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 sentinelConfiguration,
ObjectProvider 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 nodes;
/**
* Maximum number of redirects to follow when executing commands across the
* cluster.
*/
private Integer maxRedirects;
public List getNodes() {
return this.nodes;
}
public void setNodes(List 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。
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