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@interfaceSpringBootApplication{
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
#AutoConfigure 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)
publicclassRedisAutoConfiguration{
/**
*Redisconnectionconfiguration.
*/
@Configuration
@ConditionalOnClass(GenericObjectPool.class)
protectedstaticclassRedisConnectionConfiguration{
privatefinalRedisPropertiesproperties;
privatefinalRedisSentinelConfigurationsentinelConfiguration;
privatefinalRedisClusterConfigurationclusterConfiguration;
publicRedisConnectionConfiguration(RedisPropertiesproperties,
ObjectProvidersentinelConfiguration,
ObjectProviderclusterConfiguration){
this.properties=properties;
this.sentinelConfiguration=sentinelConfiguration.getIfAvailable();
this.clusterConfiguration=clusterConfiguration.getIfAvailable();
}
@Bean
@ConditionalOnMissingBean(RedisConnectionFactory.class)
publicJedisConnectionFactoryredisConnectionFactory()
throwsUnknownHostException{
returnapplyProperties(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")
publicclassRedisProperties{
/**
*Databaseindexusedbytheconnectionfactory.
*/
privateintdatabase=0;
/**
*Redisurl,whichwilloverrulehost,portandpasswordifset.
*/
privateStringurl;
/**
*Redisserverhost.
*/
privateStringhost="localhost";
/**
*Loginpasswordoftheredisserver.
*/
privateStringpassword;
/**
*Redisserverport.
*/
privateintport=6379;
/**
*EnableSSL.
*/
privatebooleanssl;
/**
*Connectiontimeoutinmilliseconds.
*/
privateinttimeout;
privatePoolpool;
privateSentinelsentinel;
privateClustercluster;
publicintgetDatabase(){
returnthis.database;
}
publicvoidsetDatabase(intdatabase){
this.database=database;
}
publicStringgetUrl(){
returnthis.url;
}
publicvoidsetUrl(Stringurl){
this.url=url;
}
publicStringgetHost(){
returnthis.host;
}
publicvoidsetHost(Stringhost){
this.host=host;
}
publicStringgetPassword(){
returnthis.password;
}
publicvoidsetPassword(Stringpassword){
this.password=password;
}
publicintgetPort(){
returnthis.port;
}
publicvoidsetPort(intport){
this.port=port;
}
publicbooleanisSsl(){
returnthis.ssl;
}
publicvoidsetSsl(booleanssl){
this.ssl=ssl;
}
publicvoidsetTimeout(inttimeout){
this.timeout=timeout;
}
publicintgetTimeout(){
returnthis.timeout;
}
publicSentinelgetSentinel(){
returnthis.sentinel;
}
publicvoidsetSentinel(Sentinelsentinel){
this.sentinel=sentinel;
}
publicPoolgetPool(){
returnthis.pool;
}
publicvoidsetPool(Poolpool){
this.pool=pool;
}
publicClustergetCluster(){
returnthis.cluster;
}
publicvoidsetCluster(Clustercluster){
this.cluster=cluster;
}
/**
*Poolproperties.
*/
publicstaticclassPool{
/**
*Maxnumberof"idle"connectionsinthepool.Useanegativevaluetoindicate
*anunlimitednumberofidleconnections.
*/
privateintmaxIdle=8;
/**
*Targetfortheminimumnumberofidleconnectionstomaintaininthepool.This
*settingonlyhasaneffectifitispositive.
*/
privateintminIdle=0;
/**
*Maxnumberofconnectionsthatcanbeallocatedbythepoolatagiventime.
*Useanegativevaluefornolimit.
*/
privateintmaxActive=8;
/**
*Maximumamountoftime(inmilliseconds)aconnectionallocationshouldblock
*beforethrowinganexceptionwhenthepoolisexhausted.Useanegativevalue
*toblockindefinitely.
*/
privateintmaxWait=-1;
publicintgetMaxIdle(){
returnthis.maxIdle;
}
publicvoidsetMaxIdle(intmaxIdle){
this.maxIdle=maxIdle;
}
publicintgetMinIdle(){
returnthis.minIdle;
}
publicvoidsetMinIdle(intminIdle){
this.minIdle=minIdle;
}
publicintgetMaxActive(){
returnthis.maxActive;
}
publicvoidsetMaxActive(intmaxActive){
this.maxActive=maxActive;
}
publicintgetMaxWait(){
returnthis.maxWait;
}
publicvoidsetMaxWait(intmaxWait){
this.maxWait=maxWait;
}
}
/**
*Clusterproperties.
*/
publicstaticclassCluster{
/**
*Comma-separatedlistof"host:port"pairstobootstrapfrom.Thisrepresentsan
*"initial"listofclusternodesandisrequiredtohaveatleastoneentry.
*/
privateListnodes;
/**
*Maximumnumberofredirectstofollowwhenexecutingcommandsacrossthe
*cluster.
*/
privateIntegermaxRedirects;
publicListgetNodes(){
returnthis.nodes;
}
publicvoidsetNodes(Listnodes){
this.nodes=nodes;
}
publicIntegergetMaxRedirects(){
returnthis.maxRedirects;
}
publicvoidsetMaxRedirects(IntegermaxRedirects){
this.maxRedirects=maxRedirects;
}
}
/**
*Redissentinelproperties.
*/
publicstaticclassSentinel{
/**
*NameofRedisserver.
*/
privateStringmaster;
/**
*Comma-separatedlistofhost:portpairs.
*/
privateStringnodes;
publicStringgetMaster(){
returnthis.master;
}
publicvoidsetMaster(Stringmaster){
this.master=master;
}
publicStringgetNodes(){
returnthis.nodes;
}
publicvoidsetNodes(Stringnodes){
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，谢谢大家对毛票票的支持。