SpringBoot自动配置源码探究

1. @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注册到Spring容器中供后续进行实例化

                   @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

           @Configuration
           @ConditionalOnWebApplication
           @ConditionalOnClass({ Servlet.class, DispatcherServlet.class,
                   WebMvcConfigurerAdapter.class })
           @ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
           @AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
           @AutoConfigureAfter({ DispatcherServletAutoConfiguration.class,
                   ValidationAutoConfiguration.class })
           public class WebMvcAutoConfiguration {
                   @Bean
                   @ConditionalOnBean(View.class)
                   @ConditionalOnMissingBean
                   public BeanNameViewResolver beanNameViewResolver() {
                       BeanNameViewResolver resolver = new BeanNameViewResolver();
                       resolver.setOrder(Ordered.LOWEST_PRECEDENCE - 10);
                       return resolver;
                   }
           }        

            

           @Configuration：表示这是一个配置类，IOC会进行CGLIB代理；
           @ConditionalOnWebApplication：Spring底层@Conditional注解（Spring注解版），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；

           @Target({ ElementType.TYPE, ElementType.METHOD })
           @Retention(RetentionPolicy.RUNTIME)
           @Documented
           @Conditional(OnWebApplicationCondition.class)
           public @interface ConditionalOnWebApplication {
           
           }

           @ConditionalOnClass：判断当前项目有没有类Servlet（服务器），类DispatcherServlet（前端控制器），WebMvcConfigurerAdapter（MVC配置适配器）；

           @ConditionalOnMissingBean：保证IOC容器中只有一个WebMvcConfigurationSupport类型的bean；

           @AutoConfigureOrder：加载的优先级别

           @AutoConfigureAfter：在DispatcherServletAutoConfiguration, ValidationAutoConfiguration 类加载后进行装配。

           WebMvcAutoConfiguration中有3个内部类：WebMvcAutoConfigurationAdapter、EnableWebMvcConfiguration、ResourceChainCustomizerConfiguration；

           其中
           WebMvcAutoConfigurationAdapter：声明在WebMvcAutoConfiguration中,是为了确保当该类不在类路径下时不会被读取到，其中@EabaleConfigurationProperties是将配置文件中的值和对应的Bean类进行绑定，并将对应的Bean类加载到IOC容器中；
           EnableWebMvcConfiguration：配置类，相当于@EnableWebMvc的功能，对 spring mvc 进行自动化配置；

            

           比如@EnableConfigurationProperties往IOC容器加载了 WebMvcProperties类，该类通过@ConfigurationProperties注解将配置文件和自身属性进行绑定到WebMvcProperties类中，如下图application.propertis声明的spring.mvc.view.suffix就是通过@ConfigurationProperties和setPrefix进行绑定到WebMvcProperties类的；

            

             

   • 总结：

         @SpringBootApplication注解导入了两个类，@SpringBootConfiguration和@EnableAutoConfiguration，前者表示一个配置类，后者进行自动装配；
         其中@EnableAutoConfiguration注解有两个功能：
         @AutoConfigurationPackage：自动导入包，将@SpringBootApplication所在的包名下的所有类都注册到Spring容器中供后续进行实例化；
         AutoConfigurationImportSelector：给容器导入组件的自动配置类，并配置好这些组件；
         最终每个自动配置类通过@EnableConfigurationProperties({xxx.class})导入配置类，并把配置类加入到IOC容器中，其中配置类的属性值配置是通过@ConfigurationProperties(prefix = "xxx")和setXX方法将配置文件的值进行绑定；

2. @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();
           }
       }

3. 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；

4. @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)
        ...........