概念

springboot auto configuration的本質就是自動配置spring的各種bean。然后使應用可以通過@Autowired等注入方式來直接使用bean。
比如自動配置redisTemplate，jdbcTemplate等bean。

原理

1. 開啟@EnableAutoConfiguration注解

如果寫了@SpringBootApplication注解，不必再寫@EnableAutoConfiguration。

@SpringBootApplication = @Configuration + @EnableAutoConfiguration + @ComponentScan
@SpringCloudApplication = @SpringBootApplication + @EnableDiscoveryClient + @EnableCircuitBreaker

2. 開啟@EnableAutoConfiguration注解后，會引入一個AutoConfigurationImportSelector類

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

   String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

   /**
    * Exclude specific auto-configuration classes such that they will never be applied.
    * @return the classes to exclude
    */
   Class<?>[] exclude() default {};

   /**
    * Exclude specific auto-configuration class names such that they will never be
    * applied.
    * @return the class names to exclude
    * @since 1.3.0
    */
   String[] excludeName() default {};

}

其會去掃描classpath下所有jar包內的META-INF/spring.factories文件.

3. spring.factories文件

spring.factories文件路徑是
spring-boot-autoconfigure-2.3.4.RELEASE.jar下的META-INFspring.factories，spring.factories文件內容：

# Initializers
org.springframework.context.ApplicationContextInitializer=
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

# Application Listeners
org.springframework.context.ApplicationListener=
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=
org.springframework.boot.autoconfigure.condition.OnBeanCondition,
org.springframework.boot.autoconfigure.condition.OnClassCondition,
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

# 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.context.ConfigurationPropertiesAutoConfiguration,
org.springframework.boot.autoconfigure.context.LifecycleAutoConfiguration,
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.CassandraReactiveDataAutoConfiguration,
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRestClientAutoConfiguration,
org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,
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.r2dbc.R2dbcDataAutoConfiguration,
org.springframework.boot.autoconfigure.data.r2dbc.R2dbcRepositoriesAutoConfiguration,
org.springframework.boot.autoconfigure.data.r2dbc.R2dbcTransactionManagerAutoConfiguration,
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,
org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,
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.ElasticsearchRestClientAutoConfiguration,
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,
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.http.HttpMessageConvertersAutoConfiguration,
org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,
org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,
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.jersey.JerseyAutoConfiguration,
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,
org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,
org.springframework.boot.autoconfigure.avAIlability.ApplicationAvailabilityAutoConfiguration,
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.mongo.embedded.EmbeddedMongoAutoConfiguration,
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,
org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,
org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,
org.springframework.boot.autoconfigure.r2dbc.R2dbcAutoConfiguration,
org.springframework.boot.autoconfigure.rsocket.RSocketMessagingAutoConfiguration,
org.springframework.boot.autoconfigure.rsocket.RSocketRequesterAutoConfiguration,
org.springframework.boot.autoconfigure.rsocket.RSocketServerAutoConfiguration,
org.springframework.boot.autoconfigure.rsocket.RSocketStrategiesAutoConfiguration,
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,
org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,
org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,
org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,
org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,
org.springframework.boot.autoconfigure.security.rsocket.RSocketSecurityAutoConfiguration,
org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyAutoConfiguration,
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,
org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,
org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,
org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,
org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,
org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,
org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,
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.client.RestTemplateAutoConfiguration,
org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,
org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,
org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,
org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,
org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,
org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,
org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,
org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,
org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,
org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,
org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

# Failure analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=
org.springframework.boot.autoconfigure.data.redis.RedisUrlSyntaxFailureAnalyzer,
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,
org.springframework.boot.autoconfigure.flyway.FlywayMigrationScriptMissingFailureAnalyzer,
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,
org.springframework.boot.autoconfigure.r2dbc.ConnectionFactoryBeanCreationFailureAnalyzer,
org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer

# Template availability providers
org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=
org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,
org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,
org.springframework.boot.autoconfigure.web.servlet.JspTemplateAvailabilityProvider

spring boot 會自動配置
org.springframework.boot.autoconfigure.EnableAutoConfiguration的key下所有自動配置類。可以看到這里面包含RabbitMq，MongoDB，Redis，JdbcTemplate，Thymeleaf，WebFlux，WebSocket等等模塊的自動配置類。

4. 一般自動配置類都有@Configuration注解，相當于一個spring的xml文件，里面可以通過@Bean注解配置bean。具體例子見org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration類。

package org.springframework.boot.autoconfigure.data.redis;

import JAVA.NET.UnknownHostException;

import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;
import org.springframework.data.redis.connection.RedisConnectionFactory;
import org.springframework.data.redis.core.RedisOperations;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.data.redis.core.StringRedisTemplate;

/**
 * {@link EnableAutoConfiguration Auto-configuration} for Spring Data's Redis support.
 *
 * @author Dave Syer
 * @author Andy Wilkinson
 * @author Christian Dupuis
 * @author Christoph Strobl
 * @author Phillip Webb
 * @author Eddú Meléndez
 * @author Stephane Nicoll
 * @author Marco Aust
 * @author Mark Paluch
 * @since 1.0.0
 */
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(RedisOperations.class)
@EnableConfigurationProperties(RedisProperties.class)
@Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class })
public class RedisAutoConfiguration {

   @Bean
   @ConditionalOnMissingBean(name = "redisTemplate")
   public RedisTemplate<Object, Object> redisTemplate(RedisConnectionFactory redisConnectionFactory)
         throws UnknownHostException {
      RedisTemplate<Object, Object> template = new RedisTemplate<>();
      template.setConnectionFactory(redisConnectionFactory);
      return template;
   }

   @Bean
   @ConditionalOnMissingBean
   public StringRedisTemplate stringRedisTemplate(RedisConnectionFactory redisConnectionFactory)
         throws UnknownHostException {
      StringRedisTemplate template = new StringRedisTemplate();
      template.setConnectionFactory(redisConnectionFactory);
      return template;
   }

}

@ConditionalOnXXX：

• @ConditionalOnBean（僅僅在當前上下文中存在某個對象時，才會實例化一個Bean
• @ConditionalOnClass（某個class位于類路徑上，才會實例化一個Bean）
• @ConditionalOnExpression（當表達式為true的時候，才會實例化一個Bean）
• @ConditionalOnMissingBean（僅僅在當前上下文中不存在某個對象時，才會實例化一個Bean）
• @ConditionalOnMissingClass（某個class類路徑上不存在的時候，才會實例化一個Bean）
• @ConditionalOnProperty(value = “spring.cloud.config.enabled”, matchIfMissing = true)（當某個配置的值為一定條件時，才會實例化一個Bean）

5. 配置bean的時候，少不了要獲取properties或者yml文件的屬性值

5.1 通過@ConfigurationProperties(prefix=”xxxx”)映射配置文件的值到POJO

application.yml

定義一個POJO類，例：ZookeeperProperties.java，通過prefix指定配置屬性的前綴，加上@Component，這可以變成spring bean，可以直接用@Autowired注入。

spring：
    zookeeper:
        address: 172.16.0.100:2181
        timeout: 4000

@ConfigurationProperties(prefix = "spring.zookeeper")
public class ZookeeperProperties {

    RedisAutoConfiguration redisAutoConfiguration;
    private String address;

    private Integer timeout;

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public Integer getTimeout() {
        return timeout;
    }

    public void setTimeout(Integer timeout) {
        this.timeout = timeout;
    }
}

5.2 將配置文件的POJO類轉化為spring的bean

5.2.1. 在要使用配置的地方使用@EnableConfigurationProperties(ZookeeperProperties.class)將其轉化為spring的bean，也可以使用@component，二選一。

我們使用EnableConfigurationProperties模式：

import org.Apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import java.util.concurrent.CountDownLatch;

@Configuration
@EnableConfigurationProperties(ZookeeperProperties.class)
public class ZookeeperConfig {
    private static final Logger logger = LoggerFactory.getLogger(ZookeeperConfig.class);

    @Autowired
    ZookeeperProperties zookeeperProperties;
    
    @Bean(name = "zkClient")
    public ZooKeeper zkClient(){
        ZooKeeper zooKeeper=null;
        try {
            final CountDownLatch countDownLatch = new CountDownLatch(1);
            //連接成功后，會回調watcher監(jiān)聽，此連接操作是異步的，執(zhí)行完new語句后，直接調用后續(xù)代碼
            //  可指定多臺服務地址 127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183
            zooKeeper = new ZooKeeper(zookeeperProperties.getAddress() , zookeeperProperties.getTimeout(), new Watcher() {
                @Override
                public void process(WatchedEvent event) {
                    if(Event.KeeperState.SyncConnected==event.getState()){
                        //如果收到了服務端的響應事件,連接成功
                        countDownLatch.countDown();
                    }
                }
            });
            countDownLatch.await();
            logger.info("【初始化ZooKeeper連接狀態(tài)....】={}",zooKeeper.getState());

        }catch (Exception e){
            logger.error("初始化ZooKeeper連接異常....】={}",e);
        }
        return  zooKeeper;
    }
}

5.2.2. 通過@Bean配置將其轉化為spring的bean

@Configuration

public class ZKConfiguration {
      @Bean
      public ZookeeperProperties zookeeperProperties() {
          return new ZookeeperProperties();
      }
}

5.2.3. 通過@Component配置將其轉化為spring的bean

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.stereotype.Component;


@Component
@ConfigurationProperties(prefix = "spring.zookeeper")
public class ZookeeperProperties {

    private String address;

    private Integer timeout;

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public Integer getTimeout() {
        return timeout;
    }

    public void setTimeout(Integer timeout) {
        this.timeout = timeout;
    }
}

轉化為spring的bean后，就可以在想用的地方通過任意注入方式注入bean來使用了。

5.2.4. 總結

在寫spring boot starter的時候，因為所有類都不在啟動的@ComponentScan目錄下，所以使用一般使用 5.2.1 或者 5.2.2 的方式獲取配置文件信息。在寫具體的spring boot應用服務時，配置類都在啟動的@ComponentScan目錄下，使用 5.2.3 的方式比較方便