前言

我们经常会被问到这么一个问题：SpringBoot相对于spring有哪些优势呢？其中有一条答案就是SpringBoot自动注入。那么自动注入的原理是什么呢？

我们进行如下分析。

1：首先我们分析项目的启动类时，发现都会加上@SpringBootApplication这个注解，我们分析这个继续进入这个注解会发现，它是由多个注解组成的，如下

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@SpringBootConfiguration@EnableAutoConfiguration@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })public @interface SpringBootApplication {

2：服务启动会扫描 org.springframework.boot.autoconfigure下的 META-INF/spring.factories 这个文件，这个文件中保存着springboot 启动时默认会自动注入的类，部分如下

# Auto Configureorg.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

3：你是不是在其中发现了自己常用的redis，mysql等相关的类？没错，springboot会尝试加载这些类，我们以 org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration 这个类为例，进去看一下它的源码，部分示例如下

@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 { private final RedisProperties properties; private final RedisSentinelConfiguration sentinelConfiguration; private final RedisClusterConfiguration clusterConfiguration; public RedisConnectionConfiguration(RedisProperties properties, ObjectProvider<RedisSentinelConfiguration> sentinelConfiguration, ObjectProvider<RedisClusterConfiguration> clusterConfiguration) { this.properties = properties; this.sentinelConfiguration = sentinelConfiguration.getIfAvailable(); this.clusterConfiguration = clusterConfiguration.getIfAvailable(); } @Bean @ConditionalOnMissingBean(RedisConnectionFactory.class) public JedisConnectionFactory redisConnectionFactory() throws UnknownHostException { return applyProperties(createJedisConnectionFactory()); }

我们能看到这个类上加了这个注解 @ConditionalOnClass({ JedisConnection.class, RedisOperations.class, Jedis.class }) 意思就是如果你的classpath中没有这些类的话，那么这个类就不能被加载，那么这些被依赖的类在哪出现呢？没错，就在我们在pom.xml中引入的依赖所对应的包里。

看到这里你因该就明白了，META-INF/spring.factories 文件中被列出来的那些类都会被springboot去尝试加载，但是有些模块我们没引入相关的依赖，那么这个类就会加载失败。即这个模块没有被成功加载。

4：我们通过上面的redis的自动加载类时，看到上面还有个 @EnableConfigurationProperties(RedisProperties.class) 注解，这个注解来注入关于redis的配置信息，这个信息都在 RedisProperties.class 中保存，我们看下 RedisProperties的源码

@ConfigurationProperties(prefix = "spring.redis")public class RedisProperties { /** * Database index used by the connection factory. */ private int database = 0; /** * Redis url, which will overrule host, port and password if set. */ private String url; /** * Redis server host. */ private String host = "localhost"; /** * Login password of the redis server. */ private String password; /** * Redis server port. */ private int port = 6379; /** * Enable SSL. */ private boolean ssl; /** * Connection timeout in milliseconds. */ private int timeout; private Pool pool; private Sentinel sentinel; private Cluster cluster; public int getDatabase() { return this.database; } public void setDatabase(int database) { this.database = database; } public String getUrl() { return this.url; } public void setUrl(String url) { this.url = url; } public String getHost() { return this.host; } public void setHost(String host) { this.host = host; } public String getPassword() { return this.password; } public void setPassword(String password) { this.password = password; } public int getPort() { return this.port; } public void setPort(int port) { this.port = port; } public boolean isSsl() { return this.ssl; } public void setSsl(boolean ssl) { this.ssl = ssl; } public void setTimeout(int timeout) { this.timeout = timeout; } public int getTimeout() { return this.timeout; } public Sentinel getSentinel() { return this.sentinel; } public void setSentinel(Sentinel sentinel) { this.sentinel = sentinel; } public Pool getPool() { return this.pool; } public void setPool(Pool pool) { this.pool = pool; } public Cluster getCluster() { return this.cluster; } public void setCluster(Cluster cluster) { this.cluster = cluster; } /** * Pool properties. */ public static class Pool { /** * Max number of "idle" connections in the pool. Use a negative value to indicate * an unlimited number of idle connections. */ private int maxIdle = 8; /** * Target for the minimum number of idle connections to maintain in the pool. This * setting only has an effect if it is positive. */ private int minIdle = 0; /** * Max number of connections that can be allocated by the pool at a given time. * Use a negative value for no limit. */ private int maxActive = 8; /** * Maximum amount of time (in milliseconds) a connection allocation should block * before throwing an exception when the pool is exhausted. Use a negative value * to block indefinitely. */ private int maxWait = -1; public int getMaxIdle() { return this.maxIdle; } public void setMaxIdle(int maxIdle) { this.maxIdle = maxIdle; } public int getMinIdle() { return this.minIdle; } public void setMinIdle(int minIdle) { this.minIdle = minIdle; } public int getMaxActive() { return this.maxActive; } public void setMaxActive(int maxActive) { this.maxActive = maxActive; } public int getMaxWait() { return this.maxWait; } public void setMaxWait(int maxWait) { this.maxWait = maxWait; } } /** * Cluster properties. */ public static class Cluster { /** * Comma-separated list of "host:port" pairs to bootstrap from. This represents an * "initial" list of cluster nodes and is required to have at least one entry. */ private List<String> nodes; /** * Maximum number of redirects to follow when executing commands across the * cluster. */ private Integer maxRedirects; public List<String> getNodes() { return this.nodes; } public void setNodes(List<String> nodes) { this.nodes = nodes; } public Integer getMaxRedirects() { return this.maxRedirects; } public void setMaxRedirects(Integer maxRedirects) { this.maxRedirects = maxRedirects; } } /** * Redis sentinel properties. */ public static class Sentinel { /** * Name of Redis server. */ private String master; /** * Comma-separated list of host:port pairs. */ private String nodes; public String getMaster() { return this.master; } public void setMaster(String master) { this.master = master; } public String getNodes() { return this.nodes; } public void setNodes(String nodes) { this.nodes = nodes; } }}

发现里面的配置项基本都是有默认值的，通过上面的注解可以明白，如果配置文件中存在 spring.redis 开头的配置项，则使用配置文件中的，如果没有的话则使用文件中默认写死的配置。你是不是想到了springboot的另外一个优势：约定大于配置。

这里我们大概了解了SpringBoot自动配置的原理和流程，里面的那些细节我们不在分析。同理，对于那些第三方或者我们自己写的starter，springboot启动的时候会扫描starter的jar包下 META-INF/spring.factories 这个文件，解析内容并加载里面对用的类。

由此，我们总结出以下几点

　　1：SpringBoot的自动配置是如何实现的

　　2：有关的那些注解，如@EnableAutoConfiguration, @ConditionalOnClass, @Configuration等也是SpringBoot的核心注解，也值得我们了解其用法和原理。

总结

以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，谢谢大家对的支持。