这里主要的看调用这个方法getCandidateConfigurations，返回的就是要自动加载的Bean
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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; }通过META-INF/spring.factories配置文件里的EnableAutoConfiguration获取配置的Bean
# 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,\ .....太多了，有兴趣的可以查看Spring的xxx-autoconfigure包。将读取到的配置最终返回给selectImports，然后通过工具类StringUtils.toStringArray转换为字符串数组返回给@Import，从而实现自动配置。第三方包只要是xxx-autoconfigure结尾的包，META-INF都有spring.factories，这个名字是固定写法。都可以被SpringBoot识别并且进行自动配置，前提是需要配置到org.springframework.boot.autoconfigure.EnableAutoConfiguration下。
从以上总结来看，SpringBoot的自动配置原理如下：
@EnableAutoConfiguration注解内部使用Import(AutoConfigurationImportSelector.class)来加载配置类
通过配置文件：META-INF/spring.factories，配置大量的配置类，SpringBoot启动时就会自动加载这些类并初始化的Bean。
这里需要说明一点，并不是所有配置到配置文件的Bean都会被初始化，需要符合配置类中使用Condition来加载满足条件的Bean。比如我们打开RedisAutoConfiguration的源码查看：
@Configuration( proxyBeanMethods = false ) @ConditionalOnClass({RedisOperations.class}) @EnableConfigurationProperties({RedisProperties.class}) @Import({LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class}) public class RedisAutoConfiguration { public RedisAutoConfiguration() { } @Bean @ConditionalOnMissingBean( name = {"redisTemplate"} ) @ConditionalOnSingleCandidate(RedisConnectionFactory.class) public RedisTemplate<Object, Object> redisTemplate(RedisConnectionFactory redisConnectionFactory) { RedisTemplate<Object, Object> template = new RedisTemplate(); template.setConnectionFactory(redisConnectionFactory); return template; } @Bean @ConditionalOnMissingBean @ConditionalOnSingleCandidate(RedisConnectionFactory.class) public StringRedisTemplate stringRedisTemplate(RedisConnectionFactory redisConnectionFactory) { StringRedisTemplate template = new StringRedisTemplate(); template.setConnectionFactory(redisConnectionFactory); return template; } }类上面有这么个注解@ConditionalOnClass({RedisOperations.class})，意思就是需要RedisOperations类存在的情况下，才自动加载；这还不算完，继续查看下面的方法上有个@ConditionalOnMissingBean(name = {"redisTemplate"})，这里的意思是，当其他地方没有redisTemplate实例化这个Bean时，才自动加载。符合这两个条件，SpringBoot才会进行自动加载并初始化。