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@Configurationpublic @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@Componentpublic @interface Configuration {@AliasFor(annotation = Component.class)String value() default \"\";}

@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

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

@Overridepublic 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)@ConditionalOnMissingBeanpublic 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_v2person.name=张三person.age=11person.maps.k1=v1person.maps.k2=v2person.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(value = {\"classpath:person.properties\"})

@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

@Configurationpublic class MyConfiguration {@Beanpublic 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)；

#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 Configureorg.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.springframe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@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@ConditionalOnMissingBeanpublic 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 {}