配置文件到底能写什么？怎么写？自动配置原理：
配置文件能配置属性参照
1）、SpringBoot启动的时候加载主配置类，开启了自动配置功能 @EnableAutoConfiguration
2）、@EnableAutoConfiguration作用：
利用EnableAutoConfigurationImportSelector给容器中导入一些组件？
可以查看selectImports()方法（选择器父类的方法）的内容:
List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);获取候选的配置
SpringFactoriesLoader.loadFactoryNames(); 扫描所有jar包类路径下的"META-INF/spring.factories" 把扫描到的这些文件的内容包装成properties对象 从properties中获取到EnableAutoConfiguration.class类（类名）对应的值，把他们添加到容器中将类路径下的"META-INF/spring.factories里面配置的所有EnableAutoConfiguration的值注入到容器中:
# 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.cloud.CloudAutoConfiguration,\\ org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\\ 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.CassandraRepositoriesAutoConfiguration,\\ org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\\ org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\\ org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\\ org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\\ org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\\ org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\\ org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,\\ org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\\ org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\\ 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.redis.RedisAutoConfiguration,\\ 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.jest.JestAutoConfiguration,\\ org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\\ 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.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.flyway.FlywayAutoConfiguration,\\ org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\\ org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\\ org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\\ org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\\ 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.mobile.DeviceResolverAutoConfiguration,\\ org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\\ org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\\ org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\\ org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\\ org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\\ org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\\ org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\\ org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\\ org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\\ org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\\ org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\\ org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\\ org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\\ org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\\ org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\\ org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\\ org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\\ org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\\ 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.DispatcherServletAutoConfiguration,\\ org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\\ org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\\ org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\\ org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\\ org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\\ org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\\ org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,\\ org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\\ org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\\ org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\\ org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration每一个这样的xxxAutoConfiguration类都是容器中的一个组件，都加入到容器中，用它们来做自动匹配
3）、每一个自动配置类进行自动配置功能；
4）、以HttpEncodingAutoConfiguration(Http编码自动配置)为例解释自动配置原理：
@Configuration //表示这是一个配置类，像以前编写的配置文件一样，也可以给容器中添加组件 @EnableConfigurationProperties({HttpEncodingProperties.class}) /*启动指定类的 ConfigurationProperties功能；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*/ @ConditionalOnWebApplication /*Spring底层有@Conditionnal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； 作用：判断当前应用是否是web应用*/ @ConditionalOnClass({CharacterEncodingFilter.class}) /* 判断当前项目是否有CharacterEncodingFilter 类（SpringMVC中进行乱码解决的过滤器） */ @ConditionalOnProperty( prefix = "spring.http.encoding", value = {"enabled"}, matchIfMissing = true ) /* 判断配置文件中是否存在某个配置，如spring.http.encoding.enabled;如果不存在，判断也成立（返回true或false） */ public class HttpEncodingAutoConfiguration { /* 他已经和SpringBoot的配置文件映射了*/ private final HttpEncodingProperties properties; /* 只有一个有参构造器的情况下，参数的值就会从容器中拿 */ public HttpEncodingAutoConfiguration(HttpEncodingProperties properties) { this.properties = properties; } @Bean /*给容器添加一个组件，这个组件的某些值需要从Properties中获取*/ @ConditionalOnMissingBean({CharacterEncodingFilter.class}) public CharacterEncodingFilter characterEncodingFilter() { CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter(); filter.setEncoding(this.properties.getCharset().name()); filter.setForceRequestEncoding(this.properties.shouldForce(Type.REQUEST)); filter.setForceResponseEncoding(this.properties.shouldForce(Type.RESPONSE)); return filter; }根据当前不同的条件判断，决定这个配置类是否生效？
一旦这个配置类生效，这个配置类就会往容器中添加各种组件，这些组件属性是从对应的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
5）、所有在配置文件中能配置的属性都是在xxxxxProperties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
@ConfigurationProperties( prefix = "spring.http.encoding" ) //从配置文件中获取指定的值和bean的属性进行绑定 public class HttpEncodingProperties { public static final Charset DEFAULT_CHARSET = Charset.forName("UTF-8");3、我们再来看这个自动配置类中到底配置了哪些组件（只要我们要用的组件有，我们就不需要再来配置了）
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，我们可以在配置文件中指定这些属性的值
xxxxxAutoConfiguration:自动配置类
给容器中添加组件
xxxxxxxProperties：封装配置文件中相关属性
1、@Conditional派生注解（Spring注解原生的@Conditional作用）
作用：必须是@Conditional指定的条件成立，才给容器中添加组件，配置类里面的所有内容才生效
@Conditional扩展注解
自动配置类必须有一定的条件才生效，
我们怎么知道哪些自动配置类生效：
我们可以通过启用debug=true属性，来让控制台打印自动配置报告，这样就可以方便的知道哪些配置类已生效
========================= AUTO-CONFIGURATION REPORT ========================= Positive matches（自动配置类已启用的）: ----------------- DispatcherServletAutoConfiguration matched: - @ConditionalOnClass found required class 'org.springframework.web.servlet.DispatcherServlet'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition) - @ConditionalOnWebApplication (required) found 'session' scope (OnWebApplicationCondition) DispatcherServletAutoConfiguration.DispatcherServletConfiguration matched: - @ConditionalOnClass found required class 'javax.servlet.ServletRegistration'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition) - Default DispatcherServlet did not find dispatcher servlet beans (DispatcherServletAutoConfiguration.DefaultDispatcherServletCondition) Negative matches（没有启动没有匹配成功的自动配置类的）: ----------------- ActiveMQAutoConfiguration: Did not match: - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.ActiveMQConnectionFactory' (OnClassCondition) AopAutoConfiguration: Did not match: - @ConditionalOnClass did not find required classes 'org.aspectj.lang.annotation.Aspect', 'org.aspectj.lang.reflect.Advice' (OnClassCondition)