@EnableAutoConfiguration

在spring boot项目中如果在@Configuration类（说明这个类是配置类，能被自动扫描到）同时添加注解@EnableAutoConfiguration就会获得很多很方便的开箱即用的配置，同时@EnableAutoConfiguration放在哪里就隐含了搜索自动化配置类的路径@EnableAutoConfiguration注解中@Import了EnableAutoConfigurationImportSelector

虽然自动配置是非侵入的，且会被你自身的配置覆盖（主要采用了条件注入的方式，见@EnableAutoConfiguration），你也可以在一开始就使用@EnableAutoConfiguration注解的exclude属性，将不是你所想要的自动化配置类（auto-configure classes）禁用掉

import org.springframework.boot.autoconfigure.*;
import org.springframework.boot.autoconfigure.jdbc.*;
import org.springframework.context.annotation.*;

@Configuration
@EnableAutoConfiguration(exclude={DataSourceAutoConfiguration.class})
public class MyConfiguration {
}

如果该类不在classpath中，你可以使用该注解的excludeName属性，并指定全限定名来达到相同效果。最后，你可以通过spring.autoconfigure.exclude属性（在属性文件application.properties中，多个值用逗号隔开）exclude多个自动配置项（一个自动配置项集合）。 excludeName和exclude都是传入一个数组

通过注解级别或属性文件中的exclude属性都可以定义排除项。

EnableAutoConfigurationImportSelector使用SpringFactoriesLoader#loadFactoryNames搜索jar包中的META-INF/spring.factories.如果找到这个文件，SpringFactoriesLoader就会搜索该文件中的一个配置org.springframework.boot.autoconfigure.EnableAutoConfiguration,并读取后面的值。META-INF/spring.factories会在spring-boot-autoconfigure.jar下

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# 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.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration,\
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.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
##...省略

# Template availability providers
org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.velocity.VelocityTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.web.JspTemplateAvailabilityProvider

org.springframework.boot.autoconfigure.EnableAutoConfiguration后面配置了几十个*AutoConfiguration类，这些类就是默认各个模块的默认配置。*AutoConfiguration才是提工默认配置的地方 （这些配置原本是需要我们自己写在项目里的，比如使用了Thymeleaf，就需要有个配置类或者老做法新建一个配置xml配置Thymeleaf）

@Configuration
@EnableConfigurationProperties(ThymeleafProperties.class)
@ConditionalOnClass(SpringTemplateEngine.class)
@AutoConfigureAfter(WebMvcAutoConfiguration.class)
public class ThymeleafAutoConfiguration {

    private static final Log logger = LogFactory.getLog(ThymeleafAutoConfiguration.class);

    @Configuration
    @ConditionalOnMissingBean(name = "defaultTemplateResolver")
    public static class DefaultTemplateResolverConfiguration {

        @Autowired
        private ThymeleafProperties properties;

        @Autowired
        private ApplicationContext applicationContext;

        @PostConstruct
        public void checkTemplateLocationExists() {
            boolean checkTemplateLocation = this.properties.isCheckTemplateLocation();
            if (checkTemplateLocation) {
                TemplateLocation location = new TemplateLocation(
                        this.properties.getPrefix());
                if (!location.exists(this.applicationContext)) {
                    logger.warn("Cannot find template location: " + location
                            + " (please add some templates or check "
                            + "your Thymeleaf configuration)");
                }
            }
        }
        //other code
}

@Configuration定义该为一个配置类@EnableConfigurationProperties(ThymeleafProperties.class)表示能在ThymeleafProperties类中使用@ConfigurationProperties注解自动从配置文件中绑定属性（不需要@value方式）

查看一下ThymeleafProperties类，@ConfigurationProperties("spring.thymeleaf")表明，默认从applciation.properties中读取前缀为spring.thymeleaf的属性，例如spring.thymeleaf.prefix=指定html路径

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

}

而以下几个注解，是非侵入式配置的关键，即只有满足某些条件的情况下该配置才会生效（Spring高级装配之条件装配）

@ConditionalOnClass(SpringTemplateEngine.class)
@AutoConfigureAfter(WebMvcAutoConfiguration.class)
@ConditionalOnMissingBean
@ConditionalOnClass

conditional注解威力很强大，常见的有：

@ConditionalOnBean
@ConditionalOnClass
@ConditionalOnExpression
@ConditionalOnMissingBean
@ConditionalOnMissingClass
@ConditionalOnNotWebApplication
@ConditionalOnResource
@ConditionalOnWebApplication