可配置項參考：

https://docs.spring.io/spring-boot/docs/2.0.1.BUILD-SNAPSHOT/reference/htmlsingle/#common-application-properties

自動配置：

從@SpringBootApplication追蹤，大致追蹤過程為：

@SpringBootApplication-->@EnableAutoConfiguration-->@Import(AutoConfigurationImportSelector.class)-->
AutoConfigurationImportSelector-->List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes); -->
List
 
<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());-->
return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());-->
Enumeration<URL> urls = (classLoader != null ?
                    classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
 

FACTORIES_RESOURCE_LOCATION的值為：META-INF/spring.factories

打開spring自動配置包（spring-boot-autoconfigure-2.0.1.RELEASE.jar!/META-INF/spring.factories）找到這個文件，可以看到裏面設置了很多自動配置的項：

# 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.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.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.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.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.jest.JestAutoConfiguration,org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,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.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.groovy.template.GroovyTemplateAutoConfiguration,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.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.reactor.core.ReactorCoreAutoConfiguration,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.sendgrid.SendGridAutoConfiguration,org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,org.springframework.boot.autoconfigure.security.oauth2.client.OAuth2ClientAutoConfiguration,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.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.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
 

比如ThymeleafAutoConfiguration打開為：

@Configuration
@EnableConfigurationProperties(ThymeleafProperties.class)
@ConditionalOnClass(TemplateMode.class)
@AutoConfigureAfter({ WebMvcAutoConfiguration.class, WebFluxAutoConfiguration.class })
public class ThymeleafAutoConfiguration {

    @Configuration
    @ConditionalOnMissingBean(name = "defaultTemplateResolver")
    static class DefaultTemplateResolverConfiguration {
        private final ThymeleafProperties properties;

        @Bean
        public SpringResourceTemplateResolver defaultTemplateResolver() {
            SpringResourceTemplateResolver resolver = new SpringResourceTemplateResolver();
            resolver.setApplicationContext(this.applicationContext);
            resolver.setPrefix(this.properties.getPrefix());
            resolver.setSuffix(this.properties.getSuffix());
            resolver.setTemplateMode(this.properties.getMode());
            if (this.properties.getEncoding() != null) {
                resolver.setCharacterEncoding(this.properties.getEncoding().name());
            }
            resolver.setCacheable(this.properties.isCache());
            Integer order = this.properties.getTemplateResolverOrder();
            if (order != null) {
                resolver.setOrder(order);
            }
            resolver.setCheckExistence(this.properties.isCheckTemplate());
            return resolver;
        }
    }
}

@ConfigurationProperties(prefix = "spring.thymeleaf")
public class ThymeleafProperties {

    private static final Charset DEFAULT_ENCODING = StandardCharsets.UTF_8;

    public static final String DEFAULT_PREFIX = "classpath:/templates/";

    public static final String DEFAULT_SUFFIX = ".html";
}

可以看到默認配置的模版目錄為classpath:/templates/，默認文件後綴為 .html，同時我們自己如果要自定義配置，可以參考其中@bean的寫法。

自定義配置：

以 HttpEncodingAutoConfiguration 舉例，當在application.properties中敲 spring.http.encoding.的時候，可以看到有幾個候選項，那麽這些候選項從哪裏來的呢？

打開 HttpEncodingAutoConfiguration 可以看到它被 @EnableConfigurationProperties(HttpEncodingProperties.class)修飾，而 HttpEncodingProperties 中的代碼為：

@ConfigurationProperties(prefix = "spring.http.encoding")
public class HttpEncodingProperties {
    public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;
    private Charset charset = DEFAULT_CHARSET;
    private Boolean force;
    private Boolean forceRequest;
    private Boolean forceResponse;
}

可以看到候選項是這個類中的屬性。其它自動配置類中都會有 @EnableConfigurationProperties，它包含的類中就是自動配置項，也就是我們寫配置文件時的候選項。理解了這個，很多配置就不需要去翻官方文檔了，直接查看源碼即可，因為這些自動配置的規律都相同。

@ConditionalOnXXX，表示條件判斷，是否滿足XXX所代表的條件，如果滿足的話，當前整個配置文件才生效，否則此配置文件不生效，我們也可以自己實現@Conditional來完成一些條件判斷。

SpringBoot-自動配置原理