Spring Boot自动注入的原理分析

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 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。

来源：https://www.cnblogs.com/wanghaoyang/p/11693973.html