Spring Boot高频面试题：Spring Boot执行原理

时间 2020-02-17

原文原文链接

以前一篇文章Spring Boot快速入门文章中，咱们已经体会到Spring Boot的神器，再也不像以前Spring那样须要繁琐的XML，甚至几秒钟就能搭建出Spring的项目骨架。接下来咱们简单分析Spring Boot的基本原理，让咱们揭开它神秘的面纱吧。html

1 @SpringBootApplication

首先，我从引导类开始java

/**
 * Spring Boot引导类
 * 一点教程网 - www.yiidian.com
 */
@SpringBootApplication
public class MyBootApplication {

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

}

引导类代码很简单，但能够看出最关键的是@SpringBootApplication注解以及在main方法中运行的SpringAppliation.run()了，咱们进去@SpringBootApplication的源码：react

@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 {
  ......
}

咱们看到@SpringBootApplication实际上是一个复合的注解，它就是由@SpringBootConfiguration、@EnableAutoConfiguration以及@ComponentScan 三个注解组成，因此若是咱们把SpringBoot启动类改写成以下方式，整个SpringBoot应用依然能够与以前的启动类功能同样：web

@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);
    }
}

由于咱们每次新建项目时都要写上三个注解来完成配置，这显然太繁琐了，SpringBoot就为咱们提供了@SpringBootApplication这样注解来简化咱们的操做。接着，咱们重点分析这三个注解的做用。redis

2 @SpringBootConfiguration

咱们来看@SpringBootConfiguration注解的源码：spring

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

咱们能够看到，SpringBoot为了区别@Configuration而新提供的专属于SpringBoot的注解，功能其实和@Configuration如出一辙。而这里的@Configuration注解对于咱们来讲并不陌生，它就是是个IoC容器的配置类。看到这里，咱们其实能够把SpringBoot的启动类这样来看就清楚了：json

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

启动类SpringbootDemoApplication其实就是一个标准的Spring纯注解下的启动类，也并无什么特殊。segmentfault

3 @EnableAutoConfiguration

看到这个注解，咱们不由联想出Spring 中不少以“@Enable”开头的注解，好比：@EnableScheduling、@EnableCaching以及@EnableMBeanExport等，@EnableAutoConfiguration注解的理念和工做原理和它们其实一脉相承。简单的来讲，就是该注解借助@Import注解的支持，Spring的IoC容器收集和注册特定场景相关的Bean定义：springboot

@EnableScheduling是经过@Import将Spring调度框架相关的bean都加载到IoC容器。
@EnableMBeanExport是经过@Import将JMX相关的bean定义加载到IoC容器。
而@EnableAutoConfiguration注解也是借助@Import将全部复合配置条件的bean定义加载到IoC容器，仅此而已！@EnableAutoConfiguration注解的源码以下：websocket

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

这其中最关键的就是@Import(EnableAutoConfigurationImportSelector.class)了，它借助EnableAutoConfigurationImportSelector.class能够帮助SpringBoot应用将全部符合条件的@Configuration配置类都加载到当前SpringBoot建立并使用的IoC容器，就像下图同样。

下面咱们给出EnableAutoConfigurationImportSelector.java的父类AutoConfigurationImportSelector.java的部分源码，来解释和验证上图：

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);
    }
}

以上源码能够看出，@EnableAutoConfiguration正是借助SpringFactoriesLoader的支持，才能完成全部配置类的加载！

4 SpringFactoriesLoader

SpringFactoriesLoader属于Spring框架专属的一种扩展方案(其功能和使用方式相似于Java的SPI方案:java.util.ServiceLoader)，它的主要功能就是从指定的配置文件META-INF/spring.factories中加载配置，spring.factories是一个很是经典的java properties文件，内容格式是Key=Value形式，只不过这Key以及Value都很是特殊，为Java类的完整类名(Fully Qualified Name)，好比：

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

而后Spring框架就能够根据某个类型做为Key来查找对应的类型名称列表了，SpringFactories源码以下：

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) {
        ...
    }
    // ...
}

对于@EnableAutoConfiguraion来讲，SpringFactoriesLoader的用途和其本意稍微不一样，它本意是为了提供SPI扩展，而在@EnableAutoConfiguration这个场景下，它更多的是提供了一种配置查找的功能的支持，也就是根据@EnableAutoConfiguration的完整类名org.springframework.boot.autoconfigure.EnableAutoConfiguration做为Key来获取一组对应的@Configuration类：

# 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依赖包中的META-INF文件下的spring.factories文件中，咱们能够找到以上内容。

总结来讲，@EnableAutoConfiguration能实现自动配置的原理就是：SpringFactoriesLoader从classpath中搜寻全部META-INF/spring.fatories文件，并将其中Key[org.springframework.boot.autoconfigure.EnableAutoConfiguration]对应的Value配置项经过反射的方式实例化为对应的标注了@Configuration的JavaConfig形式的IoC容器配置类，而后汇总到当前使用的IoC容器中。

5 @ComponentScan

@ComponentScan注解在Spring Boot启动的时候其实不是必需的！由于咱们知道做为Spring框架里的老成员，@ComponentScan的功能就是自动扫描并加载复合条件的组件或Bean定义，最终将这些Bean定义加载到当前使用的容器中。这个过程，咱们能够手工单个进行注册，不是必定要经过这个注解批量扫描和注册，因此说@ComponentScan是非必需的。

因此，若是咱们当前应用没有任何Bean定义须要经过@ComponentScan加载到当前SpringBoot应用对应的IoC容器，那么，去掉@ComponentScan注解，当前的SpringBoot应用依旧能够完美运行！

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