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

SpringBoot启动加载主配置类(@SpringBootApplication)，开启了自动配置功能(@EnableAutoConfiguration)；
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；

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

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 {}

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)

   ...........