Spring Boot高頻面試題：Spring Boot執行原理

之前一篇文章Spring Boot快速入門文章中，我們已經體會到Spring Boot的神器，不再像之前Spring那樣需要繁瑣的XML，甚至幾秒鍾就能搭建出Spring的項目骨架。接下來我們簡單分析Spring Boot的基本原理，讓我們揭開它神秘的面紗吧。

1 @SpringBootApplication

首先，我從引導類開始

/**
 * Spring Boot引導類
 * 一點教程網 - www.yiidian.com
 */
@SpringBootApplication
public class MyBootApplication {

    public static void main(String[] args) {
        SpringApplication.run(MyBootApplication.class,args);
    }

}

引導類代碼很簡單，但可以看出最關鍵的是@SpringBootApplication注解以及在main方法中運行的SpringAppliation.run()了，我們進去@SpringBootApplication的源碼：

@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 {
  ......
}

我們看到@SpringBootApplication其實是一個復合的注解，它就是由@SpringBootConfiguration、@EnableAutoConfiguration以及@ComponentScan 三個注解組成，所以如果我們把SpringBoot啟動類改寫成如下方式，整個SpringBoot應用依然可以與之前的啟動類功能一樣：

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
        @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public class SpringbootDemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(SpringbootDemoApplication.class, args);
    }
}

因為我們每次新建項目時都要寫上三個注解來完成配置，這顯然太繁瑣了，SpringBoot就為我們提供了@SpringBootApplication這樣注解來簡化我們的操作。接着，我們重點分析這三個注解的作用。

2 @SpringBootConfiguration

我們來看@SpringBootConfiguration注解的源碼：

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}

我們可以看到，SpringBoot為了區別@Configuration而新提供的專屬於SpringBoot的注解，功能其實和@Configuration一模一樣。而這里的@Configuration注解對於我們來說並不陌生，它就是是個IoC容器的配置類。看到這里，我們其實可以把SpringBoot的啟動類這樣來看就清楚了：

@Configuration
@EnableAutoConfiguration
@ComponentScan
public class SpringbootDemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(SpringbootDemoApplication.class, args);
    }
}

啟動類SpringbootDemoApplication其實就是一個標准的Spring純注解下的啟動類，也並沒有什么特殊。

3 @EnableAutoConfiguration

看到這個注解，我們不禁聯想出Spring 中很多以“@Enable”開頭的注解，比如：@EnableScheduling、@EnableCaching以及@EnableMBeanExport等，@EnableAutoConfiguration注解的理念和工作原理和它們其實一脈相承。簡單的來說，就是該注解借助@Import注解的支持，Spring的IoC容器收集和注冊特定場景相關的Bean定義：

@EnableScheduling是通過@Import將Spring調度框架相關的bean都加載到IoC容器。
@EnableMBeanExport是通過@Import將JMX相關的bean定義加載到IoC容器。
而@EnableAutoConfiguration注解也是借助@Import將所有復合配置條件的bean定義加載到IoC容器，僅此而已！@EnableAutoConfiguration注解的源碼如下：

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

這其中最關鍵的就是@Import(EnableAutoConfigurationImportSelector.class)了，它借助EnableAutoConfigurationImportSelector.class可以幫助SpringBoot應用將所有符合條件的@Configuration配置類都加載到當前SpringBoot創建並使用的IoC容器，就像下圖一樣。

下面我們給出EnableAutoConfigurationImportSelector.java的父類AutoConfigurationImportSelector.java的部分源碼，來解釋和驗證上圖：

public class AutoConfigurationImportSelector
        implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
BeanFactoryAware, EnvironmentAware, Ordered {
    protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
        return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class,
                this.beanClassLoader);
    }
    protected List<AutoConfigurationImportListener> getAutoConfigurationImportListeners() {
        return SpringFactoriesLoader.loadFactories(AutoConfigurationImportListener.class,
                this.beanClassLoader);
    }
}

以上源碼可以看出，@EnableAutoConfiguration正是借助SpringFactoriesLoader的支持，才能完成所有配置類的加載！

4 SpringFactoriesLoader

SpringFactoriesLoader屬於Spring框架專屬的一種擴展方案(其功能和使用方式類似於Java的SPI方案:java.util.ServiceLoader)，它的主要功能就是從指定的配置文件META-INF/spring.factories中加載配置，spring.factories是一個非常經典的java properties文件，內容格式是Key=Value形式，只不過這Key以及Value都非常特殊，為Java類的完整類名(Fully Qualified Name)，比如：

org.springframework.context.ApplicationListener=org.springframework.boot.autoconfigure.BackgroundPreinitializer

然后Spring框架就可以根據某個類型作為Key來查找對應的類型名稱列表了，SpringFactories源碼如下：

public abstract class SpringFactoriesLoader {

    private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);

    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

    public static <T> List<T> loadFactories(Class<T> factoryClass, ClassLoader classLoader){
        ...
    }

    public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        ...
    }
    // ...
}

對於@EnableAutoConfiguraion來說，SpringFactoriesLoader的用途和其本意稍微不同，它本意是為了提供SPI擴展，而在@EnableAutoConfiguration這個場景下，它更多的是提供了一種配置查找的功能的支持，也就是根據@EnableAutoConfiguration的完整類名org.springframework.boot.autoconfigure.EnableAutoConfiguration作為Key來獲取一組對應的@Configuration類：

# 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.CloudServiceConnectorsAutoConfiguration,\
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.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.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
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.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.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
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.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.groovy.template.GroovyTemplateAutoConfiguration,\
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.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.reactor.core.ReactorCoreAutoConfiguration,\
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.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

在SpringBoot的autoconfigure依賴包中的META-INF文件下的spring.factories文件中，我們可以找到以上內容。

總結來說，@EnableAutoConfiguration能實現自動配置的原理就是：SpringFactoriesLoader從classpath中搜尋所有META-INF/spring.fatories文件，並將其中Key[org.springframework.boot.autoconfigure.EnableAutoConfiguration]對應的Value配置項通過反射的方式實例化為對應的標注了@Configuration的JavaConfig形式的IoC容器配置類，然后匯總到當前使用的IoC容器中。

5 @ComponentScan

@ComponentScan注解在Spring Boot啟動的時候其實不是必需的！因為我們知道作為Spring框架里的老成員，@ComponentScan的功能就是自動掃描並加載復合條件的組件或Bean定義，最終將這些Bean定義加載到當前使用的容器中。這個過程，我們可以手工單個進行注冊，不是一定要通過這個注解批量掃描和注冊，所以說@ComponentScan是非必需的。

所以，如果我們當前應用沒有任何Bean定義需要通過@ComponentScan加載到當前SpringBoot應用對應的IoC容器，那么，去掉@ComponentScan注解，當前的SpringBoot應用依舊可以完美運行！

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