SpringBoot自动注入分析

　　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 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

　　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。