springBoot的自动注册

整个springboot并没有在spring的基础上提供什么额外的功能。
springBoot的最大作用就是引入某些jar包后，自动为spring上下文环境生成某些特定功能的Bean，这样就可以自动提供某些相关功能。
本质上，spring通过被标记了@Configuration的类提供一些提前生成好的Bean提供特别的功能，而用@Contional系列的注解限制生成的条件，通常就是@ConditionalOnClass和@ConditionalOnMissingBean注解的配合使用。前者确定某些功能需要的class已经有了，后者确定你没有自己生成相关的bean，才提供默认的。

spring-boot-autoconfigure

spring通过spring-boot-autoconfigure这个jar提供大量的自动注册功能。
springboot启动的时候会扫描jar下是否有 META-INF/spring.factories 文件，这个文件中包含了你需要自动注册的java类。
比如spring-boot-autoconfigure这个jar下的META-INF/spring.factories文件提供了大量自动注册的配置类:

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

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

# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnClassCondition

# 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

# Failure analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer

# 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.web.JspTemplateAvailabilityProvider

由于springboot自己提供了这么多自动注册类，我们自己也可能写一些自动注册类。为了确定他们直接的生效顺序。我可以使用@AutoConfigureBefore，@AutoConfigureAfter，@AutoconfigureOrder 来确定某个自动注册类的生效顺序。作用类比@Order。

注意：自动注册的类需要放到特定的包中，避免被应用的@ComponentScan扫描到。

自动注册一般我们提供2个module：一个是autoconfigure jar包，一个是starter jar包。
其中starter这种jar里面并没有代码，是空的。唯一的作用是将需要的各种jar写入构建文件，这样引入此starter，就不要再去关心相应的其他jar了。
比如spring提供的 spring-boot-starter-data-redis 这个jar什么都没有，只是pom文件引入了使用spring-data-redis中的功能需要的相关jar。

<dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.data</groupId>
            <artifactId>spring-data-redis</artifactId>
        </dependency>
        <dependency>
            <groupId>redis.clients</groupId>
            <artifactId>jedis</artifactId>
        </dependency>
</dependencies>

而spring提供的spring-boot-autoconfigure里面有相关的类会根据相应的条件生成redis功能需要的bean，所以我们只需要加入springboot提供的spring-boot-starter-xxx 的jar就可以自动生成相应功能的bean了。

image.png

@Configuration
@ConditionalOnClass({ JedisConnection.class, RedisOperations.class, Jedis.class })
@EnableConfigurationProperties(RedisProperties.class)
public class RedisAutoConfiguration {

    /**
     * Redis connection configuration.
     */
    @Configuration
    @ConditionalOnClass(GenericObjectPool.class)
    protected static class RedisConnectionConfiguration {
           ……略
    }

    /**
     * Standard Redis configuration.
     */
    @Configuration
    protected static class RedisConfiguration {
            ……略
    }

}

这个类里面有2个子类，一个RedisConfiguration，RedisConnectionConfiguration。
先看简单的，RedisConfiguration：

/**
     * Standard Redis configuration.
     */
    @Configuration
    protected static class RedisConfiguration {
        //当spring上下文中没有name叫redisTemplate的bean就会自动生成RedisTemplate
        //当然前提是的上下文中已经有RedisConnectionFactory了，否则自动注入失败会报错
        @Bean
        @ConditionalOnMissingBean(name = "redisTemplate")
        public RedisTemplate<Object, Object> redisTemplate(
                RedisConnectionFactory redisConnectionFactory)
                        throws UnknownHostException {
            RedisTemplate<Object, Object> template = new RedisTemplate<Object, Object>();
            template.setConnectionFactory(redisConnectionFactory);
            return template;
        }
         //同理当上下文中没有类型为StringRedisTemplate的Bean就会自动生成StringRedisTemplate。
        //同理需要注入RedisConnectionFactory
        @Bean
        @ConditionalOnMissingBean(StringRedisTemplate.class)
        public StringRedisTemplate stringRedisTemplate(
                RedisConnectionFactory redisConnectionFactory)
                        throws UnknownHostException {
            StringRedisTemplate template = new StringRedisTemplate();
            template.setConnectionFactory(redisConnectionFactory);
            return template;
        }

    }

所以这个类到底干嘛呢？就是为spring上下文生成一个RedisTemplate，一个StringRedisTemp。好供用户操作redis。
第二个类, RedisConnectionConfiguration：

    /**
     * Redis connection configuration.
     */
    @Configuration
    //这个条件是指需要commons-pool这个jar,因为jedis底层依赖此jar提供连接池的能力
    @ConditionalOnClass(GenericObjectPool.class)
    protected static class RedisConnectionConfiguration {

        private final RedisProperties properties;

        private final RedisSentinelConfiguration sentinelConfiguration;

        private final RedisClusterConfiguration clusterConfiguration;
        //注入RedisProperties，就是我们autoconfigure jar里面提供的类
        //另外2个是RedisSentinelConfiguration，RedisClusterConfiguration。但这2个bean并
        //不一定有，所以，使用ObjectProvider#getIfAvailable
        public RedisConnectionConfiguration(RedisProperties properties,
                ObjectProvider<RedisSentinelConfiguration> sentinelConfigurationProvider,
                ObjectProvider<RedisClusterConfiguration> clusterConfigurationProvider) {
            this.properties = properties;
            this.sentinelConfiguration = sentinelConfigurationProvider.getIfAvailable();
            this.clusterConfiguration = clusterConfigurationProvider.getIfAvailable();
        }
        //如果没有RedisConnectionFactory的bean,生成JedisConnectionFactory
        //同时为JedisConnectionFactory注入RedisProperties中的基本属性
        @Bean
        @ConditionalOnMissingBean(RedisConnectionFactory.class)
        public JedisConnectionFactory redisConnectionFactory()
                throws UnknownHostException {
            return applyProperties(createJedisConnectionFactory());
        }
        //为JedisConnectionFactory注入host，port，password，database，timeout等基本属性
        protected final JedisConnectionFactory applyProperties(
                JedisConnectionFactory factory) {
            factory.setHostName(this.properties.getHost());
            factory.setPort(this.properties.getPort());
            if (this.properties.getPassword() != null) {
                factory.setPassword(this.properties.getPassword());
            }
            factory.setDatabase(this.properties.getDatabase());
            if (this.properties.getTimeout() > 0) {
                factory.setTimeout(this.properties.getTimeout());
            }
            return factory;
        }
        //下面几个方法的作用类似，就是检查spring上下文中是否已经有sentinel，cluster的配置
        //bean（从构造函数注入的刚才），如果有，就用已有的，没有，就用
        //从RedisProperties中写入的属性生成。
        protected final RedisSentinelConfiguration getSentinelConfig() {
            if (this.sentinelConfiguration != null) {
                return this.sentinelConfiguration;
            }
            Sentinel sentinelProperties = this.properties.getSentinel();
            if (sentinelProperties != null) {
                RedisSentinelConfiguration config = new RedisSentinelConfiguration();
                config.master(sentinelProperties.getMaster());
                config.setSentinels(createSentinels(sentinelProperties));
                return config;
            }
            return null;
        }
               
        /**
         * Create a {@link RedisClusterConfiguration} if necessary.
         * @return {@literal null} if no cluster settings are set.
         */
        protected final RedisClusterConfiguration getClusterConfiguration() {
            if (this.clusterConfiguration != null) {
                return this.clusterConfiguration;
            }
            if (this.properties.getCluster() == null) {
                return null;
            }
            Cluster clusterProperties = this.properties.getCluster();
            RedisClusterConfiguration config = new RedisClusterConfiguration(
                    clusterProperties.getNodes());

            if (clusterProperties.getMaxRedirects() != null) {
                config.setMaxRedirects(clusterProperties.getMaxRedirects());
            }
            return config;
        }

        private List<RedisNode> createSentinels(Sentinel sentinel) {
            List<RedisNode> nodes = new ArrayList<RedisNode>();
            for (String node : StringUtils
                    .commaDelimitedListToStringArray(sentinel.getNodes())) {
                try {
                    String[] parts = StringUtils.split(node, ":");
                    Assert.state(parts.length == 2, "Must be defined as 'host:port'");
                    nodes.add(new RedisNode(parts[0], Integer.valueOf(parts[1])));
                }
                catch (RuntimeException ex) {
                    throw new IllegalStateException(
                            "Invalid redis sentinel " + "property '" + node + "'", ex);
                }
            }
            return nodes;
        }

        private JedisConnectionFactory createJedisConnectionFactory() {
            JedisPoolConfig poolConfig = this.properties.getPool() != null
                    ? jedisPoolConfig() : new JedisPoolConfig();

            if (getSentinelConfig() != null) {
                return new JedisConnectionFactory(getSentinelConfig(), poolConfig);
            }
            if (getClusterConfiguration() != null) {
                return new JedisConnectionFactory(getClusterConfiguration(), poolConfig);
            }
            return new JedisConnectionFactory(poolConfig);
        }

        private JedisPoolConfig jedisPoolConfig() {
            JedisPoolConfig config = new JedisPoolConfig();
            RedisProperties.Pool props = this.properties.getPool();
            config.setMaxTotal(props.getMaxActive());
            config.setMaxIdle(props.getMaxIdle());
            config.setMinIdle(props.getMinIdle());
            config.setMaxWaitMillis(props.getMaxWait());
            return config;
        }

    }

所以RedisConnectionConfiguration的主要作用就是在没有RedisConnectionFactory的时候，通过RedisProperties提供的属性，生成前文提到的RedisConnectionFactory（为RedisTemplate使用)