Value不支持；
  • @ConfigurationProperties不支持SpEL；@Value支持，比如@Value("#{10}")；
  • @ConfigurationProperties支持JSR303数据校验，比如在配置类上添加@Validated，然后在属性上添加@Email，它表示属性值必须符合Email格式；@Value不支持，同时也不支持复杂类型封装，比如Map；
  • 如果需要JavaBean和配置文件值进行映射，使用@ConfigurationProperties；如果是单个值使用@Value；
• @PropertySource：表示加载指定路径的配置文件

  @PropertySource(value = {"classpath:person.properties"})

• @ImportResource：导入Spring的配置文件，让配置文件里面的内容生效，比如我们写了bean.xml，里面使用<bean>创建了一个实例，但没有加入容器中

  @ImportResource(locations = {"classpath:bean.xml"})

  • 使用@Configuration和@Bean来给容器添加组件，代替上面的xml文件中方式

    @Configuration
    public class MyConfiguration {
    @Bean
    public HelloService helloService(){
    return new HelloService();
    }
    }

自动配置原理

1. SpringBoot启动加载主配置类(@SpringBootApplication)，开启了自动配置功能(@EnableAutoConfiguration)；
2. 自动配置使用了@Import(AutoConfigurationImportSelector.class)导入了组件，可查看getAutoConfigurationEntry()的getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes)返回的集合，即候选的配置；
3. SpringFactoriesLoader.loadFactoryNames()扫描所有jar包路径下的META-INF/spring.factories，把扫描到的文件的内容包装成properties对象，从properties中获取到EnableAutoConfiguration.class类对应的值，然后把它们添加到容器中；

   #spring-boot-autoconfigure-2.3.0.RELEASE.jar中spring.factories的信息，EnableAutoConfiguration后面每个xxxAutoConfiguration都是一个组件，都加入到容器中
   # 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.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

4. 每一个自动配置类进行自动配置，以HttpEncodingAutoConfiguration为例：

   @Configuration(proxyBeanMethods = false)//表示这是一个配置类
   @EnableConfigurationProperties(ServerProperties.class)//启动指定类的ConfigurationProperties功能，将配置文件中对应的值和ServerProperties绑定起来，然后加入到IOC容器中
   @ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)//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；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CharacterEncodingFilter类；SpringMVC中进行乱码解决的过滤器
   @ConditionalOnProperty(prefix = "server.servlet.encoding", value = "enabled", matchIfMissing = true)//判断配置文件中是否存在某个配置，server.servlet.encoding，如果不存在，判断也是成立的
   public class HttpEncodingAutoConfiguration { private final Encoding properties;//已经和SpringBoot的配置文件映射了，即ServerProperties中的Encoding对象
   //只有一个有参构造器的情况下，参数的值从容器中拿，即@EnableConfigurationProperties(ServerProperties.class)
   public HttpEncodingAutoConfiguration(ServerProperties properties) {
   this.properties = properties.getServlet().getEncoding();//从ServerProperties中获取Encoding对象并进行实例化
   }
   @Bean //给容器中添加一个组件CharacterEncodingFilter
   @ConditionalOnMissingBean
   public CharacterEncodingFilter characterEncodingFilter() {
   CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
   filter.setEncoding(this.properties.getCharset().name());//从properties中获取值并赋值
   filter.setForceRequestEncoding(this.properties.shouldForce(Encoding.Type.REQUEST));
   filter.setForceResponseEncoding(this.properties.shouldForce(Encoding.Type.RESPONSE));
   return filter;
   }
   }
   //从配置文件中获取指定的值和bean的属性进行绑定（所有在配置文件中能配置的属性都是在xxxProperties类中封装着；配置文件能配置什么参数就参照该功能对应的属性类）
   @ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)
   public class ServerProperties {}

5. 一旦这个自动配置类生效，就会给容器添加各种组件@Bean，这些组件的属性是从对应的properties类中获取的，每个properties类里面的每一个属性又是和配置文件绑定的；

@Conditional派生注解

• 作用在某个类上，表示必须是@Condition指定的条件成立才会给容器中添加组件，配置类里面的内容才生效；
• 通过配置文件里面的debug=true在控制台打印出自动配置的启动和没有启用的报告进行查看：

  ============================
  CONDITIONS EVALUATION REPORT条件评估报告
  ============================Positive matches:(自动配置类启用的)
  ----------------- AopAutoConfiguration matched:
  - @ConditionalOnProperty (spring.aop.auto=true) matched (OnPropertyCondition)
  ...........
  Negative matches:(自动配置类没有启用的)
  ----------------- ActiveMQAutoConfiguration:
  Did not match:
  - @ConditionalOnClass did not find required class 'javax.jms.ConnectionFactory' (OnClassCondition)
  ...........