SpringBoot学习笔记（十八）–自动配置原理
配置文件能配置的属性参照：
https://docs.spring.io/spring-boot/docs/current/reference/html/appendix-application-properties.html
自动配置原理
1、Spring Boot 启动时，加载主配置类，开启了自动配置功能：@EnableAutoConfiguration
@EnableAutoConfiguration 作用：
利用 AutoConfigurationImportSelector 给容器中导入一些组件。
可以查看 selectImports() 方法和 getAutoConfigurationEntry() 方法的内容。其中 getAutoConfigurationEntry() 方法中有下面一段代码：
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
进入 getCandidateConfigurations() 方法中，可以找到下面一段代码：
List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
getBeanClassLoader());
进入 loadFactoryNames() 方法中，可以找到下面一段代码：
Enumeration<URL> urls = (classLoader != null ?
classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
进入 FACTORIES_RESOURCE_LOCATION，可以发现 FACTORIES_RESOURCE_LOCATION 是一个 final 变量：
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
由此可知，SpringFactoriesLoader.loadFactoryNames() 方法的作用是扫描所有 jar 包类路径下 META-INF/spring.factories，通过下面的循环语句
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry<?, ?> entry : properties.entrySet()) {
String factoryTypeName = ((String) entry.getKey()).trim();
for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
result.add(factoryTypeName, factoryImplementationName.trim());
}
}
}
把扫描到的这些文件的内容包装成 properties 对象。
在 AutoConfigurationImportSelector 类的 getCandidateConfigurations() 方法中找到的下面这段代码
List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
getBeanClassLoader());
进入传入 loadFactoryNames() 方法的 getSpringFactoriesLoaderFactoryClass() 方法
protected Class<?> getSpringFactoriesLoaderFactoryClass() {
return EnableAutoConfiguration.class;
}
从 properties 中获取到 EnableAutoConfiguration.class 类（类名）对应的值，然后把他们添加到容器中。
了解了以上流程之后，找到工程依赖的 spring-boot-autoconfigure-2.3.0.RELEASE.jar 包，打开其中的 META-INF/spring.factories 文件，可以看到所有的 Auto Configure
# 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 xxxxxxAutoConfiguration 类都是容器中的一个组件，加入到了容器中，用他们来做自动配置。
2、每一个自动配置类进行自动配置功能
以 HttpEncodingAutoConfiguration 为例，解释自动配置原理。
HttpEncodingAutoConfiguration 类有以下注解：
@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(ServerProperties.class)
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
@ConditionalOnClass(CharacterEncodingFilter.class)
@ConditionalOnProperty(prefix = "server.servlet.encoding", value = "enabled", matchIfMissing = true)
public class HttpEncodingAutoConfiguration {}
@Configuration(proxyBeanMethods = false)
表示这是一个配置类，和以前编写的配置文件一样，可以给容器中添加组件。
@EnableConfigurationProperties(ServerProperties.class)
启动指定类的 ConfigurationProperties 功能。
进入 ServerProperties 类，可以看到该类所拥有的注解：
@ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)
public class ServerProperties {}
@ConfigurationProperties(prefix = “server”, ignoreUnknownFields = true) – 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 xxxxProperties 类中封装着，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。
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(type = ConditionalOnWebApplication.Type.SERVLET)
Spring 底层有一个 @Conditional 注解，作用是根据不同的条件，当满足指定的条件时，整个配置类中的配置就会生效。按字面意思理解，这个注解就是判断当前应用是否是 Web 应用，如果是的话，当前配置类生效。
@ConditionalOnClass(CharacterEncodingFilter.class)
判断当前项目有没有这个类。
CharacterEncodingFilter：SpringMVC 中解决乱码的过滤器。
@ConditionalOnProperty(prefix = “server.servlet.encoding”, value = “enabled”, matchIfMissing = true)
判断配置文件中是否存在 server.servlet.encoding.enabled 这个配置 。
由 matchIfMissing = true 可知，如果不存在，判断也成立。即使配置文件中不配置server.servlet.encoding.enabled=true，也是默认生效的。
总结
自动配置类就是根据当前不同的条件进行判断，决定这个配置类是否生效。一但这个配置类生效，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是和配置文件绑定的。
下面是 HttpEncodingAutoConfiguration 类中的一个方法
@Bean
@ConditionalOnMissingBean
public CharacterEncodingFilter characterEncodingFilter() {
CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
filter.setEncoding(this.properties.getCharset().name());
filter.setForceRequestEncoding(this.properties.shouldForce(Encoding.Type.REQUEST));
filter.setForceResponseEncoding(this.properties.shouldForce(Encoding.Type.RESPONSE));
return filter;
}
@Bean
给容器中添加一个组件。由
this.properties.getCharset().name()
this.properties.shouldForce(Encoding.Type.REQUEST)
可知，这个组件的某些值需要从 properties 中获取。
HttpEncodingAutoConfiguration 类只有一个有参构造器
public HttpEncodingAutoConfiguration(ServerProperties properties) {
this.properties = properties.getServlet().getEncoding();
}
参数的值会从容器中拿。
由
@ConditionalOnProperty(prefix = "server.servlet.encoding", value = "enabled", matchIfMissing = true)
可知，可以在 properties 中配
server.servlet.encoding.enabled=true
由
private final Encoding properties;
和
public class Encoding {
public static final Charset DEFAULT_CHARSET;
private Charset charset;
private Boolean force;
private Boolean forceRequest;
private Boolean forceResponse;
private Map<Locale, Charset> mapping;
}
可知，可以在 properties 中配
server.servlet.encoding.charset=UTF-8
server.servlet.encoding.force=true
能配置的属性其实都来源于这个功能的 properties 类
精髓
1、Spring Boot 启动会加载大量的自动配置类；
2、看需要的功能有没有 Spring Boot 默认写好的自动配置类；
3、再看这个自动配置类中到底配置了哪些组件，只要有需要的组件，就不需要再配置了；
4、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些属性的值。
xxxxAutoConfigurartion：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
在 IDEA 中搜索 *AutoConfiguration 就可以看到全部的自动配置类。通过 @EnableConfigurationProperties 中的 xxxxProperties 可以知道具体可以在 properties 文件中配置哪些属性。
